Planning and Making CrPlanning and Making Crowns and Bridges - Smith. 4th Ed.2007owns and Bridges -...

Planning and Making

Crowns and Bridges

Fourth Edition

Planning and Making

Crowns and Bridges

Fourth Edition

This highly successful text, which has achieved wide acclaim among practitioners and is arecommended text in the major dental schools, has again been revised and updated to keepit at the forefront of clinical practice. As before, the intention is to help solve real clinicalproblems by guiding dentists to make the right initial decision and then carry it through.This new edition includes significantly expanded material on veneers and implants inparticular, so as to help dentists advise patients in their choice of what to do and how tomake informed choices.

FROM REVIEWS OF PREVIOUS EDITIONS:

'well thought out and exceedingly well presented. I can wholeheartedly recommend this work as a worthwhile additionto any practitioner's bookshelf.' British Dental Review

'the greatest strength of the book lies in the sensible and comprehensive way it provides descriptions of clinicalprocedures' British Dental Journal

'must be considered a "best buy" for every practitioner interested in a planned approach to crown and bridgework'Dental Update

‘should be on every dentist’s bookshelf ’ Dental Practice

WITH OVER 600 COLOUR AND BLACK-AND-WHITE PHOTOGRAPHS

CONTENTS:Indications and contraindications for crowns • Types of crown • Designing crown preparations • Occlusalconsiderations • Planning and making crowns • Clinical techniques for making crowns • Indications for bridgescompared with partial dentures and implant-retained prostheses • Types of bridge • Components of bridges:retainers, pontics and connectors • Designing and planning bridges • Clinical techniques for bridgeconstruction • Dental implants • Fixed splints • Crown and bridge failures and repairs

Bernard GN Smith, BDS, PhD, MSc, MRD, FDSRCSEng, FDSRCSEdin, studied at the Universities of London andMichigan and spent a number of years in general practice. He is Professor Emeritus of Conservative Dentistry at Guy’s,King’s and St Thomas’ Dental Institute, Guy’s Hospital, London, and has served as examiner at several universities inthe UK and other countries and at the Royal Colleges of Surgery of England, Glasgow and Edinburgh, as HonoraryConsultant in restorative dentistry to the British Army and as past President of the British Society of RestorativeDentistry. He has contributed to a wide range of dental journals and is the former editor of Restorative Dentistry. He isthe joint author of Pickard’s Manual of Operative Dentistry.

Leslie C Howe, BDS, FDSRCSEng, is a Consultant in Restorative Dentistry at Guy’s and St Thomas’ NHS HospitalsTrust, Guy’s Hospital, London, and a Specialist in Restorative Dentistry and Prosthodontics, and is in specialist practicein London. He is also the joint author of Implants in Clinical Dentistry.

Planning and Making

Crow

ns and Bridges

Bernard GN SmithLeslie C Howe

SmithHowe

FourthEdition

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Planning and making crowns and bridges

Planning andmaking crowns and bridges

Fourth Edition

Bernard G N SmithBDS, PhD, MSc, MRD, FDSRCSEng, FDSRCSEdin

Professor Emeritus of Conservative DentistryGuy’s, Kings and St Thomas’ Dental Institute, London

Leslie C HoweBDS FDSRCSEng

Consultant in Restorative DentistryGuy’s and St Thomas’ NHS Hospitals Trustand Private Practitioner

© 1986, 1990, 1998 Bernard GN Smith, 2007 Bernard GN Smith and Leslie C Howe

Fourth edition published in the United Kingdom in 2007 by Informa Healthcare4 Park SquareMilton ParkAbingdon, Oxon OX14 4RN, UK

Informa Healthcare is a trading division of Informa UK Ltd. Registered Office: 37/41 Mortimer Street, London W1T 3JH. Registered in England and Wales number 1072954.

Tel: +44 (0)20 7017 6000Fax: +44 (0)20 7017 6336E-mail: [email protected]: www.informahealthcare.com

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any formor by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior permission of the publisheror in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of any licencepermitting limited copying issued by the Copyright Licensing Agency, 90 Tottenham Court Road, London W1P 0LP.

The Authors have asserted their rights under the Copyright, Designs and Patents Act 1988 to be identified as the Authors ofthis Work.

A CIP record for this book is available from the British Library.

Library of Congress Cataloging-in-Publication Data

Data available on application

ISBN-10: 0 415 39850 9 ISBN-13: 978 0 415 39850 3

Distributed in the United States and Canada byThieme New York333 Seventh AvenueNew York, NY 10001, USA

Distributed in the rest of the world byThomson Publishing ServicesCheriton HouseNorth WayAndover, Hampshire SP10 5BE, UK

Composition by Scribe Design Ltd, Ashford, Kent, UKPrinted and bound in

Preface vii

Acknowledgements ix

Part 1 Crowns

1 Indications and contraindications for crowns 3

2 Types of crown 29

3 Designing crown preparations 53

4 Occlusal considerations 77

5 Planning and making crowns 105

6 Clinical techniques for making crowns 123

Part 2 Bridges

7 Indications for bridges compared with partial dentures and implant-retained prostheses 177

8 Types of bridge 197

9 Components of bridges: retainers, pontics and connectors 211

10 Designing and planning bridges 225

11 Clinical techniques for bridge construction 241

Part 3 Implants, splints and maintenance

12 Dental implants 261

13 Fixed splints 287

14 Crown and bridge failures and repairs 297

Index 319

Contents

The aim of this book is to answer at least as manyof the questions beginning with ‘why’ as thosethat begin with ‘how’. A textbook is not the idealmedium for teaching practical, clinical or techni-cal procedures. These are best learnt at the chair-side and in the laboratory. However, the mass ofmaterial which must be learnt, usually in arestricted timetable, in the clinic and laboratorymeans that there is often insufficient time toanswer the questions, ‘Why am I doing this?’ or,‘When should I not do this?’ or even, ‘What onearth can I do here?’.

The book is meant for clinicians, both under-graduate and postgraduate, and so although theemphasis is on treatment planning, crown andbridge design and the related theory, clinicaltechniques are also described in some detail.Laboratory technique is, though, almost com-pletely omitted, both to keep the book tomanageable proportions and because most clini-cians no longer undertake this themselves. It isnevertheless abundantly clear that a good stan-dard of laboratory work is as important as theother phases in the construction of crowns,bridges and implants.

The process may be divided into three stages:

Initial decision making and mouth preparationClinical proceduresTechnical procedures.

The purpose of this book is to help quite a lotwith the first stage, rather less with the second(a book cannot replace clinical experience) andhardly at all with the third.

The intention is to help solve real clinicalproblems. The student sitting in a techniquelaboratory faced with an arch of intact perfectlyformed natural or artificial teeth planning toundertake ‘ideal’ crown preparations will findlittle help here. It may be good initial teaching tocut ‘classic’ preparations, but this is only part ofthe training towards solving the real problems ofreal patients in the real world. The opinionsexpressed in a textbook can only go a little wayfurther towards solving these problems.

Undergraduate and postgraduate students needalso to take advantage of their own and others’clinical experience and learn by thinking abouttheir clinical problems and talking about themwith others. Making the right decision is as impor-tant as executing the treatment well.

There is no reference to ‘case selection’ or‘patient selection’ for the techniques described.That is not the way things are in practice. Thereit is necessary to select the appropriate techniquefor the patient in front of you rather than selectthe patient for the technique. Things are differentin dental schools. It often happens that in orderto provide a balanced range of experience forundergraduate students in a limited period oftime, patients are selected to go on to particularwaiting lists to provide a flow of ‘clinical material’for the students’ needs. This may be necessarybut the attitudes it sometimes develops areunfortunate. The essential feature of any profes-sion is that it attempts to solve the problems ofits clients before concerning itself with its ownwelfare.

Because this is the approach, clinical photo-graphs or at least photographs of extracted teethor casts, are used to illustrate the text in prefer-ence to line drawings, except where a photographis impractical. Photographs are used even whenthe work shown is not ‘perfect’. No apology ismade for this. In reality, although we should strivefor perfection (if we know what perfection is ina given case, and we often do not), we willfrequently not achieve it. It is more realistic totalk about levels of acceptability. This is not toadvocate unnecessary compromise, but to recog-nise that in many situations a compromise (fromknowledge, not ignorance) is necessary. After all,the ideal would be to prevent caries, trauma andcongenital deformity so that crowns and bridgeswere not necessary in the first place. Once theyare needed there is already a situation that is lessthan perfect.

Some of the work photographed is the authors’,some is undergraduate and postgraduate studentwork with a greater or lesser amount of help byteachers, some of the technical work is carried out

Preface

by the clinicians themselves but most by techni-cians or student technicians, and some illustrationshave been kindly lent by colleagues. In view of thelikelihood, and indeed the intention that readerswill find fault with some of the illustrations andbecause some illustrate the work of a team ratherthan an individual, no acknowledgement is given for individual illustrations. We are, however,extremely grateful to all those who have allowedus to photograph their work and in particular tothose who have lent their own illustrations. Theirnames appear in the Acknowledgements.

There are no text references. In a book of thissize, which is not intended to be a referencebook, it is not possible to be comprehensive,while it is impolite to use phrases such as ‘thereis evidence that . . .’ without making proper refer-ence to the source of the evidence. Isolatedreferences in these cases could well lead theenthusiastic student into an unbalanced readingprogramme. The further reading suggestionswhich were in previous editions have beenomitted as they so quickly became out-of-date inthis fast-developing field and because computeraccess to the literature is now very easy.

Leslie Howe has joined Bernard Smith as a jointauthor for the fourth edition. His influence canbe seen throughout the book and particularly in

the new Chapter 12 on implants. Much moreattention is given to implants than in previouseditions. Modern implants had only recently beenintroduced into the UK when the first edition waspublished. The purpose is to help dentists toadvise patients in their choice of what to do whenteeth are missing. It is also to inform dentistsabout details of implant treatment so that theycan explain to patients what to expect and helpthem to make informed choices.

Chapter 12 is not sufficient to guide dentists instarting to place implant-retained restorationsthemselves. Much more training is needed beforethat can be done. A comparison can be madewith referral for orthodontic treatment which,like implants, most dentists do not offer but theydo need to know who and when to refer and tobe able to answer patients’ questions.

Much of the material in earlier editions hasbeen omitted as being out-of-date; however,some restorations which are no longer made butwhich a significant number of patients continue towear satisfactorily are still included so thatdentists can recognise them and know somethingabout their maintenance and repair.

Because of these deletions, many illustrationshave been left out but even more have beenadded.

BGNS LCH

viii Preface

The following have lent photographs but forreasons explained in the preface, specific credit isnot given to each one. We are, however,extremely grateful to them for their generosity:Chris Allen; David Bartlett; Nicholas Capp; JohnCardwell; Russell Greenwood; GeorgeKantorowicz; Bernard Keiser; Frances Manochi;Orthomax Limited, Bradford; Richard Palmer;David Parr; Ian Potter; John Richards; DavidRicketts; Paul Robinson; Saridatum Abdul Samal;

Evelyn Sheehy; Michael Thomas; Sachim Varma;John Walter; Katherine Warren; Tim Watson.

Other help with illustrations has been given by:Ruth Allen; Osama Atta; Dennis Bailey; TrevorBrooker; Peter Chittenden; Cottrell and Co.,London; Usha Desai; Andrew Dyer; TerenceFreeman; Richard Hamill; June Hodgkin; OrodentLimited, Windsor; Peter Pilecki; Peter Rhind; BillSharpling; Nicholas Taylor; Leslie Wilcox;Geraldine Williams.

Acknowledgements

Part 1 Crowns

Before the introduction of techniques to bondrestorative materials to teeth, crowns were theonly way of restoring teeth that can now berestored by these other techniques. At the sametime, more patients are keeping more of their teethfor longer and are expecting faulty teeth to berepaired rather than extracted. Therefore, althoughthere are fewer indications for crowning teeth thanthere were, more teeth are actually being crownedthan ever before. About two million crowns peryear are made in the UK National Health Service,representing two to three crowns per week perdentist. In the latest (1998) Adult Dental HealthSurvey in the UK, a third of all dentate adults hadat least one crown and nearly half the 45–54 agegroup had one or more crowns. Similar numbersof crowns are being made in many other countries.

So, a lot of crowns are still being made.When the only choice for a tooth was a crown

or extraction, the decision was relatively simple.Now, with more options it is more difficult. Thischapter discusses the current indications forcrowns and their alternatives, and guides the readertowards a decision. However, clinical decisionmaking is the very substance of the dentist’s workand cannot be done by textbook instructions: donot expect a set of clear rules to follow. Each setof clinical judgements and decisions must be unique,taken in the context of the patient’s circumstances.

General indications andcontraindications for crowns

Crowns versus fillings in thetreatment of caries

Most dental restorations are provided as treat-ment for dental caries. Once the initial lesion haspenetrated the enamel, the caries spreads along

the enamel–dentine junction and balloons out indentine towards the pulp. The growth of thecarious lesion is much faster in dentine than it isin enamel, so the enamel becomes underminedand then suddenly collapses into the cavity.(Because of this, our forefathers thought thatcaries started inside the tooth and worked itsway to the surface.) Today, many carious lesionsare detected and treated at an early stage whilethe enamel is still largely intact. Indeed, evenmore lesions are prevented from occurring at all.

Since caries produces most of its damage insidethe tooth rather than on the surface, the common-est types of restoration are fillings (intracoronalrestorations). Often, sound enamel has to be cutaway to give access to the caries. Only very rarelyis the surface of a tooth extensively destroyed bycaries leaving a base of sound dentine, and it istherefore most unusual in the treatment of primarycaries for a crown (an extracoronal restoration) tobe made on a preparation consisting of intactdentine. When secondary caries develops aroundexisting fillings, intracoronal restorations are stillmore conservative and more closely relate to thepattern of development of caries than crowns, andare therefore preferred whenever possible. Indeed,a high caries rate is a contraindication to crowns.In these cases the caries should be removed, thetooth stabilized and a preventive regime institutedbefore crowns are made.

With larger lesions and particularly when cuspsare lost, the decision between filling and crowninga tooth becomes more difficult (see pages 20–25).

General indications for crowns forother reasons

Having established that primary caries is not acommon reason for making crowns, morecommon reasons are:

3

1 Indications andcontraindicationsfor crowns

4 Indications and contraindications for crowns

Figure 1.1

General indications for crowns.

a This mouth has been well treated in the past but therestorations are now failing. In particular the lateralincisor has lost two fillings, the pulp has died and thetooth is discoloured. It now needs a crown (Figure2.1k).

b Trauma: the result of a blow from a hockey stick.Two incisors have been lost and the upper right centralincisor is fractured, exposing the pulp, the fracture lineextending subgingivally on the palatal side. The lateralincisor is fractured involving enamel and dentine only.The pulp retained its vitality. Although it could berestored in other ways, a crown would be the mostsatisfactory solution since it would then match theother anterior restorations. If the central incisor is tobe retained, it will need to be root-filled and crowned,probably as a bridge abutment (see later).

c Gross tooth wear arising from a combination oferosion and attrition. This has passed the point wherethe patient can accept the appearance, and crowns arenecessary.

e A moderate degree of amelogenesis imperfecta(defects in the formation of enamel) in a 16 year old.The posterior teeth are affected more than theanterior teeth but the upper incisors are slightlydiscoloured and are chipping away at the incisal edge.Crowns were made for all the teeth except the lowerincisors and these will be kept under review.

d Peg-shaped upper lateral incisors.

• Badly broken-down teeth• Primary trauma• Tooth wear• Hypoplastic conditions• To alter the shape, size or inclination of teeth• To alter the occlusion• As part of another restoration• Combined indications• Multiple crowns• Appearance.

Badly broken-down teeth

Extensive composite or amalgam fillings bondedto the remaining tooth structure or retained byother means have the advantage of being directlyplaced, are conservative of tooth structure anddo not involve laboratory procedures. However,when very large, involving most of the occlusalsurface, such restorations are rarely able toproduce an acceptable occlusal and proximalcontour and have an unpredictable long-termdurability and so a crown may be the treatmentof choice.

Usually these teeth will have been restoredpreviously, and may have suffered secondarycaries or parts of the tooth or restoration mayhave broken off. Before crowns can be made the

lost dentine will often need to be replaced by asuitable core of restorative material sometimesfollowing endodontic treatment (Figure 1.1a).

Primary trauma

An otherwise intact tooth may have a largefragment broken off without damaging the pulpand leaving sufficient dentine to support a crown:see the upper right lateral incisor in Figure 1.1b.If this was the only tooth damaged then a directlyplaced composite restoration bonded to theremaining tooth structure would usually be theinitial treatment, progressing to a crown if thedirect restoration was inadequate or failed.

Tooth wear

The processes of erosion (damage from acidother than that produced by bacteria), attrition(mechanical wear of one tooth against another)and abrasion (mechanical wear by extraneousagents) occur in all patients. What is remarkableis that teeth, which have little capacity for regen-eration and which are in constant use, do notwear out long before the patient dies. Althoughtooth wear is normal, if it is excessive or occurs

Indications and contraindications for crowns 5

g Typical distribution of enamel hypoplasia, in this casedue to typhoid in the patient’s early childhood. Crownswere made rather than composite restorations orveneers because the stain was too dark to be disguisedby these means.

f Dentinogenesis imperfecta (a defect in the formationof dentine) in a teenage patient. The incisor teeth havebeen protected with acid-etch-retained composite fromshortly after their eruption and the first molar teethhave been protected with stainless-steel crowns. It isnow time to make permanent crowns for all theremaining teeth.

early in life, crowns or other restorations may beneeded (Figure 1.1c).

The lifelong management of excessive toothwear is a topic of increasing interest as patientskeep their teeth longer. In general the approachshould be:

• Early diagnosis and prevention.• Monitoring any further progression until the

patient complains of the appearance, sensitivity(which does not respond to other treatment),function is affected, or the wear reaches apoint where restorations will become techni-cally difficult.

• At this point provide minimal restorations,normally directly bonded composite restora-tions.

• If the problem continues, provide crowns.


c A large midline diastema that the patient foundaesthetically unacceptable.

d The same patient after the central incisors have beenmoved closer together orthodontically and all fourincisors crowned. The patient must be warned of anycompromise in the appearance that is anticipated – inthis case the triangular space that remains at themidline. It is possible to increase the width of the incisaledges to fill the space, but the width of the crowns atthe neck is determined by the width of the roots, sothat only minimal enlargement is possible withoutcreating uncleansable overhanging crown margins. It isunlikely that a long-term acceptable result could havebeen achieved with composite in view of the size ofthe gap.

Figure 1.2

Changing the shape and size of teeth.

a and b Increasing the size of incisors with composite:a before, b after.

Hypoplastic conditions

These are divided into congenital and acquireddefects. Examples of congenital defects arehypodontia (small teeth – see the peg-shapedlateral incisors in Figure 1.1d), amelogenesisimperfecta (Figure 1.1e) and dentinogenesisimperfecta (Figure 1.1f). Examples of acquireddefects are fluorosis, tetracycline stain and enamelhypoplasia resulting from a major metabolicdisturbance (usually a childhood illness) at the agewhen the enamel was developing (Figure 1.1g).

To alter the shape, size or inclination ofteeth

Major changes in the position of teeth can bemade only by orthodontic treatment, althoughminor changes in appearance can be achieved bybuilding up the tooth with composite or bycomposite or porcelain veneers (see pages13–20). For example, a diastema between teethwhich the patient finds unattractive can be closedor reduced by means of additions of composite orveneers following orthodontic treatment (Figure1.2a and b). However, when the space is large,oversized crowns will produce a durable andattractive result (Figure 1.2c and d). This approach

is very destructive of tooth tissue and compositeor veneers should always be attempted firstbefore irreversibly preparing the teeth for crowns.

To alter the occlusion

Crowns may be used to alter the angulation orocclusal relationships of anterior and posteriorteeth as part of an occlusal reconstruction eitherto solve an occlusal problem or to improvefunction (see Chapter 4).

As part of another restoration

Crowns are made to support bridges and ascomponents of fixed splints. They are also madeto alter the alignment of teeth to produce guideplanes for partial dentures or to carry precisionattachments for precision attachment retainedpartial dentures (see Parts 2 and 3).

Combined indications

More than one of these indications may bepresent, so that, for example, a broken-down


Figure 1.3

An attractive appearance spoiled by unsightly teeth.

posterior tooth that is over-erupted and tiltedmay be crowned as a repair and at the same timeto alter its occlusal relationships and its inclina-tion, providing a guide plane and rest seat for apartial denture.

Multiple crowns

With some of these indications, notably toothwear and hypoplastic conditions, many or all ofthe teeth may need to be crowned.


Figure 1.4

The appearance of composite restorations.

a Following trauma the right central incisor toothrequires root canal treatment.

b The appearance of the tooth has been restored byinternal bleaching and a composite repair to the mesialcorner.

d Composite restorations at the necks of all theincisor teeth. They have been present for 18 monthsand are maintaining their appearance.

c The central incisors were fractured in a ridingaccident 8 years earlier. Composite restorations wereplaced by the patient’s mother and then replaced byanother dentist and again at a dental school. Theserestorations have been in place for 3 years and arediscolouring again. The patient was 21 and refusedfurther composites and crowns were made.

Appearance

One of the principal reasons for patients seekingdental treatment is to maintain or improve theirappearance. Relative prosperity, changing socialattitudes and the success of modern dentalmaterials mean that expectations of good dentalappearance are rising. Fewer teeth are beingextracted, and when they are it is at a later age.It is much less common now to see a mouth suchas that shown in Figure 1.3) than it was in themid-1960s, when this photograph was taken. Asstandards of appearance and expectations rise,some dental defects or types of restoration,which at one time would have been tolerated, areno longer acceptable to patients.

Much more can now be done to improveappearance with the current range of compositematerials than was the case a few years ago.Composite has the advantage of being moreadaptable than porcelain. It is applied, shaped andpolished at the chair-side and later it can berepaired and resurfaced (Figure 1.4). This meansthat crowns are now less often indicated toimprove appearance.

Many patients simply wish to lighten the colourof their teeth, which may have darkened with ageor from smoking. If the teeth are substantiallysound and their position and shape are accept-able to the patient, a significant improvement canbe made by thorough cleaning and externalbleaching (see Figure 1.7). However, sometimesthe discolouration is so intense that bleachingalone will not provide a satisfactory result (Figure1.5). Initial bleaching to lighten the tooth as muchas possible helps because the crown then doesnot have to disguise too dark a preparation.

Appearance is important to the patient and istherefore important to the dentist. After the

relief and prevention of pain and infection it isprobably the next most important reason forproviding dental treatment.

Function

It is possible to eat and speak without any teeth,or with complete dentures, but most patients(and probably all dentists) do not want to. Aswith appearance, this is a question of the qualityof life. An occluding set of natural, or secondbest, restored teeth is better at coping with a fullvaried range of diet than dentures.

Mechanical problems

Sometimes, although it would be possible torestore a tooth by means of a filling, the patternof damage to the tooth gives rise to anxietiesabout the retention of the restoration, thestrength of the remaining tooth tissue, or thestrength of the restorative material. The degreeto which a crown strengthens the remainingtooth structure compared to a bonded intra-coronal restoration is not clear. Usually, howevermuch damage there is, some sound tooth tissueneeds to be removed to prepare the tooth for acrown (see Figure 1.12).

Fillings fail because they fall out, because ofsecondary caries, or because part of the tooth orpart of the restoration fractures. These failuresare upsetting to the patient and embarrassing tothe dentist, and it is therefore tempting toprescribe crowns when there is even a smallpossibility that one of these problems will arise.

However, crowns can also fail. If a filling fails,it is often possible to make a more extensive


Figure 1.5

The central incisor has a necrotic pulp and is grosslydiscoloured. This degree of discoloration could not beresolved by bleaching or veneering the tooth. Theperiodontal condition must be improved before acrown can be made successfully.


Figure 1.6

Tetracycline stain.

a Mild, uniform staining. It is unlikely that treatmentwill be necessary other than to replace the missinglateral incisors.

b Tetracycline staining with severe banding. The extentof treatment depends on the lip line. In this case thelower lip covered the gingival half of the lower incisors,and therefore treatment for the lower teeth was notnecessary.

c Darker but more uniform tetracycline staining. In thiscase a vital bleaching technique was used.

d Extreme tetracycline staining with banding.

e Darkly stained teeth with four teeth, the upper andlower left premolars, prepared for crowns.

restoration or a crown. If a crown fails, a furthercrown may not be possible and extraction maybe all that is left.

In deciding between a crown and a filling thereare two considerations to be weighed up. First,how real is the risk of mechanical failure of thefilling or surrounding tooth and what can be doneto minimize this risk? Second, how much moredestruction of sound tooth tissue is necessary tomake a crown?

In general, it is better to take the more conser-vative approach first, even if this involves somerisk of the restoration failing. The alternative is toprovide far more crowns than are strictly neces-sary and perhaps give rise to even greaterproblems for the patient later on.

Indications for anterior crowns

Caries and trauma

All the general indications listed above may applyto anterior crowns. Before the days of acid-etchretained composite restorations and compositeand porcelain veneers, anterior crowns wereindicated much more frequently for the restora-tion of carious or fractured incisors. Today manyof these teeth can be restored without crowns,which are often not needed until the pulp isinvolved (Figure 1.1a and b).

Non-vital teeth

When a pulp becomes necrotic the tooth oftendiscolours due to the haemoglobin breakdownproducts. Internal bleaching (see Figure 1.7a andb) is the initial treatment of choice and will oftenproduce a good initial result, although sometimessome of the discolouration returns. However, thediscoloration may be such that it can only satis-factorily be obscured by a crown following initialbleaching to produce a lighter core for the crown(Figure 1.5).

Tooth wear

The ideal approach to problems of tooth wear isto prevent the condition getting worse by identi-

fying the cause and eliminating it as early as poss-ible. Crowns should be made only when the causeof the tooth wear cannot be identified or cannotbe eliminated, and the damage is serious.Sometimes the rate of tooth wear slows down orstops with no obvious explanation and the teethremain stable for some years. Crowns are not agood preventive measure except as a last resort.

Hypoplastic conditions

In many of the hypoplastic conditions the patient(or parents) will seek treatment at an early age,often as soon as the permanent teeth erupt, andtreatment may be carried out in conjunction withorthodontic treatment. In some of these caseslarge numbers of teeth are affected, and so thedecision whether to crown them, offer somealternative form of treatment, or simply leave thecondition alone, is a fairly momentous one. Figure1.6 shows several cases of tetracycline stainingaffecting many teeth. Differences in the lipmorphology, the depth of uniformity of thecolour, and the patient’s age and general attitudewill all influence the decision.

Unfortunately tetracycline stain often does notrespond well to bleaching, particularly when thestaining is in bands. The success of veneersdepends on the quality of the remaining enamelfor bonding, but in suitable cases veneers are theideal treatment. The option of multiple crowns isa considerable undertaking and should not beembarked upon lightly by either patient or dentist.In particular with young patients, the lifelongmaintenance implications must be fully under-stood. It should be explained that crowns are veryunlikely to last the whole of a natural lifetime andreplacements will be costly if they are possible atall. If veneers are made first the teeth can still becrowned later but the opposite is not true.

However, if after proper consideration crownsare made, they can dramatically improve thepatient’s appearance in a way that is difficult orimpossible by any other form of treatment.

The decision often has to be made while thepatient is a teenager, when social developmentcan be seriously affected by appearance includingdental appearance. This is often an importantfactor in making decisions on whether, how andwhen to treat.



Figure 1.7

Alternatives to crowns – bleaching.

a A discoloured, non-vital lower central incisor.

b The tooth shown in a has been root-filled and inter-nally bleached to produce a satisfactory appearance.

c This young patient was unhappy with the appearanceof their crowded discoloured teeth. The discolorationis due to tetracycline given when the patient was achild. Extensive and complex restorations could beconsidered but the simplest approach would be toundertake orthodontic treatment and then improve thecolour of the teeth with external home bleaching.

d The resulting appearance following external vitalbleaching – although not perfect the patient is happywith the result and extensive restorations have beenavoided.

As part of other restorations

Sometimes crowns are needed to support partialdentures. Crowns as part of bridges and splintsare dealt with in Parts 2 and 3.

What are the alternatives toanterior crowns?

Internal and external bleaching

Bleaching agents can be used to lighten the colourof teeth by internal or external application.Controversy over the legality of some bleachingmaterials and techniques in the UK has beenresolved after much debate. Dentists should beaware of the regulations in their own countriesbefore using some of the bleaching materials andtechniques.

The advantage of tooth bleaching is that itoffers simple and conservative improvements forpatients who simply want whiter teeth or toreverse some of the effects of aging withoutdamaging tooth structure. Tooth bleachingtechniques cannot do anything other thanimprove colour and remove staining.

Internal bleaching

Discoloured non-vital teeth can be treated byremoving the contents of the pulp chamber andsealing the root canal with a glass ionomer cementat or just below the gingival level to preventleakage of the bleaching agent into the canal.Hydrogen peroxide gel or a slurry of hydrogenperoxide and sodium perborate is left sealed in thetooth for 24 hours and repeated until the desiredshade is obtained (Figure 1.7a and b). The accesscavity needs to be completely sealed to preventfuture relapse. Internal bleaching will not improvethe colour of teeth made grey by the corrosion ofdental amalgam fillings in access cavities.

External bleaching

Vital teeth can be lightened by various techniqueswith the external application of carbamide per-oxide or other bleaching agents (Figure 1.7c and d).

The home bleaching technique employs a 2%carbamide peroxide gel applied to the teethwithin a customized tray that the patient wearsovernight. Chair-side bleaching utilizes a moreconcentrated carbamide peroxide gel applied bythe dentist and activated by heat or light to actin a short period of time. The colour changesachieved by both techniques are not permanentbut can be repeated. Vital bleaching techniquesapplied over lengthy periods of time can treateven very severe discolouration such as sometypes of tetracycline staining.

Composite restorations

The appearance of modern composite restora-tions is excellent (Figure 1.4). With the rapiddevelopment of anterior restorative materials, itis better in many cases to replace and repairrestorations until such time as even more durablematerials are available, rather than make crowns.

It is clear that no absolute rules can be givenon whether crowns or fillings are indicated otherthan to say that in general the more conservativeprocedures are to be preferred.

Composite and porcelain veneers

There has been a debate in the dental professionabout the advantages and disadvantages of thesetwo materials for veneers. Considerations in thisdebate are discussed below.

Appearance

Both can have a very good appearance initially(Figure 1.8). Earlier composites tended to wearand discolour, losing the quality of their appear-ance. However, this is less of a problem with theimproving current materials. When veneers arebeing made to mask intense discoloration porce-lain veneers may be preferred because they canhave an initial layer of opaque porcelain whichhelps to mask discoloration.

Reversibility

Composite veneers can be made very thin and sooften do not need any tooth preparation. They



Figure 1.8

Alternatives to crowns – veneers.

a Broken and eroded incisor teeth.

b The same patient as shown in a with compositeveneers 3 years after being placed.

c Eroded upper central incisors.

d The same patient as shown in c with two porcelainveneers in place.


e Polyacrylic veneers which have been in the mouthfor several years. The margins are staining and chipping.

f The same patient as e. The polyacrylic veneers havebeen removed and the six anterior teeth prepared forporcelain veneers.

g An incisal view of the prepared teeth.

h Porcelain veneers on the model for the patientshown in e and f.


i The etched fit surface of the porcelain veneers.

j The teeth have been isolated with acetate strip andare about to be etched with phosphoric acid gel.

k An incisal view of the porcelain veneers in place. Inthis case the porcelain was carried over the incisaledges

l The completed porcelain veneers.


Figure 1.9

a A sectioned upper central incisortooth. Left the intact tooth, right thetooth has been prepared for aporcelain veneer and the profile ofthe veneer is illustrated in wax. Thefeatures of this preparation are thatthe gingival margin is chamfered andis in enamel and the incisal edgepreserves the bulk of the naturaltooth. Had the incisal edge beenmore worn the veneer preparationcould have been taken over it as inFigure 1.8k.

b A view through the confocalmicroscope of the margin of aporcelain veneer. From the left theveneer, the luting cement, enameland dentine. This is a good fit.

c A porcelain veneer which hasbeen grit-blasted too much in itsconstruction, leaving the margindeficient.

d Porcelain surface etched with hydrofluoric acid. Agrit-blasted surface looks similar.

e A high magnification confocal microscope image witha different contrast medium showing penetration ofcomposite luting cement into dentine tubules on theleft and etched porcelain on the right.

can therefore be removed if the result is notideal. Porcelain veneers almost always need sometooth preparation because the margins cannot bemade as thin as composite, which can be tapereddown to nothing at the margin (Figure 1.9). Thismeans that porcelain veneers are usually notreversible. Because the indications for veneersusually apply to young people the decision toprepare teeth for porcelain veneers must beconsidered very carefully. Tooth preparation isirreversible and it is often better to make, andwhen necessary refurbish, composite veneers,thus allowing the patient to keep their optionsopen for the long term.

Cost

Porcelain veneers are more costly because twoappointments are necessary and considerablechair-side time is necessary for the delicatepreparations and the bonding process. Laboratorycharges add considerably to the cost. Compositeveneers are placed at the chair side in one visit,the total clinical time is usually less than porce-lain veneers and there is no laboratory cost.

Surveys of success and failures of veneers

There have been a number of medium-termsurveys that show that the life expectancy ofporcelain veneers is commonly less than had beenhoped. Fractures and debonding are not uncom-mon and a frequent type of failure is stainingaround the margins or even frank leakage (Figure1.10).

Failures like this cannot be repaired and as thetooth has usually been prepared a new veneer orcrown is necessary. In an attempt to reduce theeffect of marginal staining some clinicians carrythe preparations for veneers right through themesial and distal contact points so that the marginis other lingual/palatal side. This is an even moredestructive preparation. There have been nogood long-term studies of the success and failureof composite veneers, partly because cliniciansknow that the materials are changing all the timeand starting a medium- or long-term survey of aparticular material will become redundant in ashort time with the introduction of new, bettermaterials. However, clinical experience suggeststhat composite veneers do have a good medium-term prognosis and also have the advantages thatthey can be repaired and polished.

See Chapter 14 for a more detailed discussionof the success and failure of restorations.

Physical properties

The modulus of elasticity of porcelain is quitedifferent to that of enamel. There is a view amongsome dental material scientists that this differencewill inevitably lead to a breakdown of the bondat the margins of veneers in due course. Initiallythe estimate was about 10 years and some of thesurveys confirm this approximate time. Thecoefficient of thermal expansion is also differentbetween porcelain and enamel and this adds tothe likelihood that marginal breakdown will occur.

Composite is nearer to enamel in terms ofthese physical properties and because thecomposite is bonded directly via a bonding resin


Figure 1.10

Failing veneers.

These porcelain veneers were made less than a yearago and are now leaking.


Figure 1.11

Resin bonded ceramic crowns.

a Peg-shaped lateral incisors.

b The upper right lateral incisor prepared for a resinbonded ceramic crown. The preparation is entirelywithin enamel. The neck of the tooth has beenprepared all round with a chamfer finishing line, similarto the preparation for a veneer, and a small amounthas been removed from the incisal edge. Nothing hasbeen removed from the labial or proximal surfacesother than blending them into the gingival finishing line.

c Palatal view of the finished crowns.

d The labial appearance.


Figure 1.12

Badly broken-down teeth, all ofwhich were vital when theywere extracted. Left: the toothon presentation. Right: afterremoving old restorations,caries and grossly overhangingenamel. Only at this stage can afinal decision be made on themost suitable restoration. Theseteeth could be treated with:

a a bonded or pin-retainedamalgam restoration;

b a gold inlay with cuspalprotection or a glass ionomer/composite layered restorationto strengthen the cusps;

c a composite core or a bondedor pin-retained amalgam coreand partial crown;

d a composite core or a bondedor pin-retained amalgam coreand complete crown.

to the etched enamel, marginal breakdown is lesscommon. Porcelain veneers require two bonds,one to enamel and the other to the porcelain.

Taking all these considerations together theemphasis is now swinging towards compositeveneers rather than porcelain. However, porce-lain veneers still have an important role to playin some cases.

An alternative to porcelain veneers –resin bonded ceramic crowns

These are, in effect, a porcelain veneer that goesright round the tooth and therefore does nothave the same problems with the differences inphysical properties between porcelain and enamelas do porcelain veneers. They require the sameamount of labial, incisal and approximal prepara-tion as a porcelain veneer, usually entirely inenamel, together with equivalent preparation ofthe palatal or lingual surface (Figure 1.11).

Restoration of badly broken-downteeth

The most common indication for a posteriorcrown is a badly broken-down tooth usuallyresulting from repeated restorations, each ofwhich fails in turn until finally a cusp or larger partof the tooth fractures off. In most cases it isnecessary to build up a core of amalgam orcomposite before the crown is made. Two suchteeth are shown in Figure 1.12c and d.

Restoration of root-filled teeth

There is a strong clinical impression and somescientific evidence that root-filled teeth are morelikely to fracture than teeth with vital pulps. Itfollows that some thin and undermined cusps ofroot-filled teeth need to be protected or removedwhere similar cusps in vital teeth would be left.Together with the original damage that necessitatedthe root filling and the access cavity, this means thatmany, but by no means all, root-filled posteriorteeth are crowned. The fact that a posterior toothis root-filled is not in itself sufficient justification fora crown. When a crown is indicated it is almostalways necessary to make a core.

As part of another restoration

In Parts 2 and 3 partial and complete crowns arediscussed as retainers for bridges and fixedsplints. In addition, they may be indicated inconjunction with conventional or precision-attachment retained partial dentures.

What are the alternatives toposterior crowns?

Bonded or pin-retained amalgamrestorations

The success of bonding amalgam to large unreten-tive cavities is improving. Some dentists now usebonded amalgam routinely while others are less


Figure 1.13

Amalgam and gold restorations.

The amalgam in the first molar, which has just beenrepolished, has been in place for 15 years. Less thanhalf the natural crown is present and so a crown couldhave been considered when the treatment was firstplanned. The decision to place an amalgam was justi-fied. The amalgam restorations in the premolar teethare more recent, and less satisfactory. The inlay in thesecond molar has been present for 20 years.

confident of the bond (and find the techniquedifficult) and use pins or a combination.

Figure 1.13 shows an amalgam restoration whichhas been present for 15 years. A crack is visibleon the mesial palatal aspect of this tooth; this hasalso been present for some years. The tooth issymptomless and remains vital. It could be arguedthat all teeth with large lesions, such as this one,should be crowned in order to prevent suchcracks occurring. However, it is impossible topredict which teeth will crack and what the effectswill be. It is therefore not justified to crown allteeth with large cavities or restorations just as apreventive measure. To do so is overtreatmentand is not cost-effective. It is better to apply ageneral policy of minimum intervention, withprophylactic restorations only when there is aclear risk of failure. When occasional failures, suchas broken cusps, do occur, these problems canusually be solved without the need for extraction.

Tooth-coloured posterior restorations

Composite materials suitable for posteriorrestorations have been developed intensively inrecent times. One reason for this is increasinganxiety in some parts of the world and in somepatients about the wisdom of continuing to useamalgam restorations in view of the possible riskof mercury toxicity or allergy. The subject hasreceived much attention in the popular press andin the rest of the media. The scientific evidenceis that mercury allergy does exist in a very smallproportion of the population, although in someparts of the world, for example Japan, it appearsto be greater, probably due to patients beingsensitized by eating fish contaminated withmercury that has got into the marine food chain.

Mercury toxicity is a proper concern ofdentists, and over the last 30 years or so consid-erable improvements have been made in mercuryhygiene. Most amalgam used now is capsulated,avoiding the need for liquid mercury to be avail-able in bulk in the dental surgery, and otherprecautions are also used to protect the staff inthe dental surgery. It is the staff, who are likelyto be exposed over a long period to mercuryvapour should mercury hygiene not be adequate,who are at risk rather than individual patients.There is no reliable scientific evidence that themercury from amalgam restorations is a serious

toxic hazard to patients, despite occasionalflurries of media hype. It is also possible that thealternatives to amalgam may have equally lowlevels of toxic effect.

Nevertheless there are some patients whorefuse to have amalgam restorations, and hencethere has been a drive to develop satisfactory,cost-effective alternatives for the restoration ofposterior teeth. Some patients also have concernsabout the appearance of amalgam restorations,even in areas of the mouth that show little, andrequest tooth-coloured restorations. The materi-als are improving year by year, but some dentistsstill feel that they are not yet comparable toamalgam for the larger posterior restoration.These dentists will therefore more commonlyprescribe crowns than composite restorations inteeth that would otherwise be treated with anamalgam restoration, for example the toothshown in Figure 1.12a. The tooth shown in Figure1.12b still has substantial buccal and palatal cuspsand a good ridge of dentine between them.However, if the tooth is subject to occlusal stress(and wear facets can be seen on the cusps) thena restoration either protecting or reinforcing thecusps is indicated with this amount of tooth loss.

There is good evidence that the layeredrestoration (a core of glass ionomer cementreplacing the dentine, with an occlusal surface ofcomposite replacing the enamel) is successful inbinding weakened cusps together and producinga stronger tooth than one restored with amalgamalone. It is used when there is a large MOD cavitywhere a crown preparation would simply removeall or most of the remaining tooth tissue.

In order to increase wear resistance and tominimize the effects of polymerization contrac-tions, which are complications of large directlyplaced composite restorations, systems have beendeveloped to process composite inlays outsidethe mouth by a combination of heat, pressure andlight. A non-undercut inlay cavity is prepared, animpression taken and the composite inlay (oronlay) made in the laboratory (Figure 1.14).

Gold inlays and onlays protectingweak cusps

The gold inlay shown in Figure 1.13 is an intra-coronal restoration and is not an alternative to acrown. Figure 1.15a shows a tooth similar to


Figure 1.12b. It has been prepared for a cuspalcoverage MOD gold inlay and Figure 1.15b showsthe sectioned tooth with a wax pattern for theinlay. The metal overlying and protecting thecusps must be thick enough to prevent distortionunder occlusal forces.

This restoration is very conservative of toothtissue and is retentive. Imagine on Figure 1.15how little of the buccal cusp would have remainedhad the tooth been restored with a core and thenprepared for a metal–ceramic crown (seeChapter 2). The buccal cusp would have been


Figure 1.14

a The large MOD amalgamrestoration in the first molar toothrequires replacement.

b The restoration has beenreplaced with a direct compositerestoration, significantly improvingits appearance.

c Several extensive restorations incomposite only 2 years after place-ment. They have worn badly withmarginal staining, loss of occlusalcontour and poor proximalcontacts.

d Well placed MOD composite restorations which are5 years old and continue to function well with a goodappearance.

continued


Figure 1.14 continued

e A failed MOD amalgam restoration with secondarycaries beneath both the boxes. The mesial surface ofthe amalgam was also unsightly.

f A laboratory-processed composite inlay shortly afterinsertion.

g The same composite inlay after 8 years.


Figure 1.15

MOD cuspal coverage inlay.

a The prepared tooth. Note the bevels on the occlusalsurface just down onto the buccal and lingual surfaces.

b Section of the prepared tooth with the wax pattern.

c Occlusal view of the completed wax-up.


Figure 1.16

Ceramic inlays.

a A laboratory-made ceramic inlay. The inlay isreturned from the laboratory with a contouredocclusal surface and occlusal staining. It should onlyrequire cementation.

b A computer aided design/computer aided manufac-ture (CAD/CAM) machine for producing ceramic inlaysat the chair side. This is an early version of the Cerecmachine but there are several other makes available.

c A failed composite restoration in the first premolartooth is to be replaced by a ceramic inlay.

d The completed ceramic inlay milled at the chair sidein the Cerec machine. The machine produces a goodfit and contact points that only require minor adjust-ment and polishing. However, the occlusal surface isnot finished, and needs to be adjusted and polished inthe mouth after cementation. The main advantage ofthe system is that the whole procedure is carried outin one visit at the chair side and there are no labora-tory stages.

even more weakened. A disadvantage of this typeof restoration is that often with premolars andsome molar teeth there is a visible display of gold,which some patients do not like.

Ceramic inlays and onlays

Posterior ceramic inlays have many of the advan-tages of posterior composite restorations in that,because they are bonded to the tooth, theystrengthen weakened cusps, and they are tooth-coloured. In addition, the porcelain occlusalsurface is more wear-resistant than compositeand there is, of course, no polymerizationcontraction.

There are two systems: one that includes alaboratory stage and one that does not. Withlaboratory-made ceramic inlays, an impression ofthe prepared tooth is sent to the laboratory anda porcelain inlay is made by one of a variety oftechniques similar to the production of porcelaincrowns using the die of the tooth (Figure 1.16a).

The chair-side system consists in milling aporcelain inlay from a design produced in acomputer from a three-dimensional video imageof the prepared tooth. This, of course, requiresa very complex, sophisticated and expensive pieceof equipment (Figure 1.16b). It is too soon to saywhether this approach to dental restorations(CAD/CAM or computer-aided design/computeraided manufacture) will be revolutionary or willstay on the fringes of dental treatment.

Choosing the right posteriorrestoration

In some of the teeth shown in Figure 1.12 thefailure is due to the restoration fracturing orbecoming lost and in others it is the tooth itselfthat has failed. In some the problem is secondarycaries. In all these cases decisions must be madebetween restoring or extracting the tooth, and ifit is to be restored, whether the pulp is healthyor whether endodontic treatment is necessary.Leaving these considerations to be discussed inChapter 3, and assuming that all these teeth willbe restored, the next decision is whether theappropriate restoration is:

• An amalgam or composite restoration• A layered restoration of glass ionomer and

composite• An amalgam with additional retention (for

example bonding or pins)• A ceramic or composite inlay • A gold inlay• A gold inlay with occlusal protection (an onlay)• A partial crown• A complete crown• A core of material to replace the missing

dentine followed by a partial crown• A core and complete crown.

A further decision that must be made is whether,if a complete crown is to be used, it should be anall-metal or a metal–ceramic crown, or even insome cases an all-ceramic crown (see Chapter 2for a description of these different types of crown).

These decisions cannot be made withoutfurther information, and some of this will begathered from the history, examination of therest of the mouth, radiographs, and so on (again,these matters will be discussed in Chapter 3).However, even with all this information it isusually also necessary to remove the existingrestorations and caries before a final decision canbe made. Figure 1.12 shows the same teethbefore and after the caries and old restorationsare removed.

The decision depends upon three factors:

• Appearance• Problems of retention• Problems of strength of the remaining tooth

tissue and the restorative material.

As far as appearance is concerned, if the surfaceof the tooth to be restored is visible duringcommon movements of the mouth, and if thepatient is concerned about appearance, a ceramicinlay, composite restoration or crown will usuallybe indicated for large restorations.

When the problem is simply one of retention,an amalgam restoration with additional retentivefeatures is usually chosen (Figure 1.12a).

When the remaining tooth tissue is weak, alayered restoration, a ceramic inlay or a cuspalcoverage gold inlay will be the choice (Figure1.12a).

A core and partial crown is a very satisfactoryrestoration where a tooth previously restored


with an MOD amalgam loses its lingual or palatalcusp. The partial crown protects the remainingbuccal cusp against occlusal forces, and this cuspcan still provide valuable retention, often inconjunction with bonding and/or pins, for thecore, as well as having an acceptable appearance.If a metal–ceramic crown is made, then the wholeor the majority of the buccal cusp will be cut offin the preparation of the tooth, and the core willneed much more substantial auxiliary retention(Figure 1.12c).

A core and complete crown is the last resort.Figure 1.12d shows a case where there is nochoice but to provide a core and completecrown.

These examples illustrate the importance ofconsidering all the alternatives in each case. Thetemptation to look rather casually at the toothand immediately decide upon a crown withoutproper investigation and consideration must beavoided as it may lead to more destructive over-treatment than necessary.


This chapter gives a general description of thevarious crown types together with their mainadvantages and disadvantages in relation to:

• Physical properties• Clinical considerations• Appearance• Cost.

Crowns are described under the followingheadings:

• Anterior complete crowns for vital teeth• Anterior crowns for root-filled teeth• Posterior complete crowns• Posterior partial crowns.

Anterior complete crowns for vitalteeth

In the anterior part of the mouth appearance isof overriding importance, and so the only typesof crown to be considered are those with atooth-coloured labial or buccal surface. These fallinto three groups:

• Ceramic crowns• Metal–ceramic crowns• Other types of crowns.

Ceramic crowns

In recent years there have been several develop-ments in the ceramics used for crowns. These canbe classified as:

• Traditional fused porcelain jacket crowns(PJCs)

• Pressed ceramic crowns• Milled ceramic crowns• Cast crowns

• Reinforced crowns• Ceramic resin bonded crowns.

Traditional porcelain jacket crowns (PJCs)

This is the oldest type of tooth-coloured crownand has been in use for more than a century. Itconsists of a more or less even layer of porce-lain, usually between 1 and 2 mm thick, coveringthe entire tooth. Figure 2.1a, b, c and d shows aselection of traditional feldspathic porcelain jacketcrowns in place.

The traditional feldspathic PJC is made byadapting a very thin platinum foil to a die madefrom an impression of the prepared tooth.Porcelain powder, mixed with water or a specialfluid, is built onto the platinum foil and fired inthe furnace. All PJCs made in this way are nowstrengthened by having alumina incorporated intothe porcelain powder. A core of high-aluminaporcelain is fired onto the platinum foil. This high-alumina core is opaque and needs to be coveredby more translucent porcelain that contains lessalumina.

Variations on fused ceramic crowns have beendeveloped but most have now been replaced bythe systems described later. Examples of thesestrengthened ceramic systems were Hi-Ceram(Figure 2.1e) and In-Ceram (Figure 2.1i).

An alternative approach is to fire an extra-strong core of ceramic material to a refractorydie and then add further layers of conventionalfeldspathic porcelain. Once finished, the refrac-tory die is grit-blasted away, leaving a fittingsurface that is slightly rough, aiding retention.Both these systems can also be used to makeporcelain veneers.

Conventional dental porcelain is physicallymore like glass than the porcelain used fordomestic purposes. It is relatively brittle, andbefore a PJC is cemented it can be broken fairlyeasily. However, once it is cemented andsupported by the dentine of the tooth, the forcerequired to fracture it is of the same order of

2 Types of crown

29

30 Types of crown

Figure 2.1

Types of anterior crown.

a Traditional porcelain jacket crowns on all four upperincisors. The preparations for the crowns are shownin Figure 6.10.

b A single traditional PJC. The upper left central incisoris the crown, the other teeth are natural.

d Both upper central incisors are traditional PJCs withsupragingival margins. Despite this, there is some gingi-val inflammation. They had been present for about 5years when this photograph was taken.

c The same crown 25 years later. It has recently beenfractured traumatically and repaired with composite.The appearance is not as good as the original crownbut the patient is happy to accept this and does notwant the crown, which has been so successful, replacedyet.

Types of crown 31

e Strengthened porcelain crowns (Hi-Ceram) on bothcentral incisor teeth.

f Pressed crowns on the upper and lower incisor teethin a patient with mild amelogenesis imperfecta. This canbe seen on the uncrowned teeth, particularly the lowerright first premolar. The system used was Empress.

g Six pressed crowns made by another system –Metalor Stylepress – in which both the pressed andapplied layers are the same material.

h and i The upper left lateral incisor tooth has beenmoved orthodontically into the position of the centralincisor and crowned with an infiltrated, reinforcedsystem (Inceram) to resemble the missing centralincisor.

32 Types of crown

j A cast ceramic crown (Dicor) on the upper rightcentral incisor.

k A metal–ceramic crown on the upper lateral incisor.This is the patient shown in Figure 1.1a.

l A metal–ceramic crown with a buccal porcelainmargin so that no metal is visible on the labial surface.

m The cemented crown shown in l has an excellentappearance and is indistinguishable from a porcelaincrown.

magnitude as the force required to fracture theenamel of a natural tooth.

However, traditional PJCs do fracture and sothe other systems have all been developed toproduce stronger all-ceramic crowns.

Pressed ceramic crowns

There are a number of systems used to producea strong ceramic core by pressing a ceramicmaterial at high temperature and pressure onto adie and then building up layers of traditionalporcelain to create the right contour and shade.Examples of crowns made by this process areshown in Figure 2.1f and g and an example of theequipment used is shown in Figure 2.2a.

Laboratory milled cores and crowns

Several computer aided designs/computer aidedmanufacture (CAD-CAM) have been developedand more are in the pipeline. One example is atechnique by which the die is scanned in acomputer and the digital record is sent electron-ically to a central laboratory, often in anothercountry, where a core is produced in a strongceramic material. This is then returned to theoriginal laboratory where additional porcelain isadded to complete the crown. The system canalso mill metal, in particular titanium (Procera).The computer equipment is shown in Figure 2.2dand crowns made by this process are shown inFigure 2.2e and f.

Zirconia cores

Porcelain crowns can also be manufactured usingcomplex laboratory techniques with a zirconiacore onto which conventional porcelain is built.Zirconia is an extremely strong material compar-able to metal and is dense white in appearance.The conventional porcelain therefore needs to berelatively thick for the translucent porcelain tomask the opaque core underneath. Zirconia-based crowns are proposed by the manufacturersfor use on posterior as well as anterior teeth dueto their great potential strength, but there is notyet sufficient evidence that they will survive forthe long term in high stress situations.

Chair-side milled restorations

Chair-side milling systems have been in develop-ment for some years. Although these systems donot yet produce reliable crowns it is anticipatedthat they will do so in the foreseeable future.

Cast ceramic systems

One of the earliest developments in the 1970swas of a system to produce strong ceramiccrowns by the Dicor process in which a waxpattern of the crown was made on a conventionaldie, invested and cast in a glass/ceramic material.The casting was then placed in a ceramming ovenfor several hours, during which it went through acrystallization conversion and became muchstronger. At this stage the casting had a cloudy-clear appearance (similar to frosted glass). It wasthen stained and characterized using conventionalfeldspathic porcelains in a porcelain furnace.Although the commercially available Dicor systemwas developed by the same company that devel-oped domestic Pyrex glassware, the manufactur-ers state that the material is not the same asPyrex. A number of other castable ceramic or hottransfer-moulded glass ceramic systems havebeen developed (Figure 2.1j).

Reinforced porcelains

One technique is to form an alumina substructureon a special plaster die and following sintering ina furnace the porous substructure is coated withglass fired powder and further fired for severalhours. This infiltrates the pores and eliminatesthem as a source of weakness. Filling the poresalso improves translucency and the final appear-ance when additional porcelain is added. A typicalsystem is In-Ceram and an example is illustratedin Figure 2.1h and i.

Thin ceramic resin bonded crowns (seealso Chapter 1)

All the previous crowns are usually cemented byconventional means (see later). However, aconservative technique is to prepare the toothwithin enamel exposing the minimum amount ofdentine and then making thin ceramic crowns, no

Types of crown 33

34 Types of crown

Figure 2.2

Making pressed, milled and fibre re-inforced crowns.

a Equipment for construction of an Empress crown.

b and c Two Empress crowns restoring severelydamaged incisor teeth.

Types of crown 35

d The scanner used in the process of making a Proceracrown.

e A Procera core of dense aluminous porcelain ready.The porcelain will be fired directly onto this.

f and g Procera crowns restoring three incisor teeth.Note how the crowns disguise the gold post and coresso the appearance is the same as the crown on thenatural tooth.

continued

thicker than a porcelain veneer (see later) butcovering the whole tooth. This is then bonded byetching the enamel surface, grit-blasting the fitsurface of the crown and bonding with anadhesive resin material (Figure 1.11). Theserestorations are becoming more popular withsome dentists, who believe that their prognosismay be better than ceramic veneers because theygo right round the tooth, thereby reducing theeffects of the difference in thermal expansionbetween porcelain and enamel, and also the riskof marginal leakage and staining is reduced.

Choosing between ceramic crowns

In this fast developing field, a list of current (atthe time of publication) materials and techniqueswould soon become out of date. Instead, whendentists look at an established or new systemthey should recognise which type it is and assessthe manufacturer’s descriptions and claims objec-tively. Not all dentists or technicians will use thefull range. Most dentists gain experience in alimited range and choose the appropriate systemfor the patient’s particular clinical circumstances.

36 Types of crown

Figure 2.2 continued

h, i and j A single molar crown made using a fibrereinforced system.

Advantages

The advantages of ceramic crowns are:

Appearance Because of their translucency andthe range of techniques and shades available, theyare better able to duplicate the appearance of anatural tooth than any other type of crown.

Stability Porcelain is dimensionally and colourstable and is insoluble in oral fluids.

Cost Some ceramic crowns are less expensive toproduce in the laboratory than metal–ceramiccrowns.

Plaque Ceramic materials resist plaque accumu-lation better than other crown materials.

Brittleness The relative brittleness of a conven-tional PJC can be regarded as an advantage, partic-ularly if the tooth being crowned was originallyfractured in an accident. Should such an accidentrecur (which is not at all uncommon among sportsplayers, cyclists, children with Class II Division Iincisor relationships, and others), the PJC is likelyto fracture rather than the root of the tooth. Thisis still true, but to a lesser extent, with the newertypes of ceramic crown. With metal–ceramiccrowns, which are stronger than the remainingtooth tissue, more serious damage such as rootfracture is likely to result from a further accident.Where possible, the weakest link in the chainshould be the one easiest to repair. The principleis similar to the fuse in an electric circuit.

Disadvantages

The disadvantages of ceramic crowns are:

Marginal fit Traditional porcelain jacket crownsmade on a platinum foil matrix that is removedprior to cementation often had a less satisfactorymarginal fit than cast metal and other crowns.However, the marginal fit of the newer types iscomparable to cast-metal restorations; if shoulderporcelain (see later) is used the fit is improved.

Brittleness Although the brittleness of porcelaincrowns was described earlier as a potential

advantage in some situations, in most it is a disad-vantage. In some patients where the crownsupports a partial denture or where the occlusalforces are excessive, ceramic crowns are notusually used.

Removal of tooth tissue To overcome theproblem of the brittleness of porcelain, and togive the crown a natural appearance, there mustbe an adequate thickness of material, and so it isnecessary to reduce the tooth fairly extensively,weakening it and threatening the pulp. This isespecially true with small teeth, for examplelower incisors. This problem is much less withresin bonded ceramic crowns.

Metal–ceramic crowns (Figure 2.1k and l)

Dental porcelain can be bonded to a variety ofmetal alloys. The process is similar to the glazingof domestic cast iron and steel baths and basins.The alloys used in dentistry fall into three groups:

• Precious metal alloys containing a high propor-tion of platinum and gold. These can be highor low gold content (75% down to 25% gold)

• Semi-precious alloys containing a high propor-tion of palladium, sometimes with silver as well

• Base metal alloys containing a high proportionof nickel and chromium.

Although this classification is still used, at the timeof writing palladium is more expensive than goldand so the term ‘semi-precious’ is not reallyaccurate. Base metal alloys should only be usedin patients with a confirmed tolerance of nickelas it is fairly common for patients to be sensitiveto cheap jewellery or coins containing nickel.

There is a difference in cost between thesealloys, but they all share the properties of a highmelting temperature so that porcelain can bebonded to the surface. This is done by firing theporcelain to the metal at a temperature whichdoes not melt or distort the metal.

The preparation for an anterior metal–ceramiccrown differs from that for a posteriormetal–ceramic crown in two ways; first rathermore tooth tissue needs to be removed from thelabial surface to allow for the thickness of the

Types of crown 37

metal as well as porcelain, and second rather lessusually needs to be removed from the palatal orlingual surface since only metal will usually coverat least part of this surface.

Advantages

The main advantages of metal–ceramic anteriorcrowns are:

Strength The metal–ceramic crown is a verystrong restoration, which resists occlusal andother forces well.

Minimum palatal reduction Some teeth,particularly those severely worn by erosion andattrition that have then over-erupted back intoocclusion, may not be sufficiently bulky for aporcelain jacket crown preparation with adequatepalatal reduction, whereas a metal–ceramic crownpreparation may be possible. Figure 2.3 illustratesthis problem in comparison with a normal incisortooth.

Adaptability The metal–ceramic crown can beadapted to any shape of tooth preparation,whereas the processes involved in makingceramic crowns require a smooth and uniformpreparation. Additional retention can be gained indifficult preparations by the use of grooves orpins, which is not possible with ceramic crowns.

38 Types of crown

Figure 2.3

Sections through three sets of casts of patients in inter-cuspal position showing the profile of crown prepara-tion.

a This is a Class I Division II incisor relationship withdeep overbite and minimal overjet. It often appears,when looking at these patients from in front, that therewill be insufficient clearance for ceramic crown prep-arations. In fact, the bucco-lingual thickness of the teethis often normal and conventional preparations arepossible.

b Gross erosion of the palatal surfaces of the upperincisor teeth due to recurrent vomiting. If crowns areto be made, there will not be room to provide a palatalporcelain surface without the occlusal vertical dimen-sion being increased by techniques described later.However a metal–ceramic crown preparation is poss-ible. Because the diagnosis is erosion (chemical damage)rather than attrition (physical damage), the additionalstrength of the metal is not particularly important.

c The lower incisors are worn to approximately one-half their original length. A conventional crown prep-aration would not be possible but a one-piecemetal–ceramic post-retained crown is. The dotted lineshows the metal–porcelain junction.

Can be cast, soldered or laser welded Forbridges, metal–ceramic crowns can be attached toother crowns or bridge pontics by casting themtogether, soldering or laser welding. This cannotbe done with ceramic crowns.

Disadvantages

The disadvantages of metal–ceramic crowns are:

Strength An accidental blow may result in thetooth preparation or root fracturing because thecrown is stronger than the natural dental tissues.

Appearance Because of the metal framework, itis sometimes more difficult to match the naturalappearance of a tooth than with a ceramic crown,particularly at the cervical margin (Figure 2.1k).However, with all-ceramic margins this can belargely overcome.

Destruction of tooth tissue Metal–ceramiccrowns require more tooth reduction labially thanceramic crowns and so the tooth preparation ismore likely to endanger the pulp. If this toothreduction is not sufficient – as is often the case –the eventual crown either has a poor, opaqueappearance or is too bulky (see Chapter 3).

Cost Even if the relatively inexpensive base metalalloys are used, the laboratory time taken toconstruct a metal–ceramic crown is more thanfor a PJC and therefore the overall cost is usuallygreater. However, when compared to the costsfor most of the other types of ceramic crowninvolving complex laboratory stages, the costs aresimilar. When precious metal alloys are used, thecost is naturally greater.

Other types of anterior completecrowns

Although the majority of anterior crowns fall intoone of the two previous groups, other alterna-tives exist:

• Cast metal crowns with acrylic or compositefacings

• Fibre-reinforced composite crowns.

Cast metal acrylic or composite facedcrowns

These are mostly used as long-term provisionalrestorations. Laboratory-grade composite iscured by an intense light in a special light box,sometimes with the addition of heat or pressure.The cast metal framework needs to be mechani-cally retentive for the facing.

Fibre-reinforced composite crowns

A range of techniques have been developed toreinforce composite as a permanent crownmaterial. However, after a few years in use, theresults have been disappointing. Nevertheless,they do make good long-term provisional crownsand bridges. The techniques require a light box inthe laboratory but this is less expensive and theprocess is quicker than firing porcelain. Figure2.2h, i and j shows an example of this type ofcrown.

Anterior crowns for root-filledteeth

Often the endodontic access cavity together withthe crown preparation will leave insufficientdentine to support a crown. In this case reten-tion is gained by means of a post fitted into theenlarged root canal. These posts are used onlyfor retention, and the idea that they add strengthto the tooth has now been discounted. For thisreason, if it is possible to obtain retention for thecrown without using a post, this is preferred.Figure 2.4a shows examples of teeth that wouldbe restored by means of a simple compositerestoration, a composite (or glass ionomer) coreand crown or a post-retained crown. There arefour groups of crowns for root-filled anteriorteeth:

• Composite (or glass ionomer) core and crown• Post and core and separate crown• One-piece post crown• Other types.

Types of crown 39

40 Types of crown

Figure 2.4

Treatment of anterior root-filled teeth.

a If these three teeth had not been extracted, theywould have had to be root-filled. The caries and oldrestorations have been removed. The left-hand toothcould be restored by a simple composite restoration,the centre tooth has sufficient dentine remaining for aglass ionomer cement or composite core followed by acrown to be satisfactory, but the right-hand tooth doesnot have sufficient dentine, and retention by means ofa post cemented into the root canal is necessary.

b Both central incisors are fractured and have beenroot-filled.

c An incisal view of the teeth shown in b with therestorations removed from the access cavities.

d The access cavities restored with glass ionomercement.

e and f The teeth prepared for ceramic crowns. Thecompleted crowns are shown in Figure 2.1e.

Composite (or glass ionomer) coreand crown

When sufficient dentine remains, the endodonticaccess cavity can be filled and the missing dentinereplaced with glass ionomer cement, which bondsdirectly to dentine. Alternatively, dentine bondingagents may be used to bond composite to thedentine.

Glass ionomer cement has the advantage thatit does not contract significantly on setting.Composite is stronger and is rather easier toprepare, since it cuts with a similar ‘feel’ todentine (Figure 2.4b, c, d and f).

Post and core and separate crown

The crown will be either a ceramic ormetal–ceramic crown as described previously.

Posts and cores may either be made in thelaboratory from an impression of the post hole(indirect posts) or ready-made posts can bepurchased and cemented into the post hole whichis prepared with a matching drill of the samediameter as the post (direct posts). With anindirect post, the core (and sometimes adiaphragm) is usually cast with the post. With adirect post a composite core is built up directlyonto it in the mouth after the post is cemented.

Direct posts have the advantage of normallybeing fitted at the same time as the tooth isprepared, thus avoiding the need for a temporarypost crown. They are usually stronger and can bemuch more retentive than the laboratory-madeposts and cores. There is evidence that thetemporary cement used to cement temporarycrowns can contaminate the walls of the posthole and reduce the retention of the permanentpost.

When the clinical conditions are suitable directposts are preferred to indirect for the reasonsset out later.

Laboratory-made posts have the advantage ofadaptability and can be used in very tapered rootcanals that have suffered caries in the coronalpart of the root canal, in root canals with an ovalcross-section, and in two rooted teeth where theroots are parallel.

Direct posts are available in precious metal,non-precious metals, carbon-fibre, quartz-fibre or

titanium. Some of the advantages and disadvan-tages of these are:

• Precious metal: These can have a gold core anddiaphragm cast onto them and are strongerthan a cast gold post. Unless a diaphragm isneeded they have little advantage over non-precious posts.

• Non-precious metal: These are the mostcommonly used and are usually stainless-steel.They are stronger for the same cross-sectionthan cast posts and so a smaller diameter posthole is needed, leaving more root dentine.They are available from several manufacturersand in a wide range of diameters.

• Carbon-fibre: These were the first of the fibre-based posts to be introduced and there hasbeen more research on them than the othertypes. They have the advantage of being slightlyflexible and so are a nearer match to rootdentine. However, they are black and are diffi-cult to disguise beneath a composite core andceramic crown.

• Quartz-fibre: These are white and may havesimilar advantages to the carbon-fibre posts.However, they have only been available for arelatively short time with little research tosupport them. Some dentists are optimistic thatthey will prove to be a useful addition to therange of posts.

• Titanium: These have been available for evenless time and little is known about their advan-tages and disadvantages at the time of writing.

Although it may take longer to fit a direct postand build up a core at the chair side than to takean impression for an indirect post, this can bedone in one visit rather than two and so, overall,there is a saving of surgery time together with asaving of laboratory charges compared withindirect posts.

Post shapes

There are four shapes of post (Figure 2.5):

• Parallel: smooth or serrated• Tapered: smooth or serrated• Parallel: threaded• Tapered: threaded.

Types of crown 41

42 Types of crown

a b

c d

e

Figure 2.5

Post shapes.

a Cast gold post and core (tapered-smooth shape).The surface has been grit-blasted to improve retention.

b A parallel-sided, serrated post system. This manufac-turer produces five diameters ranging from 0.9 to1.75 mm. This is the middle of the range, 1.25 mmdiameter. From the left: a twist drill with a rubber discwhich can be moved up and down to set the length, asmooth plastic impression post, a stainless steelserrated post which can be used in the direct technique(see Chapter 6), an aluminium post with a small headused for making temporary post crowns and a serratedplastic burn-out post used in the laboratory as part ofthe pattern for the indirect system.

c A post system from a different manufacturer with thesame five diameters. This is 1.5 mm diameter. From theleft: the twist drill has three permanent marks tomeasure the length of the post hole, two impressionposts (separated before use), serrated burn-out postsand a stainless steel post for use in the direct, chair-side technique.

d Left: A parallel, coarsely threaded post with attachedhead which will be prepared as the core after the posthole is threaded and the post is cemented. Right: atapered, coarsely threaded post which is no longerrecommended (see text).

e Radiograph of a tapered-threaded (Dentatus) screwin the distal canal of a lower molar tooth. The crownneeds to be replaced. On removal of the crown thepost should be removed from the tooth by unscrew-ing it and it can be replaced with a new post of a differ-ent design.

Comparisons of post shapes

Parallel: smooth or serrated (Figure 2.5band c)

• Either preformed metal to which a compositecore is added, or made with a preformedplastic post, which is incorporated into apattern for a cast post and core.

• More retentive than tapered-smooth posts, andserrations further increase retention.

• Greater risk of lateral perforation of the root(see Figure 6.15b).

Tapered: smooth or serrated

• Laboratory-made in cast gold or other alloy.• Least retentive design, but if long enough and

a good fit, the retention is sufficient in mostclinical circumstances (serrations increaseretention but weaken the post).

Types of crown 43

f Radiograph of a quartz-fibre post in the secondpremolar tooth. Note that it is radiolucent and wouldbe easy to miss.

g The same post in place with a composite core.

h The completed crown.

• Easy to prepare and easy to follow the rootcanal.

• Similar to the shape of the root and thereforeless likely to perforate through to theperiodontal membrane.

• Adaptable technique and therefore can be usedwith oval, irregular-shaped or multiple-rootcanals.

• A diaphragm may be added to cover the rootface and extended as a bevel around themargin. This reduces the risk of root fractureand may also replace areas of dentine lostthrough caries or trauma. This is the mostcommon indication for a cast post.

• Cast posts are not as strong as direct posts,which are factory made and are strengthenedby processes that are common in engineering.

Parallel: threaded

There are two types:

• A very fine thread which can be self-tappedinto the walls of a parallel-sided post hole. It isclaimed that the threads are sufficiently fine asnot to produce too much stress in the dentineand therefore risk fracture. The cemented postis very retentive.

And

• An older and now seldom used system with amuch coarser thread. A thread is cut into thewalls of the prepared root canal with anengineer’s tap. The post is then cemented andscrewed in with minimal force (like assemblinga nut and bolt) so that stresses are not intro-duced into the dentine. Many patients had thistype of post fitted and so it is important torecognise their radiographic appearance.Because of the very retentive thread, theycannot be removed by the techniques used toremove other types of post – see Chapter 14.Attempting to do so will almost invariablyfracture the root (Figure 2.5d left).

Tapered-threaded (Figure 2.5d right)

• Made of base metal.

• Cuts its own thread as it is inserted (like awood screw) and therefore introduces consid-erable stresses into the dentine.

• Roots liable to split either as the post is beinginserted or subsequently.

• Because of the difficulty of inserting withoutroot fractures, retention is unreliable.

• Should not be used for permanent posts, butmany were used in the past and their radio-graphic appearance should be recognised(Figure 2.5e). Removal is difficult and risks split-ting the root.

Choosing between post systems

Many successful posts of all types have beenmade, and although each dentist has his or herown preference, and certain sets of clinicalcircumstances dictate that one type or another ispreferable, there is no one type that is uniformlysuperior to the others.

However, in most cases with sufficiently longroots where the coronal part of the root canalhas not been excessively tapered, the first choiceis usually either a preformed serrated or a castparallel-serrated post.

A preformed post is preferable for single,uncomplicated crowns, since it avoids the need fora temporary post crown and also avoids the riskof a casting failure. A relatively common cause ofsuch casting failures is porosity in the cast metalat the junction of the post and the core. As themetal cools, after being cast, it contracts and themore rapid cooling of the relatively thin postcompared with the bulkier core can produceporosity at the junction between them. Thisporosity is within the casting and is not visible.

Cast parallel-serrated posts and cores arepreferred when the core needs to be extendedas a diaphragm to cover part or all of the rootsurface or when a stronger core is needed.

The next best choice is usually the tapered-smooth cast post and core, which is used whenthe root canal is particularly tapered or oval orwhen the root is very tapered so that there is agreater risk of lateral perforation.

Parallel coarsely threaded posts are used whenthe available root canal is very short due to anobstruction or when particularly robust retentionis necessary. The technique for inserting them is

44 Types of crown

rather precise and time-consuming, particularly ifit is only used occasionally, and so it is notemployed as frequently as the other types.

The tapered-threaded type should not be used.The post material is relatively weak and thetechnique often introduces excessive stress intothe dentine, which may then fracture. If the self-threading characteristics of the post are not usedin order to avoid these stresses then the post isnot as retentive as other systems.

One-piece post crown

In some cases, for example with very short clini-cal crowns or with lower incisors, there is insuf-ficient space within the crown of the tooth tomake both a retentive core and a separate crown.Then, a crown made of metal–ceramic materialwith the post cast as part of the crown is oftenthe solution (Figure 2.3c).

Other types of crown for root-filledteeth

Occasionally the root canal is obliterated by afractured post that cannot be dislodged, or theroot canal is completely closed with secondarydentine. A crown can still be made by building upa core, usually in composite retained by pins:alternatively a metal–ceramic crown retained bypins cast together with the base of the crown canbe used.

Neither of these two techniques is likely to beas retentive as a post crown, and particular atten-tion must be paid to avoiding excessive occlusalforces.

Posterior complete crowns

Cast metal crowns

Although traditionally a gold alloy is used forcomplete metal posterior crowns, the cost ofgold and the considerable improvements thathave been made in the alternative alloys haveresulted in a rapid increase in the number of

crowns made with alloys containing less gold, andin some cases none. The term ‘metal’ is thereforeused as more accurate than ‘gold’ in most cases.

The British Standard BS 4425 for dental castingalloys has been replaced by BS EN ISO Standardsto reflect these improvements and the wide rangeof alloys now available. The American DentalAssociation (ADA) classification has also largelybeen replaced by BS EN ISO Standards.

This is now a complicated and confusing areaand so for everyday clinical decisions it is best torevert to the original classification of dentalcasting alloys. All modern dental casting alloys fitinto one or other of these groups.

1 (or I or A) soft used for small inlays2 (or II or B) medium used for large inlays3 (or III or C) hard used for crowns and

bridges4 (or IV or D) extra used for larger bridges,

hard implant frameworksand denture clasps

However, some of the low-gold alloys are pro-duced in a gold colour, which can be confusing.

Metal crowns are used either when the patientdoes not mind the appearance of metal or whenthe tooth does not show during the normalmovements of the patient’s mouth. When acomplete crown is necessary, it is the restorationof choice since it requires the minimum reduc-tion of tooth tissue, the margins are uncompli-cated by the presence of facing material, theocclusal surface is readily adjusted and polished,and the time taken to produce the restoration inthe laboratory is less than other types of crownso the cost should be less. It is the most conve-nient restoration for providing rest seats, guideplanes, reciprocal ledges and undercuts inconjunction with partial dentures. It can besoldered to other structures to make bridges andsplints, and solder can be added to it to reshapeits surface. The only significant disadvantage of thecast metal posterior crown is its appearance(Figure 2.6a and b) but not all patients object tothis and some even like it.

Metal–ceramic crowns

The principal advantage of metal–ceramic crownsover metal crowns is their appearance (Figure

Types of crown 45

2.6). Porcelain can be used on the mostcommonly seen buccal and occlusal surfaces.With many patients the occlusal surfaces of thelower teeth show more than the buccal surfacesand the reverse is true in the upper jaw.Therefore these surfaces are the ones whichmost commonly need ceramic material for anideal appearance. Any or all of the other surfacesmay also be covered with porcelain.

The disadvantages of the metal–ceramic crownfor posterior teeth are that more tooth tissueneeds to be removed to allow for the thickness

of porcelain, and when this is on the occlusalsurface of a tooth with a short clinical crownthere may be difficulty with retention because ofthe reduced length of the preparation. When thisis the case, additional retention by means of pinsor grooves is necessary.

With an amalgam core retained by pins, apreparation for a metal–ceramic crown is morelikely to give rise to trouble than one for a metalcrown, because the greater reduction of the corematerial may expose the pins and thus jeopardizethe retention of the core (Figure 2.7).

46 Types of crown

Figure 2.6

a and b Gold crowns on the first molar teeth which,with this patient’s lip morphology, were aestheticallyacceptable. There are metal–ceramic crowns on theupper canine and premolar teeth and ceramic crownson the upper incisor teeth.

c A partial crown (three-quarter crown). The tooth is vital: the greycolour comes from the amalgam core. A composite core would havebeen better.

Ceramic crowns

It is sometimes reasonable to use a ceramic crownon a posterior tooth, for example in conjunctionwith a post and core on a single-rooted premolartooth. Care needs to be taken in assessing theocclusion, but if this is favourable a modernceramic crown often has a better appearance thana metal–ceramic crown. However, they should notbe used in patients who grind their teeth at night(bruxists). Several manufacturers claim thatmodern ceramic crowns – particularly those witha zirconia base – are comparable to metal–ceramiccrowns, but more tooth preparation is requiredto allow for crowns of adequate thickness.

Posterior partial crowns

‘Three-quarter’ crowns

‘Three-quarter’ posterior crowns actually coverfour-fifths of the tooth’s surface – mesial, distal,occlusal, lingual or palatal and so ‘three-quarter’is not a very good name.

They are retained by grooves on the mesial,distal and occlusal surfaces that effectively performthe same function as the buccal surface of acomplete crown (see Chapter 5). They are alwaysmade of cast metal, and are used when the buccalsurface of a tooth is intact and reducing it as partof a complete crown preparation would eitherproduce an unsightly and unnecessary display ofmetal or where reducing the buccal cusp wouldweaken it, reducing the strength of the prepara-tion. An example of a tooth where a three-quartercrown is needed is shown in Figure 1.12c and clini-cal examples are shown in Figure 2.6.

The advantages of posterior partial crowns arethat they are more conservative of tooth tissue thancomplete crowns, and the margin of the crown doesnot approach the gingival margin buccally. It is stillpossible to test the vitality of the tooth via thebuccal surface, and the appearance is preferable toa complete metal crown, without there being theneed for the extra tooth destruction.

Some dentists find the preparation difficult, butthey would do well to learn the skill involved sincethe posterior partial crown is very conservative oftooth tissue, falling between the minimum prep-aration and the complete crown preparation. It isstill a useful part of the dentist’s repertoire.

Other types of posterior partialcrowns

There are a variety of alternative posteriorcrowns. The ‘seven-eighths’ crown covers all butthe mesial buccal cusp of an upper molar tooth,the ‘half’ crown covers the mesial half andocclusal surface of a lower posterior tooth wherethe distal wall is very short, and other variationsleaving various odd bits of the tooth surfaceexposed are also made. Principles governing thedesign of all these partial restorations are the sameand are covered in Chapter 5. It is for the dentistto use these principles to plan the detailed designof each restoration to solve its particularproblems. It is not good practice to follow classiccookery book-type preparation designs, none ofwhich may be suitable.

A further variation of the posterior partialcrown is the occlusal onlay, made to alter theshape of the occlusal surface or the occlusal verti-cal dimension but without necessarily coveringany of the axial walls. It is retained by intra-coronal features or adhesive techniques, andsometimes whole quadrants of opposing teeth arerestored by these means (Figure 1.15).

Cores for posterior crowns

Badly broken-down posterior teeth are rebuilt tothe general shape of the tooth using amalgam,composite, glass ionomer cement or cast metalbefore preparing them for crowns. These coresare retained by one of the techniques describedbelow.

Cores of amalgam, composite orglass ionomer cement

The commonest type of posterior core is madeof amalgam. In vital teeth the amalgam may bebonded to the remaining dentine or retained bypins or a combination of both. With the successof bonding when there is enough dentineremaining, pins are now only used with verybroken down, but vital teeth. The pins areusually threaded self-tapping pins screwed intodentine.

Types of crown 47

48 Types of crown

a A vital tooth with a pin-retained amalgam core for thepartial crown shown in Figure 2.6c. Pins must be sitedwith a view to the eventual preparation design – in thiscase avoiding the mesial and distal surfaces, wheregrooves are to be prepared. The alternative, a completemetal–ceramic crown, would have relied entirely on pinretention, the remaining cusp having been removedduring preparation. Elective endodontic therapy and apost-retained crown would have been less conservative.

b A vital tooth prepared with pins for a pin-retainedcomposite core. The enamel margin is being etchedwith gel. Once the gel is washed off, the deep part ofthe cavity will be lined. An alternative would be to usethe ‘total-etch’ technique, avoiding pins.

d An amalgam core retained by a copper ring. Thepatient was unable to return for the crown prepara-tion until 8 months after the core was placed. Thereis some gingival inflammation distally, but apart fromthis the gingival inflammation has been minimal. Thecore is intact. A large core like this placed with amatrix band would be vulnerable to fracture until it isprepared and protected with a temporary crown.

c The composite core in place, having been built up inseveral increments of light cured composite. This cannow be prepared at the same visit and a temporarycrown placed.

Figure 2.7

Cores for posterior crowns.

Types of crown 49

e A large failed amalgam restoration. f The amalgam removed and acomposite core placed, retained bytwo pins. The heads of the pins canbe seen. They are sited within thecomposite core, away from theperiodontal membrane and pulpand will resist lingual displacementof the core.

g The partial (three quarter) crownon the die.

h A premolar tooth prepared with two preformedparallel-sided posts for a post-retained core.

50 Types of crown

i Tooth prepared for a Nayyar core.

j A radiograph of the core in place.

k A clinical example of a Nayyar core.

When pins are used in a vital tooth, it is impor-tant to choose the correct number of pins and tosite them properly. In deciding the number andlocation of pins, the final design of the preparationmust be anticipated. For example, pins should notbe placed too close to the buccal margin when ametal–ceramic crown is planned, they risk beingexposed when the shoulder is prepared. Similarlythey should not be placed in the middle of themesial or distal surfaces of a core when the toothis going to be restored by means of a partialcrown. If they are, the grooves in the mesial anddistal surfaces of the preparation may expose thepins (Figure 2.7a). If a substantial cusp remains, thepins should be set at an angle relative to the innersurface of this cusp so that there is a retentiveundercut between the pins and the cusp.

When the final restoration is to be ametal–ceramic crown, the pins must be kept wellclear of the buccal shoulder area so that they arenot exposed during the preparation of the tooth.Also when a metal–ceramic crown is planned, anyremaining buccal cusp will usually be severelyweakened by the preparation, and cannot berelied upon to retain the core. Sufficient pinsmust therefore be placed to retain the corewithout this cusp, and it is sometimes goodpractice to remove the cusp completely beforethe core is placed.

Pins should not be used with root-filled teeth.The dentine of root-filled teeth is thought to bemore brittle than vital teeth, and so the stressintroduced by self-tapping pins may produce agreater risk of the tooth fracturing. In any case itseems commonsense to use the relatively largeholes that already exist down the root canals ratherthan drill yet more holes into the tooth for pins.An amalgam core in a root-filled tooth can beretained by posts in the root canals (Figure 2.7h)or by the Nayyer technique (Figure 2.7i, j and k).

With amalgam cores for both vital and non-vital teeth, a conventional amalgam matrix may beused to retain the amalgam while it is being

condensed, but when considerable tooth tissueloss has occurred it is often better to use acopper or orthodontic band that can be left insitu until the crown preparation is started. Thissupports the amalgam while it is setting, andreduces the risk of the amalgam core fracturingbefore preparation (Figure 2.7d).

Composite cores can be used in the same wayas amalgam cores, and they have the advantage thatthey can be prepared at the same visit that theyare inserted (Figure 2.7b and c). A suitable dentinebonding agent should be used and the compositebuilt up in increments, light curing each incrementto reduce the effects of polymerization contrac-tion. Despite these precautions, there is someconcern about the risks of microleakage betweenthe composite core and the dentine surface.

When posts are used to retain posterior amalgamor composite cores, the commonest type is theparallel-serrated preformed metal post (Figure 2.7h).If a substantial cusp remains and the post is placedin one of the root canals at an angle to the innersurface of the cusp, this will produce an undercutand therefore retention for the core. In other situa-tions where more of the enamel and dentine havebeen lost or will be removed in the crown prep-aration, two or even three posts should be used.Where the pulp chamber is intact the retention ofthe core using the Nayyer technique is preferableto the use of posts, which are sometimes difficultto place in the roots of molar teeth.

Cast posterior cores

With the Nayyer system, bonding and modernposts to retain amalgam and composite posteriorcores, there is little need for cast metal posts andcores in multi-rooted posterior teeth. However,cast posts and cores are still useful for single-rooted premolars when the post hole is taperedand when a diaphragm is needed.

Types of crown 51

Teeth vary so much in their general shape and inthe effects upon them of caries, trauma, toothwear and previous restorations that it is lesshelpful to describe classic ‘ideal’ preparationdesigns than it is to give the principles determin-ing the design and then show how these shouldbe applied.

The principles of crownpreparation design

The following factors need to be considered:

• Materials• Function• Appearance• Adjacent teeth• Periodontal tissues• Pulp• Retention of the crown to the tooth.

Related to materials

Metal crowns

Dental casting metal is strong in thin sections andcan be used to overlay and protect weakenedcusps against the occlusal forces. It is, however,ductile, and can be distorted if it is too thin or ifit is subjected to excessive forces. Normally nometal surface should be less than 0.5 mm thickand occlusal surfaces should be more. This usuallymeans that an equivalent amount of tooth tissuehas to be removed. When distorting forces areanticipated, the design can be modified either byreducing the tooth more, and producing a thickermetal layer, or by introducing grooves or boxesinto the preparation to stiffen the metal byproducing ridges on the fit surfaces.

Ceramic crowns

Porcelain is brittle when subjected to impactforces, and must be in sufficiently thick sectionsto withstand normal occlusal and other forces.Most of the reinforced ceramic systems (seeChapter 2) produce cores which are opaque butnot as opaque as the metal of metal–ceramiccrowns. Therefore it is necessary to provide asufficient thickness of more translucent porcelainon the labial/buccal surface of the crown tosimulate the appearance of a natural tooth. Itfollows that the minimum reduction for a ceramiccrown, made by any of the techniques, is muchgreater than for a metal crown.

The edge strength of porcelain is low, andtherefore the compromise at the cavosurfaceangle between brittle enamel and brittle porcelainis a 90° butt joint (Figure 3.1e).

Metal–ceramic materials

Even greater reduction of tooth tissue is neces-sary for metal–ceramic crowns on the visiblesurfaces, because the metal layer will need to becovered by an opaque layer of porcelain, and thisin turn will need to be covered by translucentporcelain. A thickness of 2 mm is ideal, but inmany situations, for example lower incisorcrowns, this is impossible because of the small-ness of the tooth. Where part of the crown is allmetal, for example on the lingual side, the prep-aration is as it would be for a metal crown.

The margin of the crown may be constructedin porcelain or metal, or the two materials mayjoin at the periphery. The cavosurface angle willdepend upon this decision. Figure 3.1e and fshows shoulder and bevelled shoulder finishinglines. An alternative, preferred by many dentistsand technicians, is a deep chamfer. Figure 3.1cshows a narrow chamfer suitable for a metalcrown and this can be deepened into dentine fora metal–ceramic crown.

3 Designing crownpreparations

53

Related to function

Occlusion

The occlusal relationships of the tooth to becrowned will influence the design of the prepara-tion. Those areas of the crown subjected toheavy occlusal loading in the intercuspal position(see Chapter 4) or in one of the excursions ofthe mandible should be sufficiently thick towithstand these forces without distortion if thecrown is metal, and without fracture if the crownis ceramic or metal–ceramic. This means that inan Angles Class I occlusion there should beadequate reduction of the occlusal surfaces of theposterior teeth, the palatal surfaces of upperincisor teeth and the incisal edges of lower incisalteeth. Other surfaces may also be involved indifferent occlusal relationships.

When there is posterior group function, that is,several pairs of posterior teeth slide against eachother as the jaw moves to the working side (seeChapter 4), the result of applying this principle isthat the cusps that function against each other inthis way should be reduced more than other partsof the preparation. This is often referred to as‘bevelling the functional cusp’. Before this is done,the actual relationships of the cusps in questionshould be studied during the full range ofmovements. In the majority of patients the pos-terior teeth are discluded in lateral excursion bythe canine teeth. These ‘functional’ cusps there-fore only function in the intercuspal position andare less vulnerable to wear and to lateral forces.There is therefore less need to reduce them anymore than the rest of the occlusal surface.

With tilted teeth the occlusal reduction is oftennot uniform, for example a lower second molar

tooth which has tilted forward into the space ofan extracted first molar will usually have occlusalcontact with its distal cusps but not its mesialcusps. In preparing the tooth for a crown moreocclusal enamel needs to be removed distally than

54 Designing crown preparations

Suitable for metalSuitable for porcelain

or metal–ceramicSuitable for

metal–ceramic

a b c d e fNear to 180˚ 130–160˚ Approx 90˚ Approx 90˚ 130–160˚

Figure 3.1

Margin configurations for crown preparations.

a A section of a molar tooth.

b A knife-edge margin with a cavosurface angleapproaching 180°.

c A chamfer margin with a cavosurface angle of130°–160°. This is the most commonly used margin formetal finishing lines. It can be deepened formetal–ceramic margins.

d A finishing line with minimal tooth reduction butwith a sharp step, prepared with a square-ended instru-ment producing a cavosurface angle of approximately90°.

e A full shoulder with a 90° cavosurface angle which isused for ceramic crowns. When used for ametal–ceramic crown, the metal is either brought tothe margin or finished short, leaving a porcelain margin.

f A full shoulder with bevelled margin for ametal–ceramic crown when the margin cannot be seen,for example at the buccal margin of lower molar teeth.A metal margin may be easier to finish and be lessplaque retentive than porcelain. It is also easier tomake in the laboratory.

mesially and the crown built up mesially to re-establish occlusal contact.

In some cases the crown is being made to alterthe occlusal relationship, and it may be necessaryto reduce the occlusal surface less than usual ifthe intention is to increase the occlusal verticaldimension.

Future wear

All restorative materials wear in use, and the rateis determined by the occlusion, the diet andparafunctional (bruxing) habits. Where the toothsurface is intact before crown preparation isstarted, careful note should be made of any wearfacets, and these areas of the tooth surface shouldbe prepared sufficiently to allow for an adequatethickness of crown material so that future wearwill not produce a perforation of the crown.

Related to appearance

Labial, buccal, incisal and proximalreduction

Adequate reduction of the tooth surface must becarried out on those surfaces where the appear-ance of the crown will be important. Insufficientlabial or incisal reductions for ceramic crownsresults in the core material showing through(Figure 3.2) or the crown being too prominent.Proximal reduction is important to achievetranslucency at the mesial and distal surfaces ofthe crown. Further back in the mouth it is moreimportant to reduce the preparation mesio-buccally than disto-buccally since this is the moreimportant surface aesthetically.

Occlusal reduction of posterior teeth

In most patients the occlusal surfaces of thelower premolar and molar teeth are more visiblethan the buccal surfaces in normal speech andlaughter. If metal–ceramic crowns are made forlower posterior teeth, it is usually necessary toreduce the occlusal surface sufficiently for porce-lain to be carried over it.

In the upper jaw the occlusal surfaces are farless visible, the buccal surfaces being more impor-tant aesthetically. It follows that it may be neces-sary only to reduce the occlusal surfaces of upperposterior teeth sufficiently for a thickness ofmetal. The decision whether to place porcelainon the occlusal surface of posterior teeth is basedon the balance between the patient’s wishes andthe advisability of undertaking the extra toothreduction. Patients who grind their teeth will bemore at risk of porcelain fracture, especially ifthere is a group function in lateral movement (seeChapter 4).

Crown margins

The position of the crown margin in relation tothe gingival margin affects the appearance.Subgingival margins may have a better appearanceinitially but will sometimes produce a degree ofgingival inflammation that, apart from possiblyleading to more serious periodontal disease, isitself unattractive. Crown margins at the gingivalmargin or slightly supragingival need not beobvious and will be less likely to produce gingivalinflammation (Figures 2.6a and 3.3). It is alsoeasier to take impressions of supragingivalmargins, to assess the fit and to maintain them.The intention to make visible margins supragingi-val should be discussed with the patient, explain-ing the reasons, before the teeth are prepared.

Designing crown preparations 55

Figure 3.2

Inadequate reduction of the labial incisal area of thepreparation so that the core shows through theceramic crown on the upper right central incisor.

Metal–ceramic crowns with a metal margin willbe more obvious if the patient’s smile line showsthe gingival margins. The metal margin can be cutback and the margin produced in porcelain reduc-ing this problem, but there is often still a greyingof the margin and adjacent soft tissues withmetal–ceramic crowns. Patients with high expec-tations and those with high lip lines may be betterserved with ceramic restoration when restoringteeth in visible areas. If metal–ceramic crowns areindicated, for example if the patient has a heavywear pattern, it will be necessary to place thelabial gingival margins in a more subgingivalposition than normal, requiring great care intooth preparation, impressions and checking themarginal fit of the crown. With subgingivalmargins, the patient’s level of oral hygiene mustbe very good.

Related to adjacent teeth

Clearance to avoid damage to adjacentteeth

If only one tooth is being prepared for a crown,it is clearly important to avoid damage to theadjacent teeth. This is much easier said than done,and a number of studies have shown that slightdamage to adjacent teeth during crown prepara-

tion is extremely common. In preparing theapproximal surface of a tooth with burs, thetooth surface must be reduced sufficiently toallow the full thickness of the bur to pass acrossthe contact area within the contour of the toothbeing prepared, leaving a tiny fragment of enamelor amalgam core in contact with the adjacenttooth. This falls away once the bur emerges atthe other side of the tooth. This means thatextensive reduction is often inevitable at theapproximal surface (see Figures 3.11 and 6.10b).

Path of insertion

When teeth are unevenly aligned, although acrown preparation can be made and an impres-sion taken, the finished crown sometimes cannotbe seated because the overlapping adjacent teethprevent its insertion. Adjustment of the crown toallow seating results in an open contact point.The solution is either to reshape the adjacentteeth or to design the preparation at an angle thatpermits the insertion of the crown.

Oral hygiene and technical considerations

Proximal reduction should preferably be contin-ued to allow clearance between the gingivalmargins and the adjacent tooth sufficient for a fine


Figure 3.3

Crown margins.

a Six crowns made with subgingival margins.

b The same crowns, except for the central incisors,which have been replaced, photographed 9 years later.There has been extensive periodontal disease andsurgery throughout the mouth. The crowns werereplaced with slightly supragingival margins, and thegingival tissues have remained stable.

saw blade to be passed between the dies, so thatthey may be separated in the laboratory. This alsofacilitates cleaning the margins once the crown isfitted (see Figure 3.17c). This may involve placingthe margin deeper if the teeth are close togetherand this should be assessed radiographically priorto starting the preparation. With convergentroots of adjacent teeth this may not be possible.

Figure 3.17c shows the minimum space whichshould be left between the distal surface of theprepared canine and the premolar tooth.

Related to periodontal tissues

The importance of supra- or subgingival crownmargins to periodontal health has already been

discussed. The shape of the crown margin (thecavosurface angle) should be designed so that thecrown surface can conveniently be made in linewith the tooth surface. Insufficient reduction atthe margin can result in an overbuilt crown,which in turn produces a plaque retention areaat the margin (Figure 3.1b).

It is not always possible to keep crown marginssupragingival at the proximal surface. Where thegingival tissues are normal and healthy whencrown preparation starts, the interdental papillafills the space beneath the contact point.Therefore if the crown margin is to include thecontact point, it will usually be necessary to makethe crown margin subgingival or to removehealthy gingival tissue surgically.

When the gingival tissues are inflamed, as theyoften are around teeth to be crowned, because


Figure 3.4

a Gross gingival hyperplasia and enamel hypoplasia.

b The hyperplastic gingival tissue has been removedsurgically.

c Crowns on the upper incisor teeth for the samepatient.

of plaque retention around existing unsatisfactoryrestorations, it may be necessary to modify therestoration and encourage better cleaning, or tomake a well-fitting provisional crown, provideperiodontal treatment and then adjust themargins of the preparation.

When periodontal pockets are present that areso deep that they cannot be maintained byimproved oral hygiene, periodontal surgery maybe necessary, and this will usually have the effectof moving the gingival margin apically so thatmore of the clinical crown (and root) is visible.This makes crown preparation with supragingivalmargins easier, but the appearance may be poor,

with large triangular spaces between the necks ofthe teeth (Figure 7.5). This surgical procedure isoften justified where there has been alveolar boneloss through periodontal disease or in otherperiodontal conditions (Figures 3.4 and 3.5). It isessential that the gingival condition is stabilizedprior to the final cementation of the crown, ifneed be by the medium-term use of well fittingprovisional crowns and re-inforcement of oralhygiene procedures, otherwise later recession ofthe gingival margin due to further progression ofthe periodontal disease is likely to occur.

A similar procedure, ‘crown lengthening’, is alsosometimes carried out where there is a normal


Figure 3.5

a A peg-shaped lateral incisor with a low gingivalmargin.

b Elective crown lengthening. This photograph wastaken a week after the surgery and the sutures areabout to be removed.

c The upper lateral incisor crowned with a conven-tional ceramic crown. A similar case treated with aresin bonded ceramic crown is shown in Figure 1.11.

level of alveolar bone and healthy gingival tissuesbut where the clinical crown height is reduced andit is perceived that there is a problem with reten-tion. Crown lengthening usually involves theremoval of healthy alveolar bone, and is thereforedestructive, and is quite uncomfortable for thepatient. Alternative means of improving retentionshould therefore be used whenever possible, andcrown lengthening should be reserved for thosecases where part of the purpose is to move thegingival margin apically for aesthetic reasons orwhere alternative means of retention are consid-ered to have a poor prognosis.

Related to the pulp

When a vital pulp is to be retained within thecrown preparation, a minimal thickness of dentinemust be preserved to protect it. The thickness ofthis layer will depend upon the age of the patient,the condition of the dentine (i.e. the amount ofperitubular and secondary dentine) and the typeof preparation. Only an approximate estimate canbe made of the size of the pulp in a given case,even with good radiographs. So, confusingly, thedesign of the crown preparation is partly deter-mined by the need to preserve the pulp undam-aged without really knowing in detail where it iswithin the tooth.

As with all tooth preparation it is essential thatpreparation is carried out with copious watercooling, sharp burs and gentle techniques to limitpotential damage to the pulp. Even with care beingtaken and the preparation not exposing the pulpthere is a small risk that the pulp may die aftertooth preparation. There is a view that all patientsshould be warned of this potential risk, which maynot present until long after the crown is placed.There is a contrary view that warning every patientof every possible complication of treatment is notin their interest because with some patients theeffect may be to frighten them to the extent thatthey refuse treatment that would be in their inter-est. Warnings must, of course, be given of seriousrisks and frequent complications. Dentists shouldassess the patient as a whole, the clinical circum-stances and the specific risk of pulp death in decid-ing whether a warning is appropriate.

The size of the pulp will also be determined bythe condition that has necessitated the crown

preparation. If this has been a slowly progressingcondition such as caries or tooth wear then theprobability is that the pulp will have laid down asubstantial amount of secondary dentine and willtherefore be much less vulnerable than if thetooth has been fractured in an accident. If thenatural crown of the tooth is small (microdontia)and the purpose of making a crown is to increasethe size then very little tooth preparation isnecessary and the pulp is not significantly jeopar-dized even with young patients (Figure 3.5).

This need to protect the pulp often conflictswith the need for an adequate thickness of crownmaterial, particularly in extreme cases such asmetal–ceramic crown preparations on lowerincisor teeth. Here the ideal thickness of crownmaterial commonly has to be compromised infavour of the need to protect the pulp.

A good way to gain experience that should helpavoid too many dead pulps or failed crowns is tomake preparations on a variety of extracted teethand then section them to see how much dentineis in fact remaining (Figures 3.6 and 3.7).

Retention

There are two principal systems used to retainrestorations in crowns and bridge retainers.

Mechanical retention

The conventional method, which has been usedfor many years, involves preparing the tooth to aretentive shape and then cementing the crown orbridge retainer with a luting cement, which is notusually chemically adhesive to either the toothsurface or the fit surface of the crown. Thecrown is retained by a combination of themechanical design features to be discussedshortly.

‘Adhesive’ retention

The second system is to use an adhesive luting orbonding cement that bonds either chemically ormicromechanically to both the tooth surface andthe restoration. Restorations cemented in thisway therefore do not need to rely primarily on



Figure 3.6

A metal–ceramic crown prepara-tion on a lower incisor.

a An extracted tooth. The bucco-lingual width at the cement–enameljunction is 7 mm.

b Prepared for a metal–ceramiccrown. The width of the buccalshoulder is 1.2 mm and the lingualchamfer is 0.5 mm wide.

c Sectioned and the pulp filled withwax. The closest point of thepreparation to the pulp is 1 mm.

Figure 3.7

This extracted molar tooth has been prepared with ataper of 35°. This is more than the recommendedfigure (page 63). An occlusal force, a, will not dislodgea crown, however unretentive the preparation. A forcedirected at an inclined cusp plane, b, occurring in lateralexcursions of the mandible will, though, have a dislodg-ing effect if the crown preparation is unretentive as inthis case. Loss of retention is unlikely to occur as aresult of a single contact of this sort. It is more likelyto result from small repeated forces in alternate direc-tions, b and c.

the shape of the tooth preparation to retain therestoration. Examples of these restorations areporcelain veneers (Chapter 1) and minimum-preparation bridges and splints (Chapter 8). Atpresent there is a range of such adhesive bondingcements:

• Chemically active resin-based cements, whichchemically bond to a freshly grit-blastedprecious or ideally non-precious cast metalsurface and which lock micromechanically intoan etched enamel surface and also bond to it.These cements normally also include a dentinebonding agent.

• Glass ionomer luting cements, which adherechemically to both enamel and dentine but notto cast metal surfaces or other restorativematerials. They rely on a mechanical bond tothe restoration surface.

• Composite luting cement consisting of a lightlyfilled resin that retains restorations by physi-cally locking into micromechanical retentivefeatures on both the tooth surface, etchedenamel and dentinal tubules and the restora-tion (Figure 3.8).

• Resin modified glass ionomer cements whichcombine the chemical bonding attributes ofglass ionomers with the strength and commandset of a lightly filled composite resin. Thesecements work by chemical bonding to enamel

and dentine and micro-mechanical bonding toboth the tooth and restoration.

These adhesive systems have improved signifi-cantly since the last edition of this book and arestill being developed. They have had an influenceover the principles of retention used with crownsand bridges as well as allowing the developmentof new techniques. The design of preparations forporcelain veneers and minimum-preparationbridge retainers is given in Chapters 1 and 8. Thedesign for a resin bonded crown is that thepreparation should, as far as possible, be confinedto enamel to obtain the optimum bond and reten-tion. Figure 1.11 shows the preparation of twopeg shaped lateral incisors for resin bondedcrowns. Figure 3.6 shows the preparation for alower incisor for a metal–ceramic crown. Itshows the balance which must be struck betweenthe need to protect the pulp and the need forsufficient preparation of the labial surface toproduce a satisfactory appearance.

Retention for conventional crowns

Despite the important developments withadhesive cements described above, only a limitednumber of teeth can be treated in this way. The


Figure 3.8

A confocal microscope image showing composite lutingcement penetrating the dentine tubules and producinga very strong bond to the dentine.

majority of crowns are still made by conventionaltechniques and so the following paragraphs relateto these.

Retention against vertical loss

A crown inserted from an occlusal or incisaldirection can be lost in the reverse direction.Axial forces unseating crowns in this direction fallinto three categories.

First, there can be a direct pull on the crownsuch as that exerted by biting into a sticky toffeeand the jaw then being opened sharply. Otherdirect unseating forces are the removal of partialdentures and leverage in some bridge designs.

Second, there are forces arising as a compo-nent of lateral force against an inclined plane(Figure 3.7).

Third, there are forces exerted by the dentistin a deliberate attempt to remove the crown.Apart from the force exerted by the dentist, thesevertical unseating forces are less than the forcesapplied in normal function, which are in directionsthat seat the crown onto the preparation.

The path of insertion may be inclined awayfrom the long axis if an anterior crown is beingconstructed to give the appearance of proclina-tion or retroclination, or if a large amount oftooth tissue has been lost on one or other sideof the tooth due to caries or trauma so thatinclining the path of insertion would allow moreof the remaining tooth tissue to be preserved.


Figure 3.9

A section through a typical dentine/cement/crowninterface showing the irregularity of the two surfacesand the angle at which separation must occur unlessthe cement is to be crushed.

Interlocking minor undercuts

Figure 3.9 shows a section through a dentine/cement/crown interface. The surface irregularitiesof the dentine are typical of those produced byfine diamond burs. The irregularities of the castmetal surface are typical of a surface that hasbeen cleaned by light grit-blasting. Even withoutan adhesive cement, it would not be possible todetach the crown from the tooth by sliding itaway parallel to the tooth surface or at an anglefrom the tooth surface, until an angle of morethan 36° was reached, without crushing andshearing the cement within the minor undercutson the two surfaces.

Taper of the preparation

Depending upon the size of these minor under-cuts and the compressive strength of the cementused, the taper of the preparation (the anglebetween opposing walls) and its length determinethe degree of retention against axial unseatingforces. A parallel preparation is impractical, sincecement cannot be extruded from the crownduring cementation leaving an excessive thicknessof cement occlusally and at the margin. Once thetaper of the preparation exceeds 30° or so,failure through loss of retention becomescommon. Under ‘ideal’, artificial, laboratoryconditions and using artificial materials ratherthan natural teeth, a taper of 7° has been shownto be the optimum with minimum cement filmthickness and maximum retention. However, inthe mouth it is impossible to achieve consistentlya uniform 7° taper without producing someundercut preparations and damaging manyadjacent teeth. The human eye cannot, in theclinical situation, detect the difference between aparallel preparation and one of 10° or so. Severalstudies have shown that the average taper forposterior crown preparations that have beenclinically successful in a large number of cases isapproximately 20° (Figure 3.10).

Most clinicians do not have a protractor amongtheir instruments and so rather than aiming toachieve a taper of ‘x°’ – which cannot be conve-niently measured and which will vary around thetooth – the object should be to produce a prep-aration that is as conservative of tooth tissue aspossible (including adjacent teeth), but where anabsence of undercut can clearly be seen. In most

cases this will produce an acceptable taper ofbetween 10° and 20°.

Length of the preparation

The greater the length of the preparation, themore retentive the crown will be. The minimumacceptable length will depend on other circum-stances, including the nature of the occlusalforces, the number of other teeth and whetherthe crown will be subjected to withdrawingforces from a partial denture or bridge.

The relationship between length and taper isimportant. The shorter the clinical crown, themore parallel should be the taper attempted.

When the clinical crown is assessed as being tooshort for adequate retention it must be built upwith a core (if there is sufficient occlusal clearance),or a surgical crown lengthening procedure may becarried out (Figure 3.5) or additional retention maybe achieved by means of grooves or pins. Adhesivecement will also help to retain a crown on a shorttooth and the combination of optimal tooth prep-aration with minimal taper and adhesive cementmay limit the need for further more drastic action.

Retention against other displacingforces

Provided that the taper of a complete crown prep-aration is uniform between all the opposing surfaces,the only way the crown can be lost is along the pathof insertion. However, some crowns cannot bemade with uniform taper. Incisor teeth, for example,can be prepared with a small angle of taper betweenthe mesial and distal surfaces, but it is impossible toproduce a narrow angle of taper bucco-lingually –similarly with some molar teeth (Figure 3.11). Partialcrowns must also be designed to prevent loss indirections other than axial – see later.

In all these cases it is helpful to envisage thecrown as potentially being dislodged from thepreparation in one of five directions:

• occlusally• buccally• lingually• mesially• distally


or at any angle between these directions. Thepreparation needs features that prevent loss in allthese directions. These features also need to bedistributed around the preparation so that thecomplex (and not fully understood) forces appliedto the crown do not dislodge it. All crown materi-als, and certainly dentine, have a degree of flexi-bility, and unless the crown preparation has theseretentive features, this flexibility may eventuallylead to a breakdown in the cement lute, leakageand either caries or loss of retention.

These additional retentive features are usuallyeither grooves or pinholes. Both have the poten-tial not only to resist loss of the crown in a direc-tion other than in the long axis, but also toreduce the angle of the path of insertion. Forexample, a crown preparation with an excessive


Figure 3.10

Bucco-lingual sections through crown preparations onthree premolar teeth. The taper of the preparations isshown. The 7.5° and 12° preparations would be suffi-ciently retentive in virtually any clinical situation. The23° preparation would probably be satisfactory in mostclinical situations unless subjected to undue lateral oraxial withdrawing forces. Note that in the 7.5° prep-aration both buccal and lingual enamel is hardlyreduced at all towards the occlusal surface. This is alsotrue of the 12° preparation. This would result in anoverbuilt, bulbous crown in this region.


Figure 3.11

Sections through several teeth showing the difficulty ofpreparing opposing walls nearly parallel.

Figure 3.12

A complete crown preparation that is inevitablytapered mesio-distally because of the relationship withthe adjacent teeth. For an individual crown this prep-aration with a 17° taper and good length would besufficiently retentive without additional retentivefeatures. However, as an illustration the taper has beenreduced by adding buccal and lingual grooves. Groovesshould not be used unless they are judged to be neces-sary, as they bring the preparation closer to the pulp.

mesio-distal taper may be improved with buccaland lingual grooves (Figure 3.12).

Avoiding failure from other causes

Fracture or distortion of tooth tissue

The remaining tooth tissue, once the crownpreparation is completed, must be sufficientlyrobust to withstand not only the forces to whichit is subjected when the crown is completed andcemented, but also the forces that it willencounter during impression taking, while atemporary crown is in place, and during try-in andcementation of the final crown. This may be aproblem in preparations for anterior post crowns

where a rim of tooth tissue is left around the posthole and with partial crowns.

Fracture of ceramic crowns

Stresses are developed within ceramic crowns as aresult of contraction on cooling after the firing cycle.These stresses produce minute cracks, some ofwhich originate at the fit surface and propagate toproduce failure if the crown is subjected to suffi-cient force. These stresses are concentrated aroundsharp internal angles of the fit surface, so the exter-nal angles of ceramic crown preparations should berounded to reduce these stresses (Chapter 6).

Rounded angles have other advantages: it iseasier to lay down a platinum foil matrix withouttearing it, sharp corners on a refractory die might


Figure 3.13

Rigidity in partial crowns.

a The fit surface of a premolar partial crown showingthe ridge of metal running across the occlusal surfaceand connected to the mesial and distal ridges. Thepartial crown for the canine is retained by three pinsrather than grooves. Although the premolar prepara-tion is still used, the canine preparation is no longerused, being superceded by bonded restorations.However, there are still many patients with successfulbridges retained by partial crowns and so dentists needto recognise them and know how to deal with them ifthey need maintenance or repair.

b The fit surface of another design of an anteriorpartial crown using pin retention.

be damaged, and during cementation the flow isimproved, producing a thinner film of cement.

Distortion of metal

A common cause of failure of anterior partialcrowns was leakage producing discoloration andcaries behind the incisal tip, and caries starting atthe approximal gingival margin. This is one of themain reasons that anterior partial crowns are nolonger made.

For posterior partial crowns the design mustinclude features which stiffen the casting andresist distortion. The traditional posterior partialcrown preparation incorporated internal mesialand distal ridges connected to an internal occlusalridge producing a stiff U-shaped bar (Figure

3.13a). In less classic preparations the principleshould be for a ridge of metal to run all the wayround the periphery of the preparation toprevent distortion (Figure 3.13b).

Casting difficulties

The external angles of complete crown prepara-tions for metal castings should also be roundedto prevent one of the faults that may occur in thefollowing chain of events:

• Stone die material may not flow into theimpression adequately, trapping air bubbles inthe sharp angles of the impression. If thishappens and the impression material is suitable,a new stone die can be poured.


Figure 3.14

a The fit surface of a full gold crown. Small ‘blips’ inthe casting are common and prevent the casting seatingcompletely. Castings need to be inspected routinely inthe laboratory. Look in the lower right part of theocclusal surface.

b After careful trimming under magnification the fitsurface is accurate.


Figure 3.15

Posterior crown preparations.

a Preoperative preparation for a full metal crown.

b Mesial and distal preparation with a thin diamondinstrument with pointed tip to produce a chamferfinishing line.

c The axial preparation has been carried round the buccaland lingual surfaces, and grooves are now being placed inthe occlusal surface to ensure uniform reduction.

d Reduction of the occlusal surface. The distal chamferfinishing line can be seen.

e The axial walls of the preparation finished with plain-cut tungsten carbide burs. The axial–occlusal angle willnow be rounded.


f A mesio-distal section of a cast of the finished prep-aration with the diamond and tungsten carbide bursused to prepare the tooth. The relationship betweenthese burs and the adjacent teeth can be seen, and itis clear that this is the minimum achievable taper avoid-ing damage to the adjacent teeth and an excessiveshoulder preparation mesially and distally. The mesio-distal taper is 14˚.

h Impression of the preparation in g showing thechamfer finishing line.

g Occlusal view of typical complete gold crown prep-aration on an upper first molar tooth with an amalgamcore.


Figure 3.16

Posterior metal–ceramic crown preparations.

a A tooth prepared as a bridge abutment. Only a smallamalgam restoration was present. The buccal margin isa bevelled shoulder, the palatal margin is a chamferfinishing line and the entire margin is supragingival.

b and c A typical metal–ceramic crown preparation ona root-filled tooth with a post-retained amalgam core.Note the amount of occlusal clearance. The axial wallhas been finished smooth but the occlusal surface hasnot. This is of little significance, except that the mesialand distal corners of the occlusal surface should havebeen rounded.


Figure 3 17

Anterior crown preparations.

a, b and c Ceramic crown preparation on an incom-pletely erupted tilted upper canine tooth in a patientwith a repaired cleft palate. The final crown will be aretainer for an all ceramic cantilever bridge. The crownwill have a very different alignment and appearance tothe unprepared tooth. There has been almost noenamel removed from the incisal edge and the adjacentbuccal surface. A trial preparation on a study cast isessential with this type of problem (see later).

d and e The objective is to reduce the size of theupper central incisors. Preparations in c are as exten-sive as possible, allowing ceramic crowns to beconstructed that are narrower and less prominent thanthe natural teeth, except at the neck.

• The sharp edges of the die may be damaged atthe wax-up stage.

• Investment material may not flow adequatelyinto the wax pattern, producing air bubbles inthe internal angles of the wax pattern which

produce blobs of metal in the casting, prevent-ing it from seating fully (Figure 3.14).

• Cement will flow less readily around sharpangles, increasing the likelihood of an unneces-sary thick cement layer at the margins.


f An inadequate ceramic crown preparation. The previ-ous crown had broken. The angles of the preparationare too sharp and there is insufficient reduction for anadequate thickness of porcelain for strength andappearance.

g The preparation shown in f modified for a ceramiccrown. Note the rounded external and internal anglesand the greater reduction.

h Metal–ceramic crown preparations to retain a bridgeon the lower incisor and canine teeth. Although reten-tive, these preparations are very destructive and aminimum-preparation design would normally be used.

i Totally inadequate preparations on a number ofupper anterior teeth. The preparations are over-tapered, all except the central incisors are far tooshort, and the surfaces are too rough. All theserestorations failed with disastrous consequences.

j Metal–ceramic crown preparations on badly wornincisors that have been built up with composite cores(the same patient as in Figure 5.6).

Designing specific crownpreparations

The principles outlined above are common to allpreparations. Some are more important thanothers, however, with different types of crown.

Posterior complete crownpreparations

All-metal crowns

Whether the preparation is on a natural tooth oran artificial core, application of the principles willusually result in preparations as shown in Figure3.15.

Variations include additional axial grooves orpinholes to limit the path of insertion when a pairof opposing walls are more tapered than is desir-able.


Posterior metal–ceramic crown preparations willusually have an all-metal lingual surface and aporcelain buccal surface, and may have a porcelainocclusal surface. The decision where to finish theporcelain will influence the preparation. Themargin may be a deep chamfer or a bevelled shoul-der to allow a small line of metal to show, simpli-fying the finishing of the crown margin (Figure 3.1f).In the posterior part of the mouth this appearanceis usually acceptable. Figure 3.16 shows typicalposterior metal–ceramic preparations.

Anterior crown preparations: crownsfor vital teeth

Ceramic crowns

A series of ceramic crown preparations is shownin Figure 3.17, demonstrating the application of


Figure 3.18

a A full contour wax-up of a minimum-preparationbridge on the master cast.

b A silicone putty mask has been made from the wax-up which is then cut back to allow space for the porce-lain to be placed. Once cast this ensures that theporcelain will be properly supported by the metalframe.


Figure 3.19

Posts and cores.

a A stainless-steel post has been cemented and cut offflush with the palatal surface of the preparation,preserving as much dentine as possible. See also Figure6.14.

b A cast post and core with a substantial amount ofdentine remaining as part of the preparation.

c A cast post and core with no coronal dentine.

the principles in a variety of cases, includinglower incisor teeth. In all but Figure 3.17i, thecrowns will be retentive and there is sufficienttooth reduction to enable a crown to be madeof adequate thickness for strength and appear-ance.


Figure 3.17 also shows preparations formetal–ceramic crowns. Compared with ceramiccrown preparations, the buccal reduction isgreater and lingual reduction less where possible.A deep chamfer margin is required on the labialaspect extending into the proximal areas merginginto a shallow chamfer margin palatally whereonly metal is to be placed. This is more impor-tant mesially where the porcelain will show thandistally where it will not.

The metal should also support the porcelain sothat excessive thickness of porcelain is avoided.Ideally a metal–ceramic crown or bridge shouldbe waxed up to a full contour and then cut backto allow optimal porcelain thickness for an idealappearance and yet preserving sufficient metal forstrength (Figure 3.18).

Post-retained crowns

The shapes of post holes were described inChapter 2. The margin of the crown preparationwill be similar to that for a vital crown of thesame material but the difference is that in thecase of a post crown there is no pulp to protect,and therefore the shoulders can be wider and the

core thinner than for an equivalent vital toothpreparation. This is possible not only because ofthe absence of a pulp but also because the corematerial is either reinforced by a preformed metalpost or the whole post and core is cast (Figure3.19) and is therefore stronger than dentine.Besides, a laboratory-produced core can be mademore parallel-sided and retentive than a clinicalpreparation.

The dentine remaining between the post holeand the shoulder, or other margin, may beretained or removed, depending upon its thick-ness. With a preformed post and composite core,virtually all the dentine can be saved (Figure 3.19aand b). The advantage in retaining a collar ofdentine around the post hole when a cast postand a core is to be made is to guide the techni-cian in the dimensions of the core required. Therim of dentine also slightly lengthens the post andimproves the retention of the whole restoration(Figure 3.19b).

Posterior partial crown preparation

A typical posterior partial crown is much like acomplete gold crown except that the buccal wallof the tooth is left unprepared. This means thatthe crown can be inserted or lost not only fromthe occlusal direction but also lingually. A pos-terior partial crown must therefore incorporatefeatures that will prevent lingual loss, usuallymesial and distal grooves connected by anocclusal groove, the important area being thelingual wall of the groove (see Figure 2.7 andFigure 3.20). With other types of posterior partialcrowns, similar grooves or pins are used.


Figure 3.20

A posterior partial crown preparation for an upper firstmolar tooth. Mesial and distal boxes have beenprepared rather than grooves since there was a previ-ous MOD amalgam. The occlusal groove has beenprepared through the remaining amalgam. The staineddentine is firm, and further removal is unnecessary. Thetooth has been prepared as an abutment tooth for abridge, which will be fixed-movable (see Chapter 8).

There are excellent textbooks on occlusion, andthe reader who has studied these will be forgivenfor skimming this chapter. For those who are notyet conversant with the principles of occlusion,this short explanation together with the practicaltechniques for recording and reproducing occlusalrelationships is intended as an introduction to thesubject. It will be sufficient for making crowns andbridges for patients with no functional distur-bances or pathological changes in the temporo-mandibular joint or the oro-facial musculatureand no major occlusal abnormalities, i.e. mostpatients. For more difficult occlusal problems thereader is referred to the comprehensive texts.

For many years there has been considerableinterest in normal and abnormal occlusions andin the effects of abnormality. There is a rapidlyexpanding literature, both research-based andempirical. An unfortunate side-effect of thisenthusiasm is that the whole subject seems to beconfused in some dentists’ minds with the use ofcomplex articulators, recording devices andexpensive full mouth ‘rehabilitations’. The ‘all ornothing’ law seems to apply, with some dentistsapparently blaming most of the human race’s ills,and all of its dental ones, on the odd aberrantcusp, while some others remain unconvinced,throwing the baby out with the bath water –paying little or no attention to occlusal relation-ships other than when a filling or crown is ‘highin the bite’.

An understanding of a few simple principles ofocclusion related to natural teeth and in particu-lar how to examine occlusions encourages amiddle course between these extremes and willbe of great value in preventing some of thefailures that occur with restorations that replaceocclusal surfaces.

Elaborate equipment is unnecessary for theapplication of these principles to the restorationof small groups of teeth; equipment should beseen simply as a means to an end. In fact, the

principles determining the design of articulatorsand recording devices are fairly simple, but theirconversion into three-dimensional reality leads tothe complexity of the equipment.

A functional approach to occlusion

The most useful way for a restorative dentist tolook at occlusion is from the functional point ofview; the morphological details of the occlusionare less important. The fact that an occlusion isAngles Class I, II or III is less important than theway the teeth move across each other in variousmovements of the mandible. For example, inlateral excursions in some patients, the caninesare the only teeth in occlusion, and in othersseveral of the teeth are in occlusion (Figure 4.1).

The restorative dentist should also recognisethat crowns, bridges or any restorations involv-ing the occlusal surface will often affect the waythe occlusion functions. This effect should bedeliberately planned rather than be allowed toinfluence the occlusal movements by accident.

The functional compared with theorthodontic approach

Orthodontic treatment is aimed primarily atimproving the patient’s appearance and producinga stable posterior occlusion in a single staticposition. Some forms of orthodontic treatmentgo further and establish deliberate patterns ofcontact between the teeth in various movementsof the mandible.

Most orthodontic treatment is carried out onyoung people who have not yet developed rigidpatterns of involuntary neuromuscular control oftheir mandibular movements. They are therefore

4 Occlusalconsiderations

77

capable of adaptation to fairly drastic changes intheir occlusal relationships in a way that someolder patients find difficult.

From the functional point of view there shouldbe no difference in the objectives of theorthodontist and restorative dentist – only differ-ent means of achieving them.

Fixed compared with removableprosthetic approaches

The main purpose of designing the occlusion forcomplete dentures is to produce stability of thedenture bases. This is an entirely different conceptfrom the restoration of natural teeth with intact

78 Occlusal considerations

Figure 4.1

Lateral guidance.

a Canine guidance in right lateral excursion with theposterior and incisor teeth completely discluding.Contact in this position is sometimes shared betweenthe canine, lateral and central incisor teeth – incisor oranterior guidance.

b and c Group function. The patient is shown, b, inintercuspal position, and when she moves to theworking side, c, contact is shared between the pos-terior teeth. The lateral and central incisors disclude.

roots and periodontal membranes. Occlusalconsiderations for partial dentures fall somewherebetween these two positions. In complete dentureconstruction, consideration of where the occlusalsurfaces of the artificial teeth should be in relationto the ridge, the presence of balancing contactson the non-working side, and the angulation ofcusps or absence of cusps altogether, are impor-tant. None of these applies in the same way tothe construction of fixed restorations.

The methods of recording and reproducingmandibular movement are similar whether fixedor removable appliances are being made.However, the principles governing the design ofthe occlusal relationships, although similar insome respects, are different in others.

Mandibular movements anddefinition of terms

The movements that the mandible can make andthe names of the important positions within thisrange of movements are shown in Figure 4.2.

Terminal hinge axis (THA) and the retruded arc of movement (or closure)

This is an axis which passes through bothcondyles and about which the mandible rotates inits most retruded (comfortable) position of thecondyles – the retruded arc of closure. This is aclinically reproducible movement and recording itis often useful in making crowns and bridges andessential in making complete dentures. The THAcan be measured in individual patients but it issufficient for most applications to use an averageTHA such as that recorded by an ear-bow, asshown later in Figure 4.10.

Intercuspal position (ICP)

This is the position of maximum contact andmaximum intercuspation between the teeth. It istherefore the most cranial position that themandible can reach. The term ‘centric occlusion’

has been used to describe this position, but thisis confused with ‘centric relation’ (see below) andmay also imply centricity of the condyles in theirfossae, centricity of the midline of the mandiblewith the midline of the face, or centricity of thecusps within the fossae of the opposing teeth,none of which may be the case. The term ‘centricocclusion’ is therefore better not used.

Retruded contact position (RCP)

This is the most retruded position of themandible with the teeth together. It is a clinicallyreproducible position in the normal consciouspatient. Patients with conditioned patterns ofmuscle activity may not be able to manipulate thejaw into it, even with assistance by the dentist. Inless than 10% of the dentate population the RCPcoincides with the ICP. In the remainder the RCPis up to 2 mm or more posterior to the ICP. Theterm ‘centric relation’ has been used to describethis position, but it has the same disadvantages asthe term ‘centric occlusion’ and will not be used.‘Centric occlusion’ (CO) and ‘centric relation’(CR) are terms sometimes used in completedenture construction where they mean differentthings to ICP and RCP.

Mandibular movements

Those patients who have a discrepancy betweenthe RCP and ICP usually close straight into theICP from the postural or rest position when themovement is made subconsciously. Patients sittingin dental chairs making voluntary, consciousmovements when asked to do so by the dentistoften make bizarre movements rather thanclosing into the ICP directly. These aberrantmovements and contacts are the result of patientstrying too hard to help and not understandingwhat is required. Students have been heard to asknon-dental or medical patients to bite on their‘posterior’ teeth!

However, contact does occur in the rangebetween the ICP and RCP during empty swallow-ing (particularly nocturnal swallowing), during themastication of a tough bolus and during parafunc-tional activity. Thus the mandible can slide from

Occlusal considerations 79

the ICP in four main directions with the teeth incontact, or in an infinite number of directions atangles between these main pathways. The fourexcursions are:

• retrusive• protrusive• left lateral• right lateral.

Retrusive movements

Movements between the ICP and RCP are usuallyguided by a limited number of opposing pairs ofcusps of posterior teeth. Figure 4.3a, b and c illus-

trates the occlusal contacts produced in RCP andother excursions in a typical natural dentition.The angle of the slide between RCP and ICP, itslength and the individual pairs of teeth thatproduce it are important and should be examined.Of even greater importance is any unevenness ofthe movement producing bulges or lumps in thepath of movement. These disturbances to thesmooth movement of the mandible are one formof occlusal interference (see page 83).

Protrusive excursion

In forward movement of the mandible with theteeth together. It is usually the incisor teeth that


Figure 4.2

Border movements of the mandible.

a The maximum possible movement of the tip of alower central incisor. The teeth are in occlusion fromRCP to the fully protruded position (P). In openingfrom RCP to X, the mandible rotates in a pure arc ofa circle around an axis (the terminal hinge axis, THA),which passes through the condyles. X is the maximumopening that can be made without the condyles movingforwards and O is the maximum opening with thecondyles fully protruded.

b The view from above, showing RCP and ICP. Themovements are not pure arcs of circles, because whenthe mandible moves to the side the condyle on theworking side shifts laterally (Bennett movement) andthe condyle on the non-working side moves forwardsand medially (Bennett angle).

c The border movements viewed from in front. Themovement from ICP to the cusp-to-cusp contact (C)is guided either by canines, all the anterior teeth or agroup of posterior teeth (see Figure 4.1). From C tothe maximum lateral position, L, the guidance is irreg-ular and usually controlled by the anterior teeth orteeth on the non-working side. This is a non-functionalrange not usually involved in parafunctional activity, andis therefore of little importance.

d Changes in the lateral guidance will either expand the

original border movement – Y, or encroach upon it –Z. Both changes constitute occlusal interferences. Thechange, Y, would result, for example, from the fractureor extraction of a canine tooth that previouslygoverned lateral guidance. The change, Z, might resultfrom overbuilding the cusps of a posterior crown in agroup function occlusion or from the development ofnon-working side contacts.

e This occlusal interference, an irregularity developingin the smooth movement from ICP to RCP, may alsoresult from crowns. An example of extraction andover-eruption causing this change is shown in Figure4.4.

f An expansion of the border movement is often anobjective of occlusal adjustment in the range from ICPto RCP. For example, if the original movement was fromICP to RCP1, with large vertical and horizontal compo-nents to the movement, an adjustment could be carriedout to produce a ‘long centric’, so that the movementwas flat from ICP to RCP2. This would not change thehorizontal component of the movement, but wouldreduce the vertical component to zero. The alternativeof making ICP and RCP coincident (RCP3) usuallyinvolves multiple crowns or other restorations as wellas occlusal adjustment. This is because either the bordermovement space needs to be encroached upon (thedashed line) or a substantial amount of tooth tissue mustbe removed. When this is done the process is knownas ‘reorganizing’ the occlusion. If ICP is left undisturbed,the occlusal plan is known as ‘conformative’.


b The same patient contacting in RCP. There are ofcourse fewer contacts, but they are evenly distributed,both anterio-posteriorly and between left and right.

Figure 4.3

Occlusal contacts.

a Perforations in a 0.5-mm-thick sheet of soft waxproduced by the patient closing in ICP.

e A wax occlusal record of the patient shown in d in thesame right lateral excursion, showing this interference tobe the only contact at this point.

c The same patient making contact in right lateralexcursion, mainly on the canine teeth, although acontact is also present posteriorly.

d A different patient making a right lateral excursion. Theleft (non-working) side is shown, and there is contactbetween the lower second molar and the first uppermolar. This is a non-working-side occlusal interference.

guide the movement. This will not be the case inanterior open bites or in Class III incisor relation-ships.

The angle and length of movements will bedetermined by the incisor relationship so that, forexample, in a Class II Division II incisor relationshipwith an increased overbite and reduced overjet, themovement of the mandible has to be almost verti-cally downwards before it can move forwards.Anterior guidance is important when makinganterior crowns or bridges. Sometimes, when theteeth are a normal shape, it is helpful to reproducethe patient’s existing guidance as accurately aspossible; on other occasions, for example, withworn teeth, it is unnecessary or undesirable to doso, and in fact the purpose of the treatment maybe partly to alter the incisor guidance.

Left and right lateral excursions

In lateral excursions the side that the mandible ismoving to is known as the working side and theopposite side the non-working side. The term‘balancing side’ has been used to refer to the non-working side, but since it implies a balancedocclusion, balancing or stabilizing a completedenture base, it should not be used in referenceto natural teeth.

The contacts on the working side are eitherbetween the canine teeth only (canine-guidedocclusion, Figure 4.1a) or between groups ofteeth on the working side (group function, Figure4.1b and c). Occasionally, individual pairs of

posterior teeth will guide the occlusion in lateralexcursion, but this is not regarded as ideal. Thecanine-guided occlusion is considered to beprotective of the posterior teeth which discludein lateral guidance.

Contact on the non-working side in lateralexcursions should not normally occur. It doessometimes occur after extractions and over-eruption and occasionally following orthodontictreatment, particularly when this treatment hasbeen carried out with removable appliances thathave allowed the posterior teeth to tilt (Figure4.3d and e). Contact may also occur in cases ofposterior crossbite where the lower teeth areplaced buccally to the upper teeth.

Occlusal interferences and occlusalharmony

Occlusal interferences

An occlusal interference may be defined either asa contact between teeth in one of the excursionsof the mandible so that the free sliding movementof the mandible is interrupted or uneven, or asthe guidance of the mandible being carried onteeth that are unsuitable for the purposes. Inmany cases occlusal interferences develop sometime after the eruption of the permanent denti-tion and are the result of dental treatment.

Figure 4.4 illustrates an alteration to themovement between ICP and RCP resulting from


Figure 4.4

Since the lower third molar has been extracted, theupper third molar has over-erupted, changing the RCP.Previously the mandible could slide smoothly back fromICP to RCP, but it now has to make a detour to thenew RCP to circumnavigate the mesial surface of theupper third molar. This is an occlusal interference. Thispatient presented with pain diagnosed as mandibulardysfunction, which disappeared once the upper thirdmolar had been extracted. Caries, although present inthe mesial surface of the upper third molar well belowthe original contact point, was not the cause of thepain.

the over-eruption of a tooth. This constitutes anocclusal interference in this excursion. Figure 4.3dand e illustrates an interference in lateral excur-sion.

These interferences are often difficult to detectbecause the sensory mechanism within theperiodontal membranes of the teeth involveddetects the interference and triggers a condi-tioned pattern of mandibular movement to avoid it.This accounts for the difficulty many patients havein permitting their mandible to be manoeuvredinto the RCP and also the difficulty they have involuntarily making lateral excursions with theteeth together.

An interference in the intercuspal positionresulting from a ‘high’ restoration involving theocclusal surface will be readily detected by thepatient, who will usually comment on an occlusalchange as soon as the restoration is inserted.These instant, entirely artificial interferences areobviously easier to deal with than occlusal inter-ferences in the various excursions of the mandible– which may be artificial but can also developslowly and naturally following extractions, toothmovements, occlusal wear and over-eruption.

Interferences should be suspected if the patienthas difficulty in making voluntary protrusive andlateral excursions with the teeth in contact orthere is difficulty in manoeuvring the mandibleinto a reproducible RCP. Interferences can alsobe detected by the dentist resting a finger gentlyunder the patient’s chin while the various excur-sions are performed. Irregular movements, whichthe finger will feel, indicate interferences thatneed fuller investigation.

Occlusal harmony

Occlusal harmony is the absence of occlusal inter-ferences. A harmonious occlusion allows smoothcomprehensive movements of the mandible in allexcursions with the teeth together without strainor discomfort. The movements will not causeharmful effects to the teeth (for example toothmobility, fractured cusps or excessive wear). Aharmoniously functioning occlusion will usuallyalso involve fairly shallow angles of movement inthe guidance from ICP in all four directions.

Patients who do not have free slidingmovements may not have symptoms. They have

adapted to their occlusal interferences and thereis no need for treatment. However, if the occlu-sion is altered to produce new and differentocclusal interferences (for example by unsatisfac-tory crowns or bridges), the patient’s neuromus-cular mechanism may well experience difficulty inadapting to these, resulting in damage to therestorations or teeth, or a dysfunctional disorderleading to temporomandibular joint pain, musclepain and spasm, or postural and functionalproblems. These disorders arising from occlusaldisharmony are described later and are more fullydescribed in specialist textbooks.

‘Premature contact’

The term ‘premature contact’ should not be usedin relation to the natural dentition – only withcomplete dentures. With complete denturesthere is no natural ICP, and ICP and RCP aremade to coincide, i.e. the artificial teeth interdigi-tate on the retruded path of closure. The patientlearns, subconsciously, to close on this repro-ducible path of closure and closes into intercus-pation. When, as a result of inaccurate occlusalrecords, the ICP and RCP of the completedentures do not coincide, the patient may closeon the retruded path of closure, and then mayslide into maximum intercuspation or alternativelythe dentures may move. This is known as ‘prema-ture contact’ and is clearly unsatisfactory. Theartificial teeth do not have a periodontal proprio-ceptive system, and so the position of an artificialICP cannot readily be detected. With a naturaldentition the ICP is well recognised by the neuro-muscular mechanism and the mandible closesdirectly into ICP in the great majority of involun-tary closing movements. It may not do so in theartificial environment of the dental chair when themandible is brought under voluntary rather thaninvoluntary control. In these circumstances, evenif the first contact appears to be a prematurecontact, this should not be assumed to be thenormal pattern of closure, but only the result ofthe patient concentrating on a movement that isusually entirely automatic.

For these reasons the term occlusal interfer-ence as defined and described above should beused for the dentate patient and not ‘prematurecontact’.


Occlusal stability (not the same asharmony)

A stable occlusion is one in which over-eruption,tilting and drifting of teeth cannot occur andtherefore cause new occlusal interferences. Foran occlusion to be stable, there must also be suffi-cient posterior contacts to prevent a generalcollapse of the posterior occlusion resulting in aloss of occlusal vertical dimension. Figure 4.5shows a disordered but stable occlusion, a disor-dered and unstable occlusion, and an occlusionthat has lost posterior support to the pointwhere collapse and loss of occlusal verticaldimension has occurred.

In a stable occlusion all the teeth should haveocclusal contact with either another tooth or aprosthesis (occlusal stops). Mesial drifting shouldbe prevented by the presence of contact points,either with other teeth or a prosthesis, or byadequate cuspal locking with the opposing teethin intercuspal position.

Not all partially edentulous occlusions areunstable. For example, if all the molar teeth areextracted, the remaining teeth may still be instable occlusion. Therefore extracted teeth arenot always replaced (see Chapter 7). In any case,a degree of instability is sometimes acceptable.

Occlusal vertical dimension (OVD)

The occlusal vertical dimension is the relationshipbetween the mandible and the maxilla with theteeth in ICP, that is, the face height with the teethin occlusion. It is usual to measure the differencebetween rest position and ICP (the freewayspace) to give an indication as to whether theOVD is within the normal range. However, restposition is difficult to measure with any precision,particularly in dentate patients, and the normalfreeway space may be 2–5 mm or more. OVD istherefore judged as much as it is measured by thepatient’s general facial appearance with the teethtogether and apart. In most cases requiringcrowns or bridges the OVD is satisfactory. Insome it has been reduced by the extraction ofteeth, by tilting, drifting and collapse of the pos-terior occlusion or by rapid wear of the teeth(Figures 4.5c and d and 5.5a). In these it is neces-sary to restore the original occlusal level for both

aesthetic and technical reasons. In other casesgradual wear of the dentition has resulted in veryshort teeth so that making aesthetic and reten-tive crowns is a problem, yet there is no loss offacial height because the tooth wear has beencompensated by over-eruption. In these cases adecision must be made between:

• Accepting that the natural or artificial crownswill have a short appearance

• Creating interocclusal space between the teethto be crowned without altering the otherocclusal relationships

• Artificially increasing the OVD by restoring orreplacing all the occlusal surfaces in one orboth jaws

• Artificially lengthening the clinical crowns bygingival surgery and sometimes alveolar surgery(crown lengthening)

• A combination of these approaches.

When a change in OVD is planned, whether ornot it is to restore lost facial height, it is usual toassess the tolerance of the patient’s neuromus-cular mechanism to the change. A removableacrylic plate covering all the occlusal surfaces ofone arch and increasing the OVD by at least thesame amount as is proposed for the final restora-tions may be fitted. Alternatively the teeth maybe temporarily built up with composite, amalgamor temporary crowns. The temporary adjustmentto the OVD should be left for several weeks toensure that problems do not arise with theneuromuscular mechanism or the teeth beforethe change is made permanent (Figure 5.6, page116).

Creating interocclusal space for teethto be crowned

In certain circumstances, particularly extremewear of anterior teeth, it is helpful to carry outminor orthodontic treatment to enable crownpreparations to be carried out without furtherpreparation of the worn surfaces. Often the wearhas been sufficiently slow that over-eruption haskept pace with it, carrying the gingival marginalong with the over-eruption. Conventionalorthodontic treatment can be used, but a simple,reliable and rapid technique is to use either a



Figure 4.5

Occlusal stability.

a A disordered but stable occlusion. Several teeth aremissing and there have been a number of toothmovements, some producing aesthetic problems andpotential occlusal interferences. However, the occlu-sion is now stable and study casts taken 5 years beforethis photograph show that no change has occurred inthat period. There are no symptoms of mandibulardysfunction and no other complaints by the patient,even of the appearance of the missing upper teeth.Treatment of this occlusion is therefore not justified.

b There have been recent extractions in the upperarch and undesirable tooth movements, including over-eruption of the lower teeth, can be anticipated.

c and d A ‘collapsed bite’ with loss of a number ofposterior teeth, periodontal disease, drifting upperincisors, an increase in overbite and a reduction inocclusal vertical dimension.

removable or fixed appliance commonly known asa ‘Dahl’ appliance. Dahl originally described aremovable anterior biteplane made of cast cobalt-chromium but modern adhesive technology hasenabled a simpler fixed appliance to be made thatis well tolerated by most patients and that usuallyachieves the required result in about 3 months.Figure 4.6 shows the appliance in use.

Alternatively the same effect can be producedby building up the teeth with composite or bypreparing the teeth and fitting acrylic provisionalcrowns, made in the laboratory, which are highin the occlusion.

Temporomandibular dysfunction(TMD)

Many terms are used to describe this condition,for example mandibular dysfunction, cranio-mandibular dysfunction, temporomandibular jointdysfunction, myofascial pain dysfunctionsyndrome, muscle hyperactivity disorder andothers. This illustrates the fact that the conditionis poorly understood and that there are manysuggested explanations for it. Some explanationsblame the joint itself for the symptoms, some themuscles of mastication and their control systems,and some the occlusion, which in turn affects thecontrol system and again in turn the muscles andthe joints. Some clinicians believe that thesymptoms arise mainly or entirely from psycho-logical stress and anxiety.

The least pejorative term is therefore‘temporomandibular dysfunction (TMD)’, which issimply used to label a common combination ofsymptoms often including tenderness, pain andtension in the muscles of mastication and pain,clicking and limitation of movements of thetemporomandibular joints.

In many cases the symptoms resolve sponta-neously with or without treatment. The reportedincidence is higher in young adult female patientsthan in other groups. These two facts suggest thatthe condition is more commonly of functional andpsychogenic origin than it is to do with irrevers-ible physical changes in the joints themselves.

Changes do occur in the joints, and these canbe demonstrated by conventional radiographictechniques designed for the purpose or bymagnetic resonance imaging (MRI), which does

not involve ionizing radiation or physical invasionof the joint. When MRI is available (it is a veryexpensive technique) it is the best way to inves-tigate the possibility of internal joint derange-ments or pathology. Confusingly, MRI surveys ofnormal patients with no symptoms of TMD haveshown that a significant proportion of them havedisplaced discs within the temporomandibularjoints, which, when such displacements werediscovered by the previous, invasive techniques(which could only ethically be used on patientswith symptoms) were considered to be the causeof the symptoms. Undoubtedly some of theseinternal joint derangements, or more frankpathology of the joints, do cause symptomssimilar to those arising from the purely functionaldisorder: TMD.

There are clearly cases in which the cause ofthe symptoms is dysfunction, others in whichthere is some organic, physical explanation andmany where the cause is less clear. Sadly somedentists align themselves with one or other of therather narrow and exclusive regimes for themanagement of mandibular dysfunction. This isunfortunate and unscientific. With differentschools of thought about the aetiology andmanagement of mandibular dysfunction, eachsupported by some, but incomplete, researchevidence, the sensible dentist will keep an openmind. However, some lines of treatment aremore interventive than others and so it is wiserto take a conservative approach to the manage-ment of mandibular dysfunction and assume that,in the absence of firm evidence to the contrary,most cases of mandibular dysfunction arefunctional rather than organic in nature.

An attractive hypothesis is that occlusal inter-ferences (described on page 83) produce condi-tioned patterns of muscle activity that avoid theseinterferences. This increases the basic level ofmuscle activity, which, when it is furtherincreased by anxiety or stress, brings the level ofmuscle tension above a threshold and symptomsdevelop. Therefore treatment aimed at removingthe occlusal interferences is aimed at the causeof the problem rather than the symptoms.Similarly, treatment aimed at reducing anxiety andstress is also aimed at the cause, but this shouldbe limited to sympathy and explanation of thecause together with a caring approach to treat-ment rather than, in the hands of the generaldental practitioner, the use of drugs.



Figure 4.6

a and b Palatal erosion affecting the upper incisorteeth. The lower teeth have over-erupted and aremaking contact with the worn palatal surface.

c The posterior teeth in occlusion.

d The cast Dahl appliances. They are left separate atthe midline because of the diastema.

e The Dahl appliances in place propping the occlusionopen on the anterior teeth.

Occlusal interferences are not always easy todetect clinically because of the set of conditionedreflexes that avoid contact on the occlusal inter-ference. A simple way to detect whether alterationof the occlusion is likely to reduce the symptomsof mandibular dysfunction is to provide a hard

acrylic biteplane covering all the surfaces of onejaw (sometimes called a Michigan splint when it ismade for the upper jaw, and a Tanner appliancewhen in the lower jaw). If the symptoms improveafter a few weeks of wearing the appliance atnights (or all day if it is tolerated) then this is a


f The appliances have worked in depressing the lowerincisors and the four upper incisor teeth have beencrowned without any further reduction of their palatalsurfaces.

g Another typical Dahl appliance, this time in one pieceand with a more pronounced shelf palatally to produceaxial tooth movement.

h Reflux erosion with considerable loss of anteriorOVD.

i Directly placed composite to give a Dahl effect. Thishas the advantage that it can be adjusted while it isworking but it needs more maintenance than a castappliance.

clear indication that the occlusion has somethingto do with the symptoms and justifies the expen-diture of further time and effort on identifying anddealing with the occlusal interferences.

The acrylic biteplane should be used in thisdiagnostic way rather than as a long-term treat-ment of the condition. However, some patients,despite advice given to them, continue to wearthe appliance because it has reduced theirsymptoms, and for this reason biteplanes makingcontact with only a limited number of anterior orposterior teeth should not be used. If they are,they will act as orthodontic appliances andproduce unplanned and damaging depression orover-eruption of teeth.

The treatment of occlusal interferences in themanagement of mandibular dysfunction is usuallyfairly simple once the interference has beenidentified. It usually involves occlusal ‘adjustment’by grinding selected parts of the occlusal surfaces.When this only involves one or two clinicallyobvious interferences, it can be done directly inthe mouth. However, if the adjustment needs tobe more extensive it is best carried out first onarticulated study casts set up in a semi-adjustablearticulator, a mock adjustment, to see the effectsof the adjustment before irreversible changes aremade to the teeth.

This is not the same as occlusal ‘equilibration’,which is the recontouring of the entire occlusionto fit some preconceived, idealized concept ofwhat the occlusion should be. Similarly the treat-ment of mandibular dysfunction only very seldomjustifies the construction of multiple crowns orbridges. Crowns and bridges may be necessaryfor other reasons, and if the patient has mandibu-lar dysfunction then this will complicate the treat-ment and definitive restorations should not beprovided until the symptoms have resolved.

Further detailed management of mandibulardysfunction is beyond the scope of this book, andthe remainder of this chapter deals with practicalaspects of dealing with the occlusion in a patientwithout symptoms of mandibular dysfunction.

Examination and analysis of theocclusion

In most cases it is sufficient to examine the occlu-sion clinically, but in more extensive occlusal

reconstructions or where there are conditionedpatterns of movement preventing clinical exami-nation, study casts should be articulated. Providedthat the clinician understands what he is lookingfor, there is no need to articulate study casts forthe majority of individual crowns and smallbridges at this stage.

Clinical examination of the occlusion

The following points should be noted:

• Any complaints the patient may have oftemporomandibular joint pain, muscle spasm orunexplained chronic dental pain

• The ease or difficulty with which the variousexcursions can be made voluntarily by thepatient

• Any occlusal interferences and whether theproposed restorations will influence these

• Mobility of teeth during excursions of themandible with the teeth in contact

• The presence, angle and smoothness of anyslide from RCP to ICP

• The type of lateral guidance and particularly thedegree of contact in lateral excursion of anyteeth that are to be restored, or the likelydegree of contact for any teeth to be replaced

• The presence of any contact on the non-working side

• The location, extent and cause of any facetingof the teeth to be restored or signs of exces-sive wear to restorations or teeth

• The degree of stability of the occlusion andwhether the proposed restorations will influ-ence stability

• Over-erupted and tilted teeth, particularly ifthey are the teeth to be restored or if theyoppose the teeth to be restored.

Clinical aids

Articulating paper or foil

Flexible articulating paper or plastic foil ofdifferent colours may be used to mark occlusalcontacts in different excursions. For example,


the ICP may be recorded in one colour and theRCP in a second colour (Figure 4.7).Articulating paper is rather difficult to use,having a tendency to mark the tips of cuspswhether or not they are in occlusion, and oftenit does not register contacts on polished goldand glazed porcelain. The thickness of the paperwill have an influence on the degree of markingthat occurs and ideally it should be as thin aspossible. Figure 4.8 shows a selection ofmarking materials. The teeth are very sensitiveto the thickness of material between them andcan easily detect the differences between thematerials shown in Figure 4.8.

Wax

Thin, fairly soft wax with an adhesive on one sideis marketed as a material for registering occlusalcontacts. This is useful but rather expensive. Abetter alternative is to use 0.5 mm thick, dark-coloured sheet wax. Occlusal registrations in thismaterial are shown in Figure 4.3. It has the advan-tage that it can be removed from the teeth andplaced over the study casts for the occlusalcontacts to be studied more closely. It can alsobe used in full arch-sized pieces. Areas of contactin the mouth may be marked through the per-forations with a chinagraph pencil.


Figure 4.7

a and b Occlusal contacts marked in two differentcolours of foil. The black marks on the marginal ridgesof the upper premolars, a, and on the buccal cusp ofthe lower first premolar, b, were made with the patientin ICP. Movement to RCP produced the contactsmarked in red.

Figure 4.8

A selection of articulating papers used to mark occlusalcontacts. The red paper is held in Millers forceps.Shimstock in the lower left corner is used to verify thatcontacts are present as it will be held where contactsexist.

Occlusal registration silicones

Fast setting silicone rubber materials can besyringed between teeth and the occlusal contactsrecorded. These materials are soft initially so offer

no resistance to the closure of the mandible,which can be a problem with more viscous materi-als such as wax if not softened properly. The resis-tance felt on biting into a more viscous materialmay guide the mandible into a different position.


Figure 4.9

a Plaster casts with a silicone occlusal record used torelate the casts together. The excess of materialobscures the relationship between the casts.

b The occlusal record is trimmed so that just thematerial on the occlusal surface remains.

c The casts can now be brought together and thecorrect relation seen and verified.

Once set the silicone material is flexible but suffi-ciently rigid to be used as an accurate interocclusalrecord to articulate casts. It can be placed on themodel to show, by means of the perforations,which parts of the occlusal surface are makingcontact. Because it can be replaced onto themodel or the patient’s teeth without damage it isa better material than wax (Figure 4.9).

Plastic strips

Plastic strips may be used to test whether teethare making contact in various excursions. Thethinnest of these materials (shimstock) is opaqueand silver-coloured and is only 8 micrometres(µm) thick. The strip is placed between opposingteeth and pulled aside once occlusal contact hasbeen made. Often two pieces are used onopposite sides of the jaw to test the symmetry ofthe occlusion, or between the crowned tooth andits opponent, and the adjacent tooth and itsopponent to test that the crown is in contact butis not ‘high’.

Less accurate (40 µm thick) but more manage-able mylar matrix strips, used for compositerestorations, are sometimes an acceptablealternative.

Study casts

Unarticulated study casts are useful for assessingthe stability of the occlusion in ICP and forexamining wear facets, which are often easier tosee on the cast than in the mouth. They are oflittle value in assessing contacts in the excursionsof the mandible. It is important that the casts area good quality with no air bubbles or blips on theocclusal surfaces of the teeth and that they aretrimmed distally to allow the teeth to come intocontact. If carefully used, alginate impressions maybe adequate but alternatively the casts should beproduced from conventional silicone or polyetherimpression materials used for impressions of thecrown preparations.

Articulated study casts

When sufficient information cannot be obtainedby clinical examination or examination of hand-

held study casts, it is unlikely that study castsmounted on a simple hinge articulator will giveadequate additional information; a semi-adjustableor fully adjustable articulator is necessary.

Figure 4.10 shows a set of study casts beingmounted on a semi-adjustable articulator. Forregistration of the occlusion the following arerequired:

• A facebow record which records the relation-ship of the maxillary teeth to the terminal hingeaxis (THA) in three dimensions.

• A record of the RCP in one of the materialsdescribed more fully in Chapter 6.

• Sometimes an RCP record is not needed andan ICP record is sufficient, or the interdigita-tion of the casts is clear and stable and nointerocclusal record is needed.

• Protrusive excursion record.• Lateral excursion records.

The semi-adjustable articulator has a number oflimitations and produces only an approximationto the tooth movements in the mouth. However,for most purposes it is quite sufficient.

Occlusal adjustments prior to toothpreparation

Once the occlusion has been assessed, adjustmentprior to tooth preparation must be considered.This may be necessary in cases where the teethopposing a proposed bridge have over-erupted orwhere the occlusal plane is going to be altered bymeans of crowns. If the tooth to be prepared hasan occlusal interference it should be adjustedprior to starting the preparation to ensure thatthere is sufficient enamel and dentine remainingfor the final preparation to be completed withoutendangering the pulp. Adequate tooth preparationfor the proper thickness of occlusal crownmaterial is necessary, otherwise there is a riskthat the interference will be reproduced.Sometimes the incisal plane of the lower incisorsis adjusted and levelled out before making upperincisor crowns.

Occlusal adjustment is also indicated in manycases of mandibular dysfunction (see earlier).

There is no justification for prophylactic adjust-ments unless there is evidence of damage or



Figure 4.10

Articulating casts on a semi-adjustable ‘arcon’ articula-tor, in this case a Denar articulator.

a A facebow being used to relate the maxillary teethto an approximation of the terminal hinge axis (THA).The anterior part of the facebow carries a bitefork withan impression seated on the upper teeth. The posteriorpart has earpieces which fit into the ears. An assump-tion is made that the THA is an average distance infront of the external auditory meatus.

b The facebow record removed from the mouth. Thepointer to the side of the nose orientates the facebowto the FM plane.

c The upper cast is placed into the bitefork record andthe jig placed and supported in the articulator. Theundersurface of the cast has been indexed with notchesso that it can be removed later and repositioned in thesame place.


d and e Quick-set, low expansion plaster is placed onthe upper cast and the upper arm of the articulator isbrought down so that when set, the upper cast issecured in place in the correct relationship to the axispoints.

f and g The articulator in inverted and the lower castsecured to the upper using the interocclusal, ICP orRCP silicone record and sticky wax. The lower cast isthen secured to the articulator with plaster and thearticulator closed.


h and i The completed casts in the articulator. Theycan be removed as required and replaced back in thecorrect relationship due to the indexing between thecasts and the white plaster mounts.

h The casts in RCP. The condylar guidance is nowadjusted using protrusive or lateral excursion interoc-clusal records. The record is placed between the teethand the condylar guidance angle is adjusted until thecasts are fully seated in the record.

i The articulator open.

pathology arising from the occlusion; our level ofunderstanding of occlusal problems is not yetsufficient to warrant arbitrary prophylactic alter-ations in an established, comfortable, functioningocclusion.

Occlusal objectives in makingcrowns and bridges

There are two main objectives:

• To leave the occlusion with no additionalocclusal interferences, i.e. harmonious

• To leave the occlusion stable.

In addition, there may be secondary objectives,for example:

• To distribute the guidance in one of the excur-sions more evenly between a number of teeth,for example, by modifying the anterior guidanceso that a number of anterior teeth share theocclusal forces in protrusive excursion.

• When a canine tooth that previously guided theocclusion is extracted, lateral forces should bedistributed as evenly and as widely as possiblebetween the remaining posterior teeth.

These latter objectives may be described asocclusal engineering, planned to produce occlusalrelationships that achieve the first two majorobjectives of occlusal harmony and stability.

Most crowns and small bridges are made inmouths with an established RCP and ICP. Theseshould be left unaltered by the restorations (i.e.a ‘conformative’ approach) unless:

• So many of the occluding surfaces are beingrestored that ICP will inevitably be altered

• ICP is unsatisfactory for some reason• OVD is being altered, or• There are symptoms of TMD.

In all these cases the occlusion is usually restoredwith the ICP made to coincide with the RCP (i.e.a ‘reorganized’ approach). This is mostly forpractical reasons and does not imply that RCP ispreferable to an established, comfortable,functional ICP. The reorganized occlusion isestablished either at a vertical dimension which

the dentist considers is close to the original OVDor, more commonly, at an increased OVD toreplace the lost OVD and to allow sufficientinterocclusal space for the restorations to bemade.

With all major reconstructions, well madeprovisional restorations should be worn for suffi-cient time to establish that the new OVD andocclusal functional relationships are comfortable.This is usually between 3 and 6 months.

Clinical and laboratory managementof the occlusion

Avoiding loss of occlusal relationships

When sufficient occluding teeth will remain toregister the ICP and other occlusal relationshipsafter tooth preparation, there is no need to takeany precautions to record the occlusal relation-ships beforehand. However, when the occlusalsurfaces are being removed from a number ofteeth, or when one or more of these teeth arecrucial to the guidance of mandibular movements,the occlusal relationships should be registeredbefore the tooth preparations are begun.

When large numbers of posterior teeth arebeing prepared or when several teeth are missingthere is a risk of losing any record of the origi-nal OVD. A pair of opposing teeth on either sidemay be left unprepared, the remaining teethprepared and then the opposing teeth adjusted sothat the ICP is the same as the RCP. Impressionsand occlusal records are taken with these teethstabilizing the jaws during the occlusal registra-tion. They are then prepared and further impres-sions are taken. Alternatively, one pair ofopposing crowns can be made for each side ofthe arch before the other teeth are prepared andthese pairs of crowns serve the same purpose.

Maintaining occlusal relationships withtemporary restorations

Prepared teeth and their opponents will over-eruptunless occlusion is re-established by means ofadequate temporary restorations; and the preparedtooth and the teeth either side of it can drift together unless contact points are


maintained in this way. The longer the periodbetween the impression and fitting of the restora-tions, the more important are temporary restora-tions. They are probably also more important inyounger patients, where tooth movement mayoccur more quickly. For this reason, individuallymade temporary restorations in plastic arepreferred to preformed types unless the preformedtemporary restoration happens to be an excellentfit at the contact points and in the occlusion.

Recording the occlusion

A decision must be made on the type of articu-lator to use for the working casts. Once this isdone, the appropriate occlusal records will beobvious. The choice is between the following fouroptions:

Hand-held models

These are not usually satisfactory. The mostcommon problem is that posterior restorationsare made high and are not detected because it isvery difficult to see the tiny spaces between pairsof opposing teeth adjacent to the restoration.

It is unfortunately common that the carefullycontoured occlusal surface of a posterior crownhas to be adjusted at the chair side because thecrown is ‘high’. The most common reason for thisis the use of hand-held working casts. This adjust-ment is time-consuming, frustrating and damagingto the patient’s confidence in the dentist. It ismuch better to give clear instructions to thelaboratory to use an appropriate articulator. Thiswill be more time-consuming in the laboratory(which may therefore charge more) but will oftensave clinical time and patient respect (both morevaluable than the laboratory time).

Simple-hinge articulator (Figure 4.11)

This is adequate when there are sufficient unpre-pared intercuspating teeth and the restoration isto be made occluding in ICP. For example, in astraightforward single upper anterior crown, thepalatal surface can be contoured to match theadjacent palatal surfaces so that the incisal

guidance will need very little adjustment. Similarly,for a single posterior crown when the occlusionis canine-guided, it is necessary only to reproducecontact in ICP. The crown will disclude in lateraland protrusive excursions.

The major advantage of a hinge articulator overhand-held models is that if the restoration ismade high on the working cast this can be identi-fied and adjusted prior to trying the restorationin the patient’s mouth.

Sometimes there is no need for any occlusalrecord. It is often possible to place the modelstogether entirely satisfactorily in ICP. When theremay be some doubt about ICP an occlusal recordis made in wax or one of the other materialsdescribed earlier and in Chapter 6.

Semi-adjustable articulator (see Figures 4.10and 4.12)

These have the following features:

• The condyles are on the lower member of thearticulator and the condylar guidance elementon the upper membrane, i.e. an arcon design(see later)

• The intercondylar distance is variable• The maxillary cast is related to an approxima-

tion of the THA through the condyles• Condylar guidance is variable, but only in

straight lines• Some adjustment of incisal guidance is usually

possible.

When occlusal relationships are important inpositions other than ICP a semi-adjustable artic-ulator should be used. The maxillary cast ismounted using the facebow, and the mandibularcast is related to it by hand in ICP, by an ICPrecord or by a record in the RCP, whichever isappropriate. The articulator is then adjusted usingintra-occlusal records taken in either protrusiveor lateral excursions. The records taken will beselected according to the circumstances. Forexample, if crowns on the right side are beingmade in a case with group function but wherethere is no risk of non-working side contactsoccurring in left lateral excursion, only a recordof right lateral excursion is necessary.

With this arrangement a good approximationto group function should be possible, with only


minor adjustment being necessary at the chairside because of the compromises inherent withsemi-adjustable articulators.

There are two broad categories of semi-adjustable articulator – arcon and non-arcon. Thearticulator shown in Figure 4.10 is a semi-

adjustable arcon articulator, and the fullyadjustable articulator shown in Figure 4.13 is alsoan arcon design. The type shown in Figure 4.12 isa non-arcon type. That is, the balls representingthe condyles are attached to the upper memberof the articulator and the condylar guidance to the


Figure 4.11

a An example of a simple but rigid articulator that issatisfactory for most single crowns and small bridges.

b An inadequate articulator which is not sufficientlyrigid and does not maintain the occlusal relationshipsproperly so that:

c without detaching the articulator the relationship ofthe casts can be distorted. This much distortion wouldbe obvious but minor amounts would not.

lower. This is, of course, upside down withrespect to the anatomy of the joint. The articula-tor shown in Figure 4.11 is also non-arcon.

In practical terms the difference between thearcon and non-arcon designs is not particularlyrelevant, provided that the occlusal records canbe taken with the teeth in contact or nearly so.However, if the OVD is being increased or if theocclusal records have to be taken at a degree ofopening of the mandible then this affects therelationship between the condylar guidance angleand the upper member of the articulator. Itfollows that in these circumstances an arcondesign should be used.

Fully adjustable articulator

A full description of the use of these articulatorsand the additional records needed to use them isbeyond the scope of this book. Suffice it to saythat they are sometimes, but decreasingly, usedwhen more accurate and comprehensive recordsof mandibular movements are required. Their usehas been largely superseded by making long-termprovisional restorations and adjusting these in themouth over a period time. When satisfactory theprinciples, but not necessarily the exact contours,of the occlusal design are reproduced in the finalrestoration. This can be achieved with a semi-

adjustable articulator. In other words the patient’smouth is used to refine the occlusion rather thanusing an over-complicated, and still not entirelyaccurate, artificial reproduction of the patient’smandibular movements.


Figure 4.12

A ‘non-arcon’ type of semi-adjustable articulator. Thisis still sometimes used for dentures but seldom forcrowns or bridges.

Figure 4.13

A fully adjustable articulator, now of mainly historicalinterest; see text.


Figure 4.14

a Poorly trimmed casts cannot be properly related asthe heels at the back of the casts get in the way.

b Once trimmed properly the casts will cometogether.

c Plaster ‘blips’ are common if small air bubbles arepresent in the impression. This happens more withalginate impressions.

d These need to be carefully trimmed.

Laboratory stages

Trimming the casts

One of the commonest causes of restorationsbeing high when tried in the mouth is distortionof the casts, particularly the opposing casts, whichmay be made from an alginate impression.Commonly, small air bubbles trapped in theocclusal fissures will prop the models apartslightly, so that if the restoration is made to touchthe opposing model, it will be high in the mouth.Impression techniques for crowns and bridgesshould concentrate on the crowns of the teeth,injecting impression material into the occlusalfissures or rubbing alginate into them with thefingers. If air bubbles do occur, great care shouldbe taken to trim occlusal defects from themodels, and if individual teeth are suspect, theyshould be cut right away from the model unlessthey are opposing or adjacent to the teeth beingrestored (Figure 4.14).

Articulating the casts

As small an amount of plaster as possible shouldbe used since the expansion of the plasterdistorts the relationship of the articulator withthe casts. Impression plaster or plaster containingan anti-expansion agent should be used; alterna-tively, there are plasterless designs of articulator.

Shaping the occlusal surfaces

The technique of shaping the occlusal surface willdepend upon whether the surface is to be metalor porcelain.

Wax carving With this technique wax is builtup to excess on the occlusal surface and thencarved to the required occlusal contour. Smallincrements of wax are added when necessary torepair over-carving. When completed, occlusalcontact should be checked using shimstock, bothbetween the carved tooth and its opponent andbetween adjacent teeth and their opponents(Figure 4.15).

The wax-added technique Small increments ofmolten wax are flowed from the tip of an instru-

ment to build up cones, each one forming the tipof a cusp. The other features of the occlusalsurface are then added, often with differentcoloured waxes to identify each feature. Usingthis technique, the occlusal relationships in allexcursions can be checked from the beginningand adjusted as the process continues (Figure4.15).

Occlusal shaping with porcelain Porcelainsurfaces are built up slightly to excess and thenground to shape, stained and glazed. Againshimstock or similar material is used to check theocclusal relationships in the articulator.

Adjusting the occlusion ofrestorations in the mouth beforecementation

Occlusal marking materials

Articulating paper and foil and 0.5 mm thickdarkly coloured wax have already been described.An alternative technique for metal occlusalsurfaces is to grit-blast them lightly with a mildabrasive which gives the surface a matt appear-ance. Burnish marks will then appear in areas ofcontact with the opposing teeth and the articu-lating papers will mark more clearly. For porce-lain occlusal surfaces, especially with complexocclusions or multiple crowns, it is often best toadjust the occlusion when the porcelain is in abiscuit bake stage prior to the final glaze.

Adjusting in intercuspal position

A patient who does not have a local anaestheticwill be immediately conscious of a high restora-tion in ICP. Even with a local anaesthetic theopposing teeth will normally sense a high restora-tion. The patient will not of course be aware of arestoration that is short of the occlusion, and soocclusal contact should be checked withshimstock or mylar matrix strip. If occlusal contactis not present (i.e. the restoration is not occlusallystable), the tooth or its opponent will over-eruptand occlusal interferences may be introduced.

High restorations should usually be adjusted.However, occasionally a high posterior crown (or


small bridge) may be planned in advance in orderto produce tooth movements similar to toothmovements produced by a Dahl appliance in theanterior part of the mouth. This may be done, forexample, when the tooth to be crowned is veryshort and little or no preparation of the occlusalsurface is desirable. This tooth movement (aswith a Dahl appliance) is preferably achieved withprovisional restorations and the patient warned inadvance. Patients readily accept this as part ofplanned treatment but, as stated earlier, resent itwhen it is unplanned and appears to be a mistake.

Adjustments in lateral, protrusive andretrusive excursions

The occlusion of the restoration is examined forinterferences in these excursions of the mandibleand adjusted if necessary. A simple guide to thepresence of possible interferences is whether thecrown is displaced during lateral and protrusivemandibular movement with the teeth in contact.

Stability

Following these adjustments, a final check of thestability of the occlusion is made by confirmationof the presence of centric stops on the restora-tion and the adjacent teeth. The adequacy of thecontact points is checked with dental floss.

Adjustment techniques

ICP is adjusted first and the centric stops markedwith articulating foil, paper or wax. Interferencesare marked with a different colour and adjusted.

Metal and porcelain can be adjusted withmounted stones or diamond burs. Metal can befinished with finishing burs and polished withmounted rubber wheels or points. Porcelain canbe finished with the mounted points or discs usedto finish composite and with specially producedsets of instruments. With these instruments, thefinished surface is as smooth as the glazed surface,and re-glazing is unnecessary (see Figure 14.11).


Figure 4.15

a and b Two crowns waxed up on the lower secondpremolar and first molar teeth on working casts artic-ulated in a simple but robust articulator. The lowersecond premolar has been waxed using a carvingtechnique, starting with an excess of wax on theocclusal surface. The first molar has been carved by a‘wax-added’ technique, cones of wax being built up tothe required contact with the opposing teeth and thegaps between filled in with a different-coloured wax.Shimstock is being used to check that contact justoccurs between the unprepared second molar teeth,and will be used between the wax patterns and theopposing teeth and the unprepared second premolars.

There is a natural sequence by which the historyand examination of the patient leads to a decisionon the advisability or otherwise of crowns in thecontext of the overall treatment. This generaldecision leads to a further series of stages in thedetailed planning of treatment. This sequence is:

History and examination:

• Of the whole patient• Of the mouth in general• Of the individual tooth

Decisions to be made:

• Keep the tooth or extract• If the tooth is to be kept – crown or other

restoration• If the tooth is to be crowned – preparatory

treatment necessary

Detailed planning of the crown:

• Appearance• The remaining structure of the tooth and its

environment, including any necessary core• Choice of type of crown, including material• Detailed design of the preparation

Planning and executing the clinical andlaboratory stages:

• Appointment sequence – agreement withpatient and laboratory, including agreements onfees and laboratory charges

• First clinical stage• Laboratory stage• Second clinical stage• Maintenance.

It would be very nice if life were as simple as this.It is convenient to have such a sequence of eventsin mind but it is not often possible to follow thepattern precisely. For example, if endodontictreatment is necessary as part of the preparatorytreatment then a temporary crown may well have

to be made at an early stage before the prepara-tion can be finally planned. This outline sequencemay have various repeat loops arising within it.The dentist must be prepared to rethink theoptions as new circumstances arise and allow fullfreedom to his or her professional judgement.

History and examination

Considering the whole patient

Patient attitude and informed consent

Complex and time-consuming procedures such ascrowns should not be contemplated unless thepatient is enthusiastic and cooperative about thetreatment. There is always some other way oftreating the tooth, even if it means extracting it.The patient’s attitude is particularly importantwhen crowns are being considered for purelycosmetic reasons. The dentist must be satisfiedthat the patient fully understands the limitationsof what can be achieved. Techniques for demon-strating cosmetic changes to patients before theteeth are prepared are described later.

If these explanations and demonstrations arenot given adequately then the patient cannot giveproperly informed consent – a very importantlegal concept.

Patients generally appreciate having the reasonsfor treatment explained to them together withsome of the details of treatment. A commonsource of dento-legal problems is the patient whoclaims an inadequate understanding of what wasbeing proposed and that had it been fully under-stood, he or she would not have gone ahead withthe treatment. Again this applies particularly totreatment provided mainly for cosmetic reasons.

Prior to starting any extensive treatment it isprudent to confirm to the patient in writing

5 Planning andmaking crowns

105

exactly what treatment is proposed, what com-promises may be necessary, what the alternativesare and what the costs will be. Copies of thiscorrespondence should be kept. This will help toreduce future problems if the patient is unhappywith the result and is time well spent.

Age

There is no upper age limit for crowns providedthat the patient is fit enough to undergo thetreatment and is in other ways suitable forcrowns. There are some practical problems inextensive treatment for elderly patients; forexample, the teeth tend to become more brittlewith age and this affects the design of crownpreparations. Also some older patients developphysical problems such as rheumatoid arthritis intheir hands or medical problems requiring treat-ment with drugs which reduce salivary flow andpredispose them to caries at the crown marginseven if this has not been a problem when theywere younger. Both these examples, and others,affect the patient’s ability to maintain the standardof oral hygiene which is necessary if crowns areto succeed for a useful period of time.

Neither is there a lower age limit for crowns.It is unusual to make crowns for teeth shortlyafter they have erupted, and crowns arecommonly delayed until the patient is 16 or so.However, this decision has traditionally beenbased upon three main factors:

• The size of the pulp• The degree of eruption of the tooth• The cooperativeness of the patients.

These will vary considerably among patients. Forexample, an upper incisor tooth that is fracturedat the age of 7 or 8 and restored with compos-ite, will usually develop extensive secondarydentine so that the pulp will be smaller by theage of 10 than the pulp of an undamaged toothat the age of 16. In patients with good oralhygiene, the position of the gingival margin of theincisor does not alter much after this age, andtoday’s children are far less anxious about dentaltreatment than they were a generation ago. In acase like this, therefore, there may be nocontraindication to providing a permanent crownat the age of 10.

Similarly, when a successful root canal treatmenthas been carried out so that there is no need toworry about the pulp, post-crowns can be madefor children in their early teens and even younger.

Even when the pulp has not been damaged oraffected by secondary dentine, there is nowevidence that the size of the pulp does not varysignificantly with age in the great majority ofyoung patients. The ratio of the size of the pulpto the size of the tooth is very varied, andcertainly the pulp does not suddenly shrivel to asignificantly smaller size on the patient’s 16thbirthday or at any other age. It is much moreimportant to assess pulp size from a good, clearperiapical radiograph than it is to adopt anarbitrary rule about the age at which teeth canbe prepared for crowns.

Of course, there are far fewer indications foranterior crowns in young patients than there werea few years ago, with the introduction of a varietyof new ways to restore anterior teeth and makebridges, as described elsewhere in this book.

Figure 5.1 shows bridges being made for a 13-year-old boy and a set of full mouth crowns fora girl of 12 with severe dentinogenesis imperfecta.Both patients were particularly cooperative andenthusiastic about treatment. Figure 5.1c showsthe crowns for the second patient still in place atage 30.

It is argued that boisterous children and sportsplayers who suffer damage to their teeth shouldnot have the teeth permanently restored untilthey are over this energetic period. However,many of them continue to play vigorous contactsports well into their twenties or thirties or laterand, if crowns are indicated, it is quite unaccept-able that patients should be deprived of themuntil they have become docile and sedentary. It isvery much better to provide the crowns, andwith them a mouth protector, not only for thecrowns but also, more importantly, for theremaining natural teeth.

Sex

Many male patients are just as concerned withtheir appearance as females. They may, however,be less willing to admit to this. It is more impor-tant to determine patients’ real attitudes to theirappearance than to make assumptions based upontheir gender.

106 Planning and making crowns

Social history

When extensive treatment is planned, it is impor-tant to establish that the patient will be availablefor appointments of sufficient length andfrequency to complete the treatment. Crownsshould not be started just before a patient is dueto sit important examinations, or just beforetravelling abroad. People who plan to marryusually like to have their crowns completed intime for the wedding photographs.

The patient’s occupation may be important.Wind instrument players, for example, are partic-ularly anxious to retain their incisor teeth in

order to support their embouchure (the particu-lar contraction of the lips needed to form thecontact with the mouthpiece).

Habits such as pipe smoking, where the stemof the pipe is clenched between the teeth, mayaffect the design or type of crown selected.

Cost

There is no satisfactory way of mass producingcrowns, and so they will always be labour-inten-sive and therefore costly. Whichever way thecost is borne – by the patient, or by a private or

Planning and making crowns 107

Figure 5.1

Crowns and bridges for young patients.

a Bridge preparations for a patient aged 13 witholigodontia (a combination of hypodontia – congenitallymissing teeth – and microdontia – small, malformedteeth). The teeth prepared are the canines and secondpremolars: the diminutive and unattractive canines arein the position of the missing lateral incisors. At thisstage, minimal preparations are carried out and metal-acrylic provisional restorations placed for a period of6 months to a year. This encourages secondary dentineformation so that the definitive preparations can bemade with less risk to the pulp (see the text for anexplanation of the poor gingival condition).

b Threaded-pin retention for composite cores in apatient aged 12 with dentinogenesis imperfecta. In thiscondition the pulps recede rapidly, and this procedurewas carried out without the need for local analgesia.

c These are the crowns made for the patient in b 18years after they were fitted. The porcelain at the incisaledge of the lower left central incisor has chipped andthere has been some gingival recession and repairs withglass ionomer cement, otherwise the pin-retainedcomposite cores have been successful and none of thecrowns has been lost. The teeth were eventuallyextracted 26 years after these crowns were fitted andreplaced by implant-retained restorations.

public insurance scheme – the cost is importantand must be taken into account in any treatmentplan. Because crowns are expensive, they shouldnot be made unless they will really contributesignificantly to the patient’s well-being and can beexpected to last for a reasonable period of time.

Considering the whole mouth

Oral hygiene

There is obviously no point in embarking upon acomplex course of treatment involving crowns(or bridges) in a mouth with rapidly progressingcaries or periodontal disease resulting from poororal hygiene. The first priority must be to arrestthe disease process and improve the oral hygiene.

That being said, however, it is impossible forany mouth to be kept absolutely plaque-free. It isalmost always possible to find some in the mouthof even the most meticulous patient. Most, despitegood intentions, achieve only a moderately goodlevel of plaque control. The problem for thedentist is therefore one of degree. He or she mustdecide whether the patient, after instruction inoral hygiene, can achieve a level of oral cleanlinessthat warrants treatment which is time-consumingand costly. It is also necessary to decide how totreat those patients who are assessed as having alevel of oral hygiene falling below this standard butwho nevertheless have teeth that can only betreated satisfactorily by means of crowns.

There is no simple guidance on these difficultdecisions. Perhaps the best advice is to assess notonly the level of oral hygiene, but the effect thatthis is having on periodontal disease and caries.Yet there is no single direct relationship betweenoral hygiene and disease – many other factorsinfluence the disease processes. The decisionwhether to crown a tooth or not should there-fore be made on an assessment of the prognosisof the tooth without the crown or with it. If, inan otherwise-intact arch, a single badly broken-down anterior tooth is ugly, does not functionwell and is difficult to restore by any means otherthan a crown then, provided that the prognosisof the alveolar support is such that the tooth isnot likely to be lost for at least a few years, it isalmost certainly better to make a crown than toextract it and provide a partial denture, even if

the oral hygiene is poor and cannot be improved.It would be quite wrong not to offer any form oftreatment, and morally dubious to attempt toblackmail the patient to maintain a betterstandard of oral hygiene by refusing the crownunless the oral hygiene improves. In any case, thiscrude psychological approach seldom produces apermanent improvement in oral hygiene.

Although every effort should be made by bothdentist and patient to improve oral hygiene whenit is poor, there are those who are simply notable to improve, but who are nevertheless fortu-nate in having a slow rate of progress ofperiodontal disease and a low caries incidence,and for these patients crowns are often justified.

Figure 5.1a shows a typical 13-year-old boywho has entered puberty. This hormonal changeaffects the gingival response to plaque, but it also– as many parents of teenagers know –sometimes leads to expressions of independenceand even rebellion. This may show as lapses incleanliness, including oral hygiene. Fortunatelymost recover. This patient had several missing ormisshapen teeth through no fault of his own. Hehad cooperated with a course of fixed orthodon-tic treatment. How cruel now to prescribe aremovable denture, which he desperately wishedto avoid, at this difficult stage in his life, justbecause, for the time being, his standard of oralhygiene has lapsed. When crowns or other formsof treatment that improve the patient’s appear-ance are provided, this and the general increasein dental awareness that comes with extendedcourses of dental treatment themselves oftenimprove the patient’s motivation and, in turn, oralhygiene (Figure 5.2).

Condition of the remaining teeth

The state of health and repair of the whole mouthmust be taken into account. When there havebeen no previous extractions and the prognosisof the remaining teeth is good, it is usually worththe patient and dentist putting a considerableamount of effort into saving an individual tooth.Conversely, when the patient has already lost anumber of teeth and is wearing a partial denturethat will need replacing fairly soon, it would befoolish to struggle to save an individual toothunless it is a crucial abutment for the denture, orof particular importance to the patient’s appear-


ance. It is usually better to extract the tooth andremake the denture.

The periodontal condition of the remainingteeth is one of the factors in assessing theirprognosis, but it is more important to determinewhether any periodontal disease is progressing orwhether treatment of it has produced a stablestate. The effects of periodontal disease, particu-larly when there has been gingival recession, canaffect the choice and design of the crowns

Assessment of the occlusion is important(Chapter 4). In particular, the adequacy of poste-rior support should be considered when anteriorcrowns are planned. Insufficient occluding naturalposterior teeth usually means that anteriorcrowns should be metal–ceramic or one of thestronger ceramic materials.

The replacement of posterior teeth by remov-able partial dentures may provide medium-termposterior support but will rely on the patientwearing the denture continuously, which may becounterproductive. It is unlikely that a denturewill maintain the posterior occlusion in the longterm due to the denture teeth becoming wornand resorption of the alveolar bone.

A fixed reconstruction of the posterior teeth,even if limited in distal extension, may beachieved with bridges or implant-supportedrestorations. It is not necessary for all teeth tobe present for the remaining dentition to beconsidered stable (see the discussion of theshortened arch concept in Chapter 7 (page 182)).A patient who is insistent on anterior crowns forreasons of appearance should be strongly advised


Figure 5.2

a and b The same patient before and after a 6-monthcourse of extensive dental treatment involvingperiodontal treatment and the construction of upperanterior crowns. During this period, the patient’sdental awareness and motivation improved consider-ably and his oral hygiene became markedly better oncehe had more attractive teeth.

of the risks to anterior crowns without initiallyproviding adequate posterior support.

In some cases where there has also beenperiodontal disease and drifting of the incisorteeth, crowns joined together may be necessary(splinting is discussed in Chapter 13).

Considering the individual tooth

The value of the tooth

Not all teeth are of equal value. Third molars arecommonly extracted with no harmful effects onappearance or function. To crown third molars inan intact dentition would probably be no morethan a display of clinical virtuosity. However, if anumber of other teeth are missing, a broken-down third molar tooth that can be crowned mayprovide an invaluable abutment tooth for adenture or a bridge. There is a similar range ofpossibilities for most other teeth.

Appearance (Figure 5.2)

The presence of failed restorations may suggestthat crowns are advisable, but since anterior fillingmaterials are continually improving, the possibil-ity of replacing the restoration rather thancrowning the teeth should normally be consid-ered first. Whether the problem is one of failedrestorations, intrinsic staining or the shape orangulation of the teeth, a realistic appraisal of thecosmetic advantages of crowns must be made.Sometimes patients expect more of crowns thancan be achieved, and are disappointed with theend result. This should be avoided by explainingthe problems, complications and compromisesassociated with crowns (see Chapter 1). A moreconservative approach such as veneers shouldalso be considered before a crown is made.

Condition of the crown of the tooth, thepulp and periodontium

The presence of caries, previous restorations orpulp pathology are not contraindications tomaking crowns, but they may well determine thetype of crown and the design of the preparation.

Caries or fractures extending deep below thegingival margin will make crown preparation diffi-cult, and it may sometimes be better to extractthe tooth. Alternatively, periodontal surgery maybe used to expose the margin of the fracture.

Unless the tooth has been root-treated, thevitality of the pulp must always be tested andwhen necessary endodontic treatment carriedout. The prognosis and acceptability of crowninga recently root-filled tooth will depend on theabsence of signs or symptoms and its radiographicappearance. If there is any anxiety about thesuccess of a root filling then a choice must bemade between a number of options:

• Leave the tooth temporarily restored until thesymptoms settle and a good prognosis can begiven

• Repeat the root filling • Proceed with the core and a temporary crown,

but delay the final crown until the symptomshave settled or the radiographic appearance hasimproved, or the tooth has been apicected.

In many cases the third option is best. If a toothis left with a temporary restoration (the firstoption) for too long, there is a risk that remainingenamel and dentine will break (if there is no risk,why is the tooth being crowned in the first place?).With anterior teeth, if the post and core areinserted immediately, there will be no risk ofdisturbing the root filling later, and with most well-condensed anterior root fillings, the treatment forfurther endodontic problems may be an apicec-tomy. It is better to cement the post and corebefore the apicectomy rather than afterwards, toavoid the risk of disturbing the apical seal.

A more liberal attitude should be taken tominor radiographic defects in the root filling whenit has been present for some years and issymptomless. Further details of the criteria forassessing root canal fillings are left to theendodontic textbooks.

Any local periodontal problems should beassessed and treated.

Occlusion

The occlusal contacts on the surface of the toothmay be important in determining the type ofcrown to be used. For example, an upper canine


tooth that is the only tooth in contact in lateralexcursion (canine guidance, see Chapter 4) willusually need a metal–ceramic crown rather thana ceramic crown. However, if the tooth is onlyone of a number that make contact in lateralexcursion (group function), it may be possible torestore the canine with a ceramic crown.

The point of contact between the tooth to becrowned and the opposing tooth is also impor-tant in determining the position of the crownmargin. It is wise to design the preparation sothat the opposing tooth contacts either toothtissue or the crown but not the junction betweenthe two. In the case of partial crowns, whenocclusal protection is required, the occludingsurfaces of the tooth to be crowned should bedetermined. These are not always the same as theocclusal surface. Occlusal is an anatomical term,and an extracted tooth still has an occlusalsurface. The occluding surfaces are those thatreally do make contact with opposing teeth inone or other excursion of the mandible.

Root length

The length of the root should be assessed fromradiographs in two ways. First, this should be donefrom the point of view of periodontal support, i.e.the ratio of the length of the root supported byalveolar bone to the length of the remainder of thetooth. Second, the length of the root is importantin providing retention for a post crown. A conven-tion is that the length of the post should be notless than the length of the artificial crown, butthere is no good evidence to support this. A betterapproach is to make the post as long as possiblewithout disturbing the apical seal or risking per-foration in a curved or very tapered root.

Variations are possible: for example, a shorterpost is acceptable in the case of reduced occlusalforces (such as incisor teeth with an anterioropen bite); and a longer post is necessary wherethere are excessive forces applied to the tooth,for example when the tooth is used as a partialdenture abutment. When this length is not avail-able, a post with improved retention, such as athreaded parallel post, should be used. An alter-native is to include a full diaphragm of gold overthe root face together with a collar around theperiphery. This improves retention and alsoreduces the likelihood of root fracture.

Decisions to be made

Is the tooth to be kept or extracted?

Usually the result of the history and examinationwill determine this question. Sometimes,however, it is necessary to proceed to furtherstages and then return to a decision to extractthe tooth if further endodontic, periodontal orother treatment is not successful.

If the tooth is to be kept, is it to berestored by a crown or a filling?

In Chapter 1 the alternatives to crowns aredescribed, and the findings of the history and exami-nation will sometimes settle this question. However,it is often necessary to proceed to a further stage,actually starting the treatment by removing previousrestorations and caries, before a properly informeddecision can be made (Figure 1.12). For example atooth with extensive decay may have the decayremoved and the need for endodontic treatmentdetermined before it can be decided if a crown ispossible. Decay in the mouth is always worse thanseen on a radiograph and if extending deeply subgin-gival or into the furcation area may make the toothunrestorable even with a crown.

If the tooth is to be crowned, is anypreparatory treatment necessary?

Preparatory orthodontic treatment may be neces-sary to move the tooth into a suitable positionfor crowning. Combinations of orthodontic treat-ment and crowns can often produce results thatcannot be achieved by either form of treatmentalone. Periodontal and endodontic treatment mayalso be necessary.

Detailed planning of the crown

Appearance

When a significant change in appearance isproposed, it is most important that the patient is



Figure 5.3

Trial or diagnostic wax-ups.

a Missing upper lateral incisors and a midline diastema.Centre: closing the diastema orthodontically and provid-ing bridges to replace the lateral incisors. Lower: the alter-native is to make four oversized crowns or veneers onthe central incisors and canines to resemble four incisors.Neither solution will produce an ideal appearance.

b A diagnostic wax-up for a similar case. The pointedcanines are unattractive and the right central incisor isfractured. The canines have been retracted orthodon-tically. The lateral incisor teeth will be replaced by two3-unit conventional bridges (see Part 2) increasing thesize of the central incisor crowns and improving theshape and the size of the canines.

c This patient has extensive palatalerosion as a result of an eatingdisorder. The teeth are alreadyshort and so diagnostic prepara-tions have been made on three ofthem to see whether sufficientdentine remains for retentivepreparations or whether someother solution is necessary.

d The preparations are judged asbeing sufficiently retentive and thisfigure also shows how short theunprepared incisor teeth have be-come. The diagnostic wax-up showsthe appearance of the plannedrestorations and also provides astarting point for more detailedplanning of the preparations and formaking temporary crowns.

fully informed of what can be achieved and whatcannot. This can best be done by a modificationof the patient’s own study casts, usually in wax.Figure 5.3 shows examples of missing upperlateral incisors that could be treated by movingthe central incisors mesially closing the diastemaand replacing the lateral incisors by means ofbridges. If this were done, all the teeth would berather small. The alternative of not moving theteeth and enlarging the central incisors by meansof crowns and crowning the canines to resemblelateral incisors is also shown.

Figure 5.3b, c and d shows satisfactory trialwax-ups demonstrating retentive preparationsand aesthetically pleasing crowns and bridges.

Trial preparations and diagnostic wax-ups areextremely valuable in predicting the final appear-ance, and should be used routinely in all but themost straightforward cases, particularly when thestudent or dentist is inexperienced.

Because the teeth and soft tissues are all repro-duced in plaster or artificial stone in the cast, itis possible to ‘cheat’ by making the diagnosticwax-up in a way that would be impossible in themouth by reshaping the gingival margin or bychanging the dimensions of the root as it emerges

from the gum. When the teeth are not to bemoved orthodontically, it is useful to draw apencil line around the gingival margin on the studycast and to ensure that this is still visible after thewax-up has been completed. When the tooth isto be moved orthodontically, the mesio-distalwidth at the gingival margin should be measuredand this width reproduced in the new position ofthe tooth on the study cast. Figure 5.4 shows anexample of study cast ‘cheating’. The plan is tocrown the upper canines to resemble upperlateral incisors. This is always difficult and oftendisappointing. One of the distinctive features ofan upper canine is the sharply curving gingivalmargin. This will be retained once the crown isin place, and will detract from the impression thatthe tooth is a lateral incisor.

These diagnostic wax-ups serve a number ofother purposes as well as informing the patientof what can be achieved. However experiencedthe clinician, each case is different and modifiedstudy casts will help in planning details of theeventual appearance. The technician will knowwhat is wanted and will have models to copyrather than have to design the patient’s newappearance in porcelain.


Figure 5.4

Diagnostic wax-up ‘cheating’. The upper cast shows apatient with ugly prominent canine teeth and missinglateral incisors with no residual space. He was notprepared to have orthodontic treatment to retract thecanines. Because of their shape and size it would notbe possible to achieve a satisfactory appearance byadjusting them and adding composite. So, leaving themalone or crowns were the only options.

The lower cast shows two trial wax-ups: left, withthe contour of the gingival margin carefully marked inpencil before the preparation and wax-up are made;right, the position of the gingival margin has been lostand a more natural-looking wax-up made. However, itwill not be possible to achieve this result in the mouthand a decision whether to proceed with crowns mustbe made on the appearance of the ‘honest’ diagnosticwax-up.


Figure 5.5

a On a study cast the possible shape of the tooth canbe seen if it is to be built with the labial surface levelwith the adjacent teeth. This will result in a very thickincisal surface and the patient needs to be fully awareof this complication. Due to the overlap of the adjacentteeth the lateral incisor tooth may appear narrower.

b Directly placed composite can be used as a trial forthe patient to assess the likely outcome. It might bepossible for this to be used as a final restoration but abetter result can be achieved by minor adjustment ofthe adjacent teeth and placing a porcelain veneer.

The person who may have difficulty in inter-preting a diagnostic waxing on a study cast is thepatient. They are not used to seeing their teethin this way and the dentist needs to explain theresults carefully. It is helpful to show the patientthe unmodified model as well as the modifiedone. Tooth-coloured wax should be used asbrightly coloured wax is distracting. Ideally themodified model should be duplicated in the samecoloured stone as the original.

The diagnostic wax-up can be used to constructa vacuum-formed matrix that can allow for a trialto be undertaken in temporary crown material in

the mouth. Alternatively a change to toothcontours can be tried out in directly placedcomposite so that the patient can see directly intheir mouths what the result will be like (Figure5.5). In patients with high expectations for theirappearance great caution should be taken instating the appearance that can be achieved andthe dentist should not be worried about seekinga second opinion on the advisability of treatment.

Techniques are available to alter photographicimages of the patient’s actual teeth to show theeffects of possible treatment. With thesetechniques it is easy to promise more than can


c Following agreement with the patient, the tooth isprepared minimally.

d The result with a porcelain veneer.

be achieved and their use as a diagnostic tool isdubious.

The modified study cast, agreed by the patient,forms part of the contract between the dentistand patient. If the final outcome is an appearancesimilar to that of the study cast, it can be usedas evidence that the contract has been fulfilled,and so dento-legal problems can be avoided. The

modified study cast may also be used to producetemporary crowns (see Chapter 6).

Shade

It is wise to select the shade at this early stage,since some shades are more difficult to match


Figure 5.6

Temporary and permanent changes to occlusal verticaldimension.

a Gross erosion of the upper incisor teeth following aperiod of chronic vomiting. The patient had a pepticulcer that had been successfully treated surgically 2years before this photograph. The OVD is reducedbecause of this wear and because the lower posteriorteeth are replaced only by a tissue-supported partialdenture. The gingival condition at this early stage oftreatment is poor.

b and c Increasing the length of the upper incisorstemporarily by means of light-cured composite placedin a vacuum-formed PVC matrix. Acrylic was added tothe lower partial denture to increase its occlusal heighttemporarily.

d After a period during which the patient becameaccustomed to the new OVD, the upper incisors werecrowned and a tooth-supported partial lower denturewas fitted. The gingival condition has improved. Notethe supragingival crown margins, which help the patientto maintain good oral hygiene. The left lateral incisorhas been extracted and replaced by a bridge.

than others. It is better to know about any diffi-culty before the teeth are prepared, both fromthe point of view of warning the patient andbecause it may be helpful to modify the prepara-tion. For example, if there is an extensive amountof incisal translucency, the preparation may needto be shorter to allow additional incisal porcelainthan if the tooth were more opaque. Manypatients these days wish to lighten the shade oftheir teeth with bleaching techniques. If this isanticipated, it is important that this is done beforethe crowns are made. Patients should be discour-aged from over-bleaching their teeth as extremely‘white’ shades can be impossible to match inporcelain.

Clinical modifications

In some cases it may be helpful to adjust theshape of teeth in the mouth by adding compos-ite material – particularly when alterations inocclusal vertical dimensions (OVDs) are planned.Figure 5.6 shows a patient with gross erosiontreated by upper anterior crowns and a newpartial lower denture with an increase in OVD.The patient’s tolerance of an increase in OVD isassessed by means of the temporary additions ofacrylic to the occlusal surfaces of the old partiallower denture and of composite material to theupper incisor teeth. Temporarily reshapingincisor teeth with composite to close diastemasand produce other changes are furtherexamples.

Assessing the remaining toothstructure and its environment

Existing restorations and caries, especially in badlybroken-down posterior teeth, should beremoved, together with any completely unsup-ported enamel, so that the amount and shape ofthe remaining tooth structure is not guesswork.Only at this point should the final restoration beplanned. This preliminary cleaning away wasnecessary in all the examples shown in Chapter1 (Figure 1.12), for a properly informed decisionto be made.

At this stage it may also be necessary to returnto a decision on further preparatory treatment.

For example, although the pulp may be vital inan anterior tooth, it may be decided that becauseof the weakness of the remaining coronal tissue,an elective root canal treatment and post crownis the preferred restoration. Similarly, wherecaries extends below the gingival margin, it maybe decided to carry out crown lengthening orother periodontal surgery to alter the gingivalcontour.

The need for a core

At this stage, when the full extent of the damageto a broken-down tooth is known, a decision ismade on whether sufficient tooth substanceremains for a crown preparation or whether itneeds to be built up by means of a core and, ifso, whether the core should be of amalgam,composite or cast metal.

At the same time, the position of the crownmargin should be settled. Almost always thecrown will extend beyond the core andcompletely cover it. However, occasionally, whenpart of an amalgam core is subgingival but is wellcondensed and smooth, it may be better to makethe crown margin supragingival, leaving part of thecore exposed at the gingival margin.

The choice of the type of crown andthe material

At this stage too the decision is taken betweenmaking a complete or partial crown, and what thematerial for the crown will be chosen.

Detailed design of the preparation

Chapter 3 described the principles of crownpreparation design. This is the point where theyare applied to the particular tooth. In cases ofdoubt, for example where there are questions onthe likely retentive qualities of the final prepara-tion or on the likelihood of exposing the pulp inremoving sufficient tooth tissue for a crown thatis planned to change the shape of a tooth, a trialpreparation on the study cast is of considerablevalue (Figures 5.3c and d and 5.7).


Planning and executing the clinicaland laboratory stages

Appointments

The treatment plan and fee having been agreedwith the patient, a series of appointments is madeand agreement reached with the laboratory thatthe technical work can be undertaken in the timeallotted. Very few dentists now carry out theirown technical work and most laboratories appre-ciate being notified in advance when their services

will be required, at least for extensive cases. Thisavoids the problem of promising the patient deliv-ery of the crown by a specified date only to findwhen the impression is taken that your favouritetechnician is on holiday.

Clinical and laboratory stages

Details of clinical techniques are given in Chapter6; at this point only the sequence of events islisted (see pages 120 and 121). Depending on thenumber of crowns involved, the experience and


Figure 5.7

Trial preparations on study casts.

a The patient is unhappy about the appearance of therotated lateral incisors and would like them crowned.

b Right: the maximum labial reduction, while preserv-ing the vitality of the pulp, would result in this prep-aration, allowing some reduction in the prominence ofthis tooth. Left: however, initial preparation quicklyshows that devitalization would be necessary if a crownis to be made. The patient should be encouraged tohave orthodontic treatment.

c The upper right central and lateral incisors werealmost identical to the unprepared worn teeth on theleft. Trial preparations show that it will be possible toachieve a retentive crown preparation for the centralincisor but not for the lateral incisor. Crown length-ening would make a retentive preparation possible.

speed of the operator and other factors, eachclinical stage may be accomplished in a singleappointment or in several. The patient should beadvised on oral hygiene techniques appropriate tothe new crown, and the crown should beexamined and, if necessary, maintenance carriedout at the patient’s general review appointments.

It may be necessary to abort the procedure

shown on pages 120 and 121 and return to anearlier stage, either in the construction sequenceor even the planning stages. For example, adamaged working model in the laboratory stagemay mean returning to the first clinical stage fora new impression, or a cusp fracturing after atooth has been prepared for a partial crownmeans returning to the planning stage.


d The same patient as shown in Figure 5.4. Half thepreparation has been completed on the upper caninetooth, showing the amount of enamel and dentine thatcould be removed without damaging the pulp. This isnot enough to achieve a successful aesthetic result, anddevitalization would be necessary. Most dentists andpatients would consider this too destructive and acompromise solution with composite additions shouldbe considered – the patient was adamant about nothaving orthodontic treatment.

e Trial preparations for a conventional fixed-movablebridge (see Part 2). These show that this design wouldwork in this case.

f An instanding lateral incisor which the patient wishesto improve. They are not prepared to have orthodon-tic treatment.


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Planning stages before preparingthe tooth

Emphasis has been placed in earlier chapters onthe general approach to planning. Here theprocess is taken to the final, practical stages. Thefollowing factors should be considered beforeclinical tooth preparation is started:

• Study casts• Photographs• Trial preparations• Appearance• The final impression• The temporary crown.

Study casts and opposing cast

Full arch study casts are useful in a variety ofplanning procedures described earlier (Chapter 5)and below. In addition, the opposing cast may bearticulated with the working cast, provided thatit has not been damaged during the planningstages and there has been no occlusal adjustmentor other restorations since the impressions forthe study casts were taken.

Alginate is usually satisfactory as an impressionmaterial for study casts, but occasionally a moreaccurate material is required for the opposingcast: if the opposing cast cannot be pouredstraightaway, a more stable material than alginateshould be used. Alginate material should berubbed into the fissures with the finger to obtaingood impressions of the occlusal surfaces of theteeth. If a rubber material is used, it should besyringed into the occlusal fissures.

If there is any doubt about the opposing studycast, a new impression should be taken at thetime of tooth preparation to produce an accurateopposing cast.

Photographs

When the main indication for crowning anteriorteeth is to change their appearance in some way,photographs of the teeth before preparation area valuable record. It is easy for both dentist andpatient to forget the exact appearance of teethwhen they have been prepared.

Good quality clinical photographs should nowbe a routine record in all cases where the appear-ance is to be changed. Digital photography hassimplified this. Clinical photographs have been astandard part of orthodontic records for manyyears.

Patients sometimes have old photographs ofthemselves showing their teeth before theysuffered. These can be helpful in reproducing thepatient’s original appearance.

Planning the tooth preparation

A trial preparation on the study cast can beinvaluable in predicting difficulties that may beencountered with preparing the tooth. Forexample, with short clinical crowns in posteriorteeth, where the intention is to provide a porce-lain occlusal surface on a metal–ceramic crown, atrial preparation on the study cast will indicatewhether there will be sufficient axial length of theremaining preparation for adequate retention orwhether additional devices are necessary toachieve retention. With thin upper anterior teeth,trial preparations will show whether it is possibleto remove sufficient palatal tooth tissue to allowa ceramic crown preparation to be made orwhether a metal–ceramic crown will be neces-sary. In some cases it will become clear thatadditional steps, such as increasing the verticalspace between the incisal edges of the upper andlower teeth, or removing gingival tissues to

6 Clinical techniquesfor making crowns

123

‘crown-lengthen’ the teeth will become necessary(see page 58).

There are so many similar problems that canbe assessed with trial preparations that an inex-perienced operator is recommended to alwaysspend a few minutes making a trial preparationbefore embarking on the natural tooth.

When the intention is to alter the shape, sizeor angulation of incisor teeth, even the moreexperienced operator is well advised to make trialpreparations first. These will show whether thepulp is likely to be endangered and whetherproblems with the appearance will arise.

Trial preparations on plaster or artificial stonestudy casts are best started with a steel bur in aslow handpiece and finished with a scalpel bladeor chisels. The air turbine is difficult to controlwhen cutting relatively soft materials like this.

It is helpful to do the preparation in stages,particularly if there is any question of the pulpbeing exposed. Figure 5.7d (page 119) shows anexample with half a preparation complete so thatthe remaining half indicates the original shape ofthe tooth. The tooth could not be prepared anymore than this without serious risk to the pulp.

Planning the appearance

Diagnostic wax-ups

Chapter 5 gave examples of diagnostic wax-upswith alternative treatment plans for the sameclinical situation.

Ivory wax is preferably to other waxes since itis easy to carve and gives a reasonably realisticappearance. However, pink wax, inlay wax orother material may also be used.

Shade

One reason for matching the shade before start-ing tooth preparation was described in Chapter5. A second, very basic reason is that when shadematching (one of the most crucial parts of theprocedure) is left to the end of a long appoint-ment, the operator and patient are tired and theoperator’s vision is not at its best, so the processmay be hurried and mistakes made. Besides, teethmay change shade slightly during a long operative

appointment. It is well known that teeth changecolour dramatically after a period of time underrubber dam (Figure 6.1). Although rubber dam isnot normally used for crown preparations, it ispossible that after an hour or so of wetting anddrying and then several minutes in contact with arubber impression material, the shade of thetooth to be matched may be altered.

Technique for shade selection

The lighting conditions are very important.Traditionally shades are selected in naturaldaylight rather than artificial light. However, thereare practical problems with this. Daylight is veryvariable in its intensity; and it is not always poss-ible to make appointments during the hours ofdaylight. It is anyway equally important that the

124 Clinical techniques for making crowns

Figure 6.1

The lower central incisors are bridge pontics for aminimum-preparation bridge (see Part 2). They are agood shade match to the natural upper incisor teethbut the lower lateral incisors and canines have becometemporarily whiter because they have been isolatedunder rubber dam for some time while the minimumpreparation bridge was bonded. This has resulted inthem drying out and the shade lightening. They willrevert to the normal shade an hour or so after therubber dam has been removed. This phenomenon mayoccur to some degree during impression taking andmaking temporary crowns. Hence the advice to takethe shade prior to starting tooth preparation.

Clinical techniques for making crowns 125

Figure 6.2

a Using a traditional shade guide to select the basichue. The upper left central incisor is in fact a ceramiccrown made from a combination of the two shadesimmediately below it. More commonly individual tabswould be removed and compared to the tooth to bematched.

b A standardized light source can be used to removethe risk that the shade is influenced by the ambientlighting.

c and d An example of a shade-taking device thatdigitally analyses the shade of a tooth and produces aprint out of the shade in detail.

e More detailed shade guides are available includingshades specially formed to match to bleached teeth(see those on the lower left).

crown appears to be the correct shade in bothartificial light and daylight. Many people spend alarge part of their working and social lives inartificially lit surroundings.

Dentists often now match the shade both indaylight and in different forms of artificial light(tungsten filament and fluorescent).

Alternatively, a standardized artificial lightsource designed to be a close approximation tonatural daylight may be used. It is of courseimportant that the technician has a similar lightsource (Figure 6.2b).

Various devices are available to record shadeautomatically with variable accuracy. The shade ofthe tooth is analysed and a prescription to copythe shade with differing porcelains is printed forthe laboratory (Figure 6.2c). With experienceboth at the chair side and in interpreting theprescription in the laboratory these systems cangive predictable results.

Involving the dental technician in taking theshade is an excellent idea; first because it givestwo opinions, but also because it allows thetechnician to meet and see the patient and theiractual clinical situation, usually producing a betterresult. Further information on shade can be givento the laboratory by taking a photograph of thetooth with the selected shade guide in position(Figure 6.2e).

Modern dental unit lights are designed toprovide a reasonable colour balance, and if this isadequate then using the unit light has the advan-tages of consistency and convenience.

Colour is said to have three dimensions: hue,chroma and value. Hue is the colour itself (e.g.red compared with green). Chroma is the amountof colour (e.g. red compared with pink) and valueis the darkness or lightness of the colour (theshade of grey that the colour would appear ifseen on a black and white photograph). Manyshade guides for dental porcelain are arranged ingroups representing different hues, with a grada-tion of chroma within each hue. Figure 6.2ashows a typical shade guide with four hues (lightbrown, yellow, blue/grey and a pinkish hue) desig-nated A, B, C and D. Within each group, differ-ences in chroma are indicated by a number. Forexample, B2 is a fairly light yellow shade, whereasC4 is a much darker blue/grey.

In addition, many shade guides have differentneck, body and incisal edge shades on them. Theincisal edge is also made more translucent.

Shade selection usually follows these lines:

• Choose the appropriate lighting conditions ortake the shade in a variety of different lightingconditions

• Look at the whole mouth and make a generalassessment of the appropriate hue – whetherthe teeth are generally brown, yellow or grey

• Look more closely at the tooth to be crowned,the adjacent teeth and the contralateral tooth,and decide the hue or mixture of hues (theletter on the shade guide)

• Select the chroma (the number)• Choose the blend of neck, body and incisal

shades. It is not necessary to select the neck,body and incisal edge shades from the sameshade button, and it is possible to mix porce-lain powders so that shades between those onthe shade guide are produced

• Decide whether any other characteristics suchas crack lines, areas of opacity or increasedtranslucency are required.

It is helpful to draw a shade map of the tooth.Figure 6.3 shows a tooth with its shade map.

Some dentists and technicians prefer a shadeguide consisting of ‘shades’ made by fusing a smallbutton from each of the single porcelain powders.

Others find the combination of shades on thecommercial shade guides confusing, and grind offthe neck (which often has surface stain added)and incisal portions, leaving a single body shade.

Planning the impression

The impression technique should be decidedbefore preparing the tooth. Some impressionmaterials are better used in a special tray, and thisis made on the study cast. Other impressionmaterials can be used in stock trays. If a stocktray is used it must be possible to seat it withoutdistortion by contact with the teeth or softtissues. It is essential that good quality stock traysare used which are not too flexible.

Generally a special tray is chosen if multiplecrowns are being made or the patient has an archsize or form that a stock tray does not fit. Specialtrays, if properly made, are rigid and will not distort.They cost more than stock trays but this is partlyoffset because they require less impression material.


Making a special tray (Figure 6.4)

When a special tray is to be used it can be madefrom self-curing acrylic or light-curing acrylic. Atone time shellac and vacuum-formed materialswere used but they are not sufficiently rigid orstable and so they should now be regarded asobsolete as far as impressions for crowns andbridges are concerned.

To make a special tray a wax spacer approxi-mately 3 mm thick is laid down over the studycast. This is perforated through to the occlusalsurface of three or four teeth that are not to beprepared for crowns. The purpose of theseperforations is to allow tray material to formstops on the occlusal surfaces of the teeth. This

will localize the tray in the mouth and prevent itmaking contact with the prepared teeth. The trayis then formed by moulding acrylic dough or asheet of light-curing acrylic over the study cast

Figure 6.4 shows the stages in making specialtrays in the two materials. Self-curing acrylic isconvenient and inexpensive and does not requirespecial equipment. Self-curing acrylic special trayscan be made in the dental surgery. Light-curingacrylic special trays are quicker and easier toproduce and give more consistent results.However, it is necessary to have a light-curingbox in which to cure the tray, and the cost ofthis would not normally be justified for surgeryuse. The trays are therefore made in the labora-tory. There is usually little difference in cost


Figure 6.3

a and b A shade map of this patient’s upper rightcentral incisor.


Figure 6.4

a and b Making a self-curing acrylic special tray.

a shows a study cast with a wax spacer with holesmade in it. The holes have been filled with self-curingacrylic. Self-curing acrylic has been rolled out andshaped together with a handle.

b The tray has been moulded over the study cast andspacer and the handle attached. The stops, locatedover teeth which are not to be prepared, prevent thetray from seating too closely onto the prepared teeth.

c–g The construction of a light-cured acrylic specialtray.

c The study cast and wax spacer together with apreformed blank of light curing acrylic material.

d The blank is roughly shaped, producing a handle.


e It is moulded over the study cast and trimmed witha scalpel.

f The light curing box with the bright blue light turnedon.

g The finished tray with stops.

between the two types of tray if they are labora-tory-produced. Light-cured materials have theadvantage of being quick and will be dimension-ally stable once light-cured, whereas self-curingtrays need to be made 24 hours previously toallow for polymerization shrinkage to occur.

Planning the temporary crown

Temporary crowns may be either purchased aspreformed units or made at the chair side in asuitable mould.

Preformed temporary crowns

The following types of preformed temporarycrown are available:

• Polycarbonate, tooth-coloured temporarycrowns for anterior and some posterior teeth

• Stainless-steel posterior temporary crowns• Aluminium posterior temporary crowns.

When one of these is to be used the appropri-ate size can be selected before the tooth isprepared using the study cast as a guide.

Polycarbonite temporary crowns are preferredas they are relined with acrylic or other resinmaterial in the mouth and can therefore betrimmed to a better marginal fit than metaltemporary crowns.

Chair-side temporary crowns

Temporary crowns can be made in the mouth,using one of the higher acrylics, usually consistingof a mixture of poly(ethyl methacrylate) andpoly(isobutyl methacrylate), sometimes with anylon fibre filler. Alternatively acrylic-poly(methylmethacrylate), epimine resin or amalgam may beused. The mould that is used to form the tem-porary crown may be one of the following:

• A preformed celluloid crown form• A vacuum-formed PVC crown form made on a

study cast or a modified study cast of thepatient’s mouth

• A silicone putty impression of the study cast

• An alginate or silicone impression of the mouthtaken before the tooth is prepared.

The use of these materials and moulds isdescribed on pages 150–153.

Laboratory temporary crowns

Temporary acrylic or composite crowns can bemade in advance from the model followingdiagnostic preparations. These will need to berelined in the mouth. The potential advantagesare minimal considering the clinical time takenand the extra laboratory costs. This is in contrastto laboratory-made provisional crowns – see later– for the difference between temporary andprovisional crowns.

Building up the core

As described in Chapter 2, cores may be madeof cast metal retained by a post, or of amalgamor composite retained by posts or pins whenrequired. Techniques for constructing post andpin-retained amalgam cores are illustrated inFigure 6.5. Other cores are shown in Figure 2.7(pages 48–50). With a composite core the toothcan be prepared at the same visit. The site andangulation of pins is crucial (see Chapter 2). Thedetailed design of the preparation must bedecided before the pins are placed; otherwise, ifthe pins are in the wrong place, they may be cutoff during the preparation of the core for thecrown.

Managing worn, short teeth

Figure 6.6a shows some very worn and shortupper anterior teeth. They have continued toerupt as they have worn, and so remain incontact with the lower incisor teeth. The gingivalmargins have migrated incisally, following thefurther eruption of the teeth. The upper andlower posterior teeth remain in contact. Theupper incisor teeth are to be crowned toimprove their appearance and prevent further



Figure 6.5

Post- and pin-retained cores and Nayyar cores.

a A copper band trimmed to shape and the marginssmoothed.b Band in place ready to receive amalgam. Retentionin this root-filled tooth is provided by a preformed post(see Figure 2.7).c The amalgam core placed. Note that the gap betweenthe remaining cusp and copper ring has been filled with

amalgam so that the edge of the copper ring does notirritate the tongue. The amalgam will fall away when thecopper ring is removed at the next visit. As the copperband will be left in place, the amalgam can be left inocclusion – note the marks from articulating paper.d Pins, lining, matrix and wedges placed. Pins are usedwhen the pulp remains vital.e Amalgam placed.f Matrix removed and amalgam roughly carved. Theamalgam is left out of occlusion to avoid undue stressesbefore the crown is placed.

a

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d

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wear (other preventive measures having beenunsuccessful), but the length of the crowns is notsufficient to produce satisfactory preparations andcrowns with a good appearance.

There are a number of ways to overcome thisproblem:

• A fixed anterior biteplane (a fixed Dahl appli-ance), is cemented to the upper incisor teeth.This is designed to hold the anterior teeth

apart by the amount that is needed for toothpreparation. Once cemented, the posteriorteeth do not occlude, but patients cope withthis very well. In 3–6 months the anterior teethare intruded and/or the posterior teeth over-erupt so that they come into occlusal contact.At this stage the fixed Dahl appliance isremoved and the teeth are prepared forcrowns without removing any more dentinefrom the worn palatal surfaces or incisal edges


Figure 6.6

a A patient who has suffered from bulimia nervosaproducing extensive palatal and incisal erosion of theupper incisor teeth. Continuing eruption has kept pacewith the erosion and so the upper and lower incisorteeth are still in contact.

b A similar case, again caused by an eating disorder,but this time predominantly affecting the incisal edgesof the upper central incisor teeth.

c The central incisors have been built up with compos-ite which improves their appearance for the time beingbut also acts as a form of Dahl appliance which willdepress both upper and lower incisor teeth. Note thatthe composite restorations are in contact with thelower incisors and yet the canine and posterior teethare out of occlusion.

(Figure 4.6a–g). The original Dahl appliance wasa removable cast metal anterior biteplaneretained by clasps on posterior teeth.However, this has now been largely supersededby the fixed appliance.

• Restoring the worn surfaces with composite toact in the same way as a Dahl appliance(Figures 4.6h and i and 6.6b and c).

• The upper anterior teeth may be prepared forcrowns, without removing any dentine fromthe worn palatal surfaces or incisal edges, andthen provisional crowns made to the lengthand shape that is required for the permanentcrowns. These will act in a similar way to thefixed Dahl appliance. However, the disadvan-tage is that the teeth are prepared before thetooth movement has been established.

• The incisal teeth may be depressed and/or theposterior teeth allowed to over-erupt byconventional orthodontic treatment. Theinter-incisal space created means that theworn incisal surfaces do not need to beprepared, and the gingival margins migrateupwards as the incisor teeth are intruded. Thisis the technique of choice in some cases, butthe disadvantage is that the patient needs towear an orthodontic appliance, which is usuallyvisible.

• Crown-lengthen the teeth by means of aperiodontal surgical procedure to removegingival tissue and usually bone. This is destruc-tive if the gingival tissues are healthy and isusually quite painful for the patient. It canproduce sufficient length for retentive crownpreparations, but the worn incisal surface has

to be prepared and so there is a risk to thepulp. The neck of the preparation is placed partway up the tapering root, and so is of a smallerdiameter than if it were at the cement–enameljunction. This means that the interdental spacesare usually greater, and this can spoil theappearance.

• Increase the whole occlusal vertical dimension(OVD) by crowning all the teeth in the upperarch. This is sometimes necessary if the OVDis reduced and if the posterior teeth needcrowning in any case. However, it is unjustifi-ably destructive if the posterior teeth do notneed crowning and is likely to fail if a normalOVD is encroached upon artificially. It is poss-ible for intact posterior teeth to be built up toan increased OVD by the direct bonding ofcomposite materials but in the long term thesewill not be stable enough to maintain theocclusal form. If the problem is the other wayround and the posterior teeth require buildingup with crowns to a new OVD and theanterior teeth do not, building up the palatalsurfaces of the upper incisors with compositemay well be stable for long periods of time asthe risk of wear and overloading is less.

Of these techniques, the fixed Dahl appliance orcomposite additions are preferred wheneverpossible. Provisional crowns or conventionalorthodontic treatment are the next best, andcrown lengthening and increasing the OVDoverall should be preserved for those caseswhere there are specific indications for thesetechniques.


Figure 6.7

A selection of burs and the surfaces prepared by them.In all cases the bur was used entirely within the contourof the tooth and would not have damaged the adjacenttooth. From the left: a square-end tapered diamond bur,a square-end tungsten carbide bur, both producingnarrow shoulders; a parallel-sided but pointed diamondbur, with the matching tungsten carbide finishing bur; around-ended parallel-sided plain cut tungsten carbidebur, these last three producing chamfers.

Tooth preparation

Choice of instruments

The major part of the preparation is carried outwith the airotor. Diamond burs are preferred forpreparing enamel, and either diamond or tungstencarbide burs for amalgam and dentine.

The shape of the bur or stone should bechosen to match the contour of the surface thatis being prepared. This includes the shape of themargin, so that if a shoulder is being prepared, asquare-ended straight or tapered bur should beused. Alternatively, if a chamfer finishing line isbeing prepared then an appropriately shaped burshould be chosen. Figure 6.7 shows a selection ofburs set against the tooth surfaces they haveprepared.

It is easier to control the preparation of theconcave palatal surface of upper incisor teeth if alarge diameter diamond bur, matching thecontour of the tooth, is used.

The finishing is an important stage and can takerather longer than the main bulk reduction. Thepurpose is to finalize the shape of the prepara-tion, rounding off angles where necessary, ensur-ing that the margin is properly located in relationto the gingival margin and is the correct contourand dimension. In addition, coarse undercutsresulting from diamond score marks should beremoved and the surface of the preparation leftreasonably smooth. Otherwise there will be diffi-culty with removing a wax pattern from the dieand with cementation. There is, however, noneed to polish preparations: a very slight rough-ness helps retention (Figure 3.9, page 62).

Slow-speed handpieces with steel finishing bursor fine stones can be used for finishing; however,

it is more commonly done at medium to highspeed with plain tungsten carbide burs or fine-grain diamond burs.

Stages in the preparation

It is usual to prepare each surface in turn so thatthe amount of tooth reduction can be controlled.Establishing how much tooth has been removedcan be done in a number of ways. At the marginthe width of the shoulder or other finishing linecan be seen directly. Where the tooth beingprepared occludes with opposing teeth, and otheradjacent teeth also occlude, the amount of toothtissue removed from the occluding surface isassessed by direct observation or by the patientbiting through soft wax; the thickness of theremaining wax shows how much tooth has beenremoved (Figure 6.8). On other surfaces, half maybe prepared first, leaving a step between theprepared and unprepared areas so indicating thatamount of tooth tissue removed (Figure 6.10c).Alternatively, a groove may be prepared acrossthe surface to the intended depth of the prepa-ration and the remainder of the surface thenprepared to the depth of the groove (Figure6.10b).

Another useful technique to ensure thatadequate tooth reduction has been made is tomake a silicone putty matrix of the tooth or itsdiagnostic wax-up prior to starting and sectionthis through the centre of the tooth. Placing itonto the preparation shows whether furtherreduction is required (Figure 6.10g).

The order in which the tooth surfaces areprepared will depend upon the circumstances and


Figure 6.8

An occlusal record taken in 2 mm thick soft wax, whichdoes not require warming. The occlusal contacts of theunprepared first and third molar teeth can be seentogether with the imprint of the second molar prep-aration. It is clear that there is nearly 2 mm clearance,and this is sufficient for a metal–ceramic occlusalsurface.

the operator’s preference. However, for theinexperienced, some basic guidelines may beuseful. Surfaces that are easy to prepare and thatwill improve access to more difficult surfacesshould be prepared first. For example, withincisor teeth some operators prepare the incisaledge first in order to remove part of the approx-imal surfaces and improve access to the remain-der of them. Similarly, the most difficult surfaceshould be left until last.

It is best to initially prepare the tooth, leavingthe margin well supragingival until the basic prepa-ration is completed, and then assess and refinethe preparation taking the margin down to theappropriate gingival level.

Sometimes with a difficult path of insertion, thedirection of one surface is critical. In this case itshould be prepared first and the other surfacesprepared relative to it. When grooves are to beused as part of the preparation, they are left untillast and aligned with other prepared surfaces toform part of the overall retentive design.

Preparing teeth for completeposterior crowns

Figure 3.15 (page 68) shows a typical sequence inthe preparation for a complete gold crown of aposterior tooth that has been built up with apinned amalgam core. Figure 6.9 shows a pre-molar with a composite core prepared for ametal–ceramic crown.

Occlusal reduction

The shape of the prepared occlusal surface shouldfollow the general contours of the original toothsurface. In some cases, with heavily worn teeth,this will be flat, but in others the general shapeof the cusps should be reproduced. This allowsthe crown to be of reasonably uniform thicknesswith minimum preparation of tooth tissue.

A convenient instrument to prepare theocclusal surface is a dome-ended parallel-sideddiamond bur held on its side. With this instru-ment it is possible to form the cuspal inclinestogether with a rounded shape to the fissurepattern.

The occlusal relationships of the tooth beingprepared should be studied in function. Forexample, in preparing a posterior tooth, if theguidance in lateral excursion is carried by thetooth being prepared, the cusp (or cusps) thatcarry this guidance should be prepared rathermore, so that there is a greater thickness ofcrown material covering them. This will producegreater strength in this stressed area and will alsoallow for future wear. In these circumstances thecusp in question is known as the ‘functional cusp’.However, in most natural dentitions the poste-rior teeth disclude in lateral excursion and sonone of the cusps can be described as ‘functional’in the same sense. Therefore they do not needto be reduced any more than the remainder ofthe occlusal surface.

The term ‘functional cusp’ is sometimes usedinaccurately as an anatomical term. For examplethe buccal cusps of lower molar teeth aresometimes called ‘functional cusps’ even if theydisclude in lateral excursions. They would not bedescribed as ‘functional’ on an extracted tooth.‘Functional’ is a physiological term. It is moreimportant to assess what the cusp does ratherthan where it is before deciding whether it needsto be reduced more than the other cusps.

One advantage of minimizing the reduction ofthe occlusal surface is to maintain the axial wallsof the preparation as long as possible, therebyimproving retention.

The amount of reduction will depend on thematerial being used on the occlusal surface. Metalocclusal surfaces should be at least 0.5 mm thick,or more if they are in occlusal contact in a patientwho grinds their teeth. Metal–ceramic crownsrequire a minimum of 1.5 mm and all ceramiccrowns 2 mm.

Axial reduction

Buccal and lingual surfaces

These may be prepared with parallel-sided ortapered diamonds of appropriate length and withthe end shaped to produce the required shape ofmargin. It may be possible to use a diamond ofknown taper held at a constant angle on thebuccal and lingual sides, so that the taper of thepreparation can be controlled. However, this



Figure 6.9

Crown preparation for a metal–ceramic crown.

a The pin-retained composite core has been present for severalmonths. The appearance is better than an amalgam core.

b and c The finished preparation.

often has to be modified because of the curva-ture of the tooth’s surface, previous restorationsor the presence of a core. A fairly large diame-ter instrument is convenient and reduces thelikelihood of vertical ridges in the preparation.

Mesial and distal surfaces

These are the most difficult surfaces to prepareif there is an adjacent tooth in contact; withoutone, they are prepared like the buccal and lingualsurfaces. Sometimes both adjacent posteriorteeth are to be crowned, and then the surfacesin contact should be prepared simultaneously, thereduction of each being minimized.

Unfortunately, damage to adjacent teeth iscommon, with some studies showing over 90% ofadjacent teeth damaged even by careful operatorswho knew their work would be inspected. Whenthe preparation is finished the adjacent toothsurface should always be checked for damage, andif necessary smoothed, polished and fluorideapplied.

It is almost impossible to prepare the approx-imal surfaces of a posterior tooth when there areteeth in contact on either side without eitherover-tapering the preparation, removing moretooth tissue than is desirable or damaging theadjacent teeth (Figures 3.12 and 6.7).

Very thin long tapered diamond burs arepassed through the approximal surface in anattempt to leave a sliver of enamel (or core)between the bur and the adjacent tooth.Controlling the angle, position and depth of thisbur without wavering or going off course is oneof the most skilful procedures in operativedentistry and deserves many hours of practice onextracted teeth in models before it is attemptedin the mouth.

A matrix band may be applied to the adjacenttooth to protect it, but this interferes with visionand access, and is in any case cut through veryeasily. A wooden wedge at the gingival margin toseparate the teeth slightly may help.

Margins

The shape of the margin will be determined bythe shape of the end of the bur used for the axialreduction. This may be flat, producing a shoulder,

or chamfered if the bur has a rounded end. Aknife-edge finishing line is produced by the side ofthe bur only being used, the tip not cutting thetooth. It is more efficient to produce the requiredshape of margins during the bulk preparationstage rather than as a secondary procedure.

Finishing

Suitable finishing instruments are used asdescribed on page 140. It is important that theangles between the axial and occlusal surfaces arerounded for reasons described in Chapter 3.

Preparing teeth for completeanterior crowns

Figure 6.10 shows stages in preparing an upperincisor tooth for a ceramic crown. The stages ofpreparation of a tooth for a metal–ceramic crownare similar, although the end result is ratherdifferent, complying with the principles describedin Chapter 5. If a tapered or parallel-sideddiamond bur of appropriate length and diameteris selected, the first three stages of the prepara-tion can all be carried out with the same instru-ment, with only the incisal-palatal reduction andfinishing left to be done with different instru-ments.

Incisal and proximal reduction

When only one tooth is being prepared the incisalsurface can be reduced with the shank end of thetapered diamond bur and the adjacent teeth usedas a guide to the amount of reduction necessary.When a series of teeth is being prepared, eitheralternate teeth are reduced first with the unpre-pared teeth used as a guide, or half the incisaledge is reduced followed by the second half tothe same depth.

In patients with a Class I incisor relationshipthe upper incisor teeth have their incisal edgesinclined lingually and the lower incisors labially.The same inclinations are preserved in theprepared teeth.



Figure 6.10

Stages in the preparation of upper incisors for ceramiccrowns. The indication for crowns was progressiveerosion of the buccal surfaces and unsightly restora-tions that rapidly discoloured after replacement. Thefirst three stages were carried out with a long-tapereddiamond bur. Finishing was with plain-cut tungstencarbide burs in a 1:4 ratio speed increasing contra-anglehandpiece. The stages in the preparation formetal–ceramic crowns would be very similar exceptthat the palatal surfaces would be prepared with asuitably shaped bur.

a Reference grooves are cut in the buccal and incisalsurfaces to establish the depth of the preparation.

b Distal surfaces being prepared. Note that a sliver ofenamel has been preserved at the contact point of thelateral incisor to protect the canine from damage. Thiswill fall away as the preparation is carried further gingi-vally.

c The incisal reduction of the central incisor has beencompleted together with half the incisal reduction ofthe lateral incisor.

d Palatal reduction with a round-edged wheel bur inan airotor.

Approximal reduction may be continued withthe same bur. Because so much more tooth isbeing removed than is necessary for a posteriormetal crown and since incisor teeth are a morefavourable shape and the buccal/lingual dimensionat the contact point is smaller, it is much easierto prepare the approximal surfaces withoutdamaging the adjacent teeth than in the case ofposterior crowns. Passing the bur through themesial and distal approximal surfaces (leaving asliver of enamel) establishes the taper of thesesurfaces as well as the location and width of theapproximal shoulders.

Labial reduction

The contour and depth of the labial shoulder isestablished with the tip of a diamond bur. A

common mistake in preparing upper incisor teethfor crowns is to remove insufficient material fromthe labial/incisal third of the preparation becausethe labial surface is curved rather than straightand so the tooth reduction needs to follow thetooth contour in all planes. This results either ina crown that is too thin, so that the opaque corematerial shows through (Figure 3.2, page 55), orin a bulbous crown. The amount of tooth reduc-tion in this area can be fixed by a buccal depthindicator groove being cut down the buccalsurface and the remainder of the surface reducedto the same depth. With large teeth or where thealignment of the buccal surface is being altered,more than one groove may be needed. In reduc-ing the remainder of the surface, the bur shouldbe used at an angle to the depth groove toprevent it dropping into the groove and deepen-ing it unintentionally. A sectioned putty matrix isvery useful.


e Checking, with the teeth in occlusion, that there issufficient clearance for porcelain.

f The finished preparations. The right lateral incisorhas lost a mesial composite. This defect will be madegood with glass ionomer cement before the impressionis taken. The finished crowns are shown in Figure 2.1a(page 30).

g A silicone putty matrix made from a diagnostic waxup is held against the teeth being prepared to ensurethat the correct amount of tooth reduction has beencarried out. There is adequate space for a metal-ceramic restoration on the premolar tooth but furtherpreparation is required to the mid-buccal area of themolar tooth behind if a metal-ceramic restoration is tobe provided to the contour of the diagnostic wax-up.

Gingival-palatal reduction

For a ceramic crown the same bur is continuedround the palatal surface, producing the palatalshoulder and a short gingival palatal wall nearlyparallel to the buccal-gingival surface.

These three stages, using the same bur, can allbe carried out very quickly provided that theoperator has planned the design properly and hasthought through the sequence.

Incisal-palatal reduction

This surface is usually concave and is bestprepared with a large diameter instrument, forexample a large wheel bur in the air turbine(Figure 6.10d). Small instruments produce anundulating surface, which is difficult to finishsmoothly. The occlusion between this surface andthe opposing teeth should be checked before thepreparation starts, and constantly recheckedduring preparation until sufficient space has beenproduced for the crown material.

Finishing

The prepared individual surfaces should beblended into each other to produce a roundedshape during the gross reduction. The axial

surfaces are finished and the angles around theincisal edge rounded, using a suitable finishinginstrument. An excellent finish can be producedby using a plain-cut tungsten carbide friction gripbur in a 1:4 speed increasing contra-anglehandpiece. The shoulder can be finished using thesame instrument or steel burs (Figure 6.11).

Preparing anterior teeth for postcrowns

There are three stages:

• The shoulder or other margin is prepared• The post hole is prepared• Any remaining dentine between the two is

reduced as necessary.

The margin is prepared as for a ceramic ormetal–ceramic crown, but because there is nopulp to protect, there can be more reduction sothat the shoulders are wider than for an equiva-lent vital tooth and therefore the crown thicker.

Post hole preparation

Removing the root canal filling

When the root canal filling consists of guttapercha (GP) and sealer, the coronal part may be


Figure 6.11

Three burs used to finish a shoulder preparation. Left: a steel slow-speedbur cutting both on the side and at the end. Centre: an end-cutting burthat has produced a ledge; it would be difficult to eradicate this withoutlifting the bur from the shoulder. End-cutting burs are now seldom used.Right: a plain-cut tungsten carbide tapered side and end-cutting bur,which is best used in a friction grip 1:4 speed increasing handpiece. Thetungsten carbide bur produces the best finish most conveniently.

removed with Gates-Glidden or round burs(Figure 6.12) or by softening it with heatedmetal instruments. Provided that the root fillingis well condensed, a convenient method is tocut out the GP root filling with a slowly rotat-ing round bur or twist drill slightly larger indiameter than the root canal. If too small aninstrument is used, or too fast a speed so thatthe GP melts, it becomes attached to the burand the whole of the root filling may be pulledout when the bur is removed. Using a bur ordrill slightly larger than the root canal enablesthe root filling to be cut away from its end

without the sides of the GP point becomingentangled in the bur. Extra long shank contra-angle burs are useful in long teeth. With normallength burs the head of the handpiece clasheswith the adjacent teeth.

GP and most sealers are softer than dentine,and so the bur will tend to follow the root fillingrather than cut into the side of the root canal,but nevertheless great care must be taken toensure that the bur stays on course. Regularinspection of the root canal using both the mouthmirror and direct vision is essential (Figure 6.13).Magnification will also help.


a

b

Figure 6.12

Post-hole preparation.

a Long-shank round burs (left) and Gates-Glidden burs for removinggutta percha from root canals.

b Extra long shank contra-angle burs allow access without the head ofthe handpiece clashing with adjacent teeth. They also improve visibility.


Figure 6.13

a A handpiece-driven twist drill to prepare a parallel-sided post hole.

b The prepared post hole and a preformed stainless-steel post being tried in.


c The posthole is 9 mm long.

d The post is shortened from its apical end to preservethe retentive tag for the core, and is tried in again.

e Glass ionomer luting cement is spun down the posthole with a rotary paste filler.


f The post cemented with glass ionomer cement.

g Light-cured composite is built up freehand.

h The completed core.

At deeper levels in the root it is wise to switchto a Gates-Glidden bur which has a blunt tip,reducing the risk of the bur going out of the longaxis of the root canal. This is followed by theappropriate diameter twist drill to match thepreformed post.

Whichever instrument is chosen it must beused at slow speed and removed frequently sothat the end of the hole can be inspected toensure that it is centred on the root filling. Theair turbine should never be used to remove GProot fillings.

Some cement fillings are more difficult toremove than GP because they set to a consis-tency harder than dentine so that the bur tendsto slip away from the root filling into the dentine.In this case the coronal end of the root filling canbe removed with a long tapered bur in the airturbine, but great care is needed to avoid lateralperforation of the root.

It is almost impossible to cut down full-lengthsilver point root fillings, and these should beremoved intact, if possible, and replaced by GProot fillings. When the silver point cannot beremoved, an alternative form of core should beused.

Shaping the post hole

The post hole needs to be shaped to match thepost selected (the different types of post weredescribed in Chapter 2). When the post is to beparallel-sided, a twist drill may be used from theoutset, and the root filling is removed and thepost hole shaped in a single operation. In somecases, once the root filling is removed, it may bedecided that a larger diameter post is needed, andso the next size of twist drill is then used toshape the post hole. Figure 6.13 shows the stagesin preparing a parallel-sided post hole and makingthe post and core.

For a tapered post hole for a cast metal post,an instrument such as that shown in Figure 6.12bis used. This not only produces the taper but mayalso be moved side to side to produce an oval-shaped canal, following the shape of the tooth.This increases the strength of the post whileleaving a uniform thickness of root (Figure 6.14).A selection of instruments, and sections of theteeth prepared using them, are illustrated inFigure 6.15.

Finishing the preparation

Once the margin, the remaining axial walls andthe post hole have been prepared, there mayremain a substantial collar of dentine, some spursor none at all. Substantial amounts of dentineshould be left, since they lengthen the post holeand define the margins. Fragile fragments,however, should be removed.

Completed post crown preparations with postsand cores in place are shown in Figure 3.19 (page74).

Temporary crowns

For a description of the difference between‘temporary’ and ‘provisional’ crowns and bridges,see page 155. Temporary crowns are describedat this stage because in a normal clinical sequence,once a crown preparation has been started, itmust be completed at least in terms of grossreduction at the same visit and a temporarycrown fitted. Often it will be possible to proceedto impressions and other stages at the same visit,but these can be deferred if necessary. Thetemporary crown, however, cannot be deferred.

Preformed temporary crowns

Polycarbonate temporary crowns

The appropriate size of a temporary crown hasalready been selected during the planning stage,and Figure 6.16 shows the stages in preparing andmodifying a polycarbonate crown for an uppercanine tooth. The crown is relined with a tem-porary crown resin.

Once the polycarbonate crown has beenrelined, it can be adjusted for incisal length, occlu-sion and marginal fit. It does not matter if thepolycarbonate is ground right through, as long asa layer of the lining material remains.

Stainless-steel temporary crowns

These are difficult to adapt and often do notproduce good contact points or occlusal contact.



Figure 6.14

a Looking up the post hole with direct vision. Note that the oval shapeof this post hole will resist rotation of the post within the hole. Notetoo that the crown margin has been exposed using electrosurgery. Thiswas necessary here because of caries beneath the previous crown. An‘anti-rotation’ notch has also been prepared. The idea was that thisprevented the post losing retention by rotating in the post hole.However, clinical evidence suggests that these notches are more likelyto be the starting point of a longitudinal root fracture. Therefore theyshould not be used.

b and c A tooth prepared for a cast post, retaining asmuch coronal dentine as possible. The remaining guttapercha can be seen at the base of the post hole.


d The completed cast post and core which has been made with a burnout plastic post for the apical two-thirds, producing a parallel-sided postwith serrations for increased retention. The coronal part is irregular inshape to fit the shape of the root canal. The core supplements the remain-ing dentine to provide adequate retention for the crown. The post is12 mm in length.

e and f The post and core cemented. A radiograph shows the post to becentred in the root canal and extends about half way down the root asthis is a long rooted tooth. In a shorter root the post may extend up totwo-thirds of the root.

d

e

f


a b c d e

Figure 6.15

a A sectioned, extracted tooth showing the preparationfor a parallel-sided post hole. The system being used isthe same as that shown in Figure 2.5b. This is a 1.75 mmdiameter drill. It is rather too big for this size of toothand is veering towards the side. Ideally the post holeshould be longer but, if continued in this direction, therewould be a risk of lateral perforation of the root.

b This tooth is thinner and the drill is 1.25 mm diame-ter; however, it is progressing down the centre of thetooth with less risk of lateral perforation.

c A sectioned, extracted tooth with a stainless-steel postand composite core in place. The composite core is about half the preparation. (The stainless-steel post is paral-lel and longer but part of it and the root filling have been lost in the sectioning process.)

d A radiograph of a post in a root-filled upper lateral incisor tooth. The tooth is an abutment for a bridge andthe bridge had been present for many years. The length and diameter of the post are suitable for this size andshape of root.

e A tapered post hole cutter with good side-cutting ability. It has been tilted back and forth to produce a taperedpost which is larger at the neck than the diameter of the bur. This is often necessary when caries has progresseddown the root canal or when a previous post was present. An impression and cast post will be necessary. As thispreparation inevitably weakens the tooth, the root face has been prepared with an external bevel so that a completediaphragm can be cast together with the post and core.

f A parallel-sided post with a very fine self-tapping thread. Top: the post. Centre: the wrench which locks onto thehead of the post so that it can be screwed in and then the wrench released. Bottom: the matching twist drill.

f


Figure 6.16

Polycarbonate temporary crowns.

a Temporary crown being tried in.

b Trimmed and relined with a temporary crown and bridge material.

c The temporary crown 2 weeks later.


Figure 6.17

Chair-side temporary crowns: injection techniques.

a The patient shown in Figure 6.10; the buccal surfacesof the central incisors are reshaped with wax in themouth.

b A temporary crown and bridge material beinginjected into an alginate impression of the modifiedteeth.

c The temporary crowns before being removed fromthe mouth. Note the thin flash which is easy to trim,leaving a good marginal fit.

d Flexible PVC slip vacuum-formed to the study cast for a differentpatient.


e The partly set material (in this case a differentmaterial) is removed from the mouth still in the mould.

f The completed temporary crowns for the secondpatient.

g This patient has lost extensive amounts of enameland dentine following many years of gastric reflux. Toimprove the appearance ceramic crowns are to beprovided.

h and i A diagnostic wax-up has been made to showthe patient the proposed end result and a silicone puttymatrix made for the temporary crowns.


j, k and l Following preparation of the teeth the puttymatrix is used to make chair-side temporary crownswhich are the same shape as the diagnostic wax-up.Temporary crown material is syringed into the matrixand placed over the preparations. It is important thatthe margins are trimmed to a good fit. The temporaryrestorations are in one piece for strength.

m The completed Procera porcelain crowns.


Figure 6.18

a–d A similar technique to Figure 6.17 using a mouldproduced in a laboratory silicone putty material andtemporary crown and bridge material mixed in anautomix gun and injected directly into the impression.

a The silicone mould made on a study cast modifiedwith wax.

b The automix gun with a very fine nozzle.

c Inserting the material.

d Ready to be placed in the mouth.

They are, however, hard and durable and can beleft in place for some time. The margins aretrimmed with stones and contoured with pliers,and the temporary crown is then cemented,usually with a rigid cement such as zinc phosphateor a reinforced zinc oxide eugenol cement.

Aluminium crown forms

Being softer, these are more readily adapted tofit contact points and occlusal contacts, but themargins are irritant to the soft tissues and aredifficult to clean so that the gingival margin


Figure 6.19

Chair-side temporary crowns: moulding technique.

a A temporary crown and bridge higher acrylic mixedto a dough consistency is moulded over the preparedtooth and the patient is asked to occlude into it.

b When nearly set it is eased off the preparation andout of the undercuts between adjacent teeth. In thiscase the fit surface was satisfactory; in others it mayneed to be relined with a further mix of material afterthe exterior surface has been trimmed.

c The temporary crown trimmed to a good fit at themargins. Articulating paper is being used to adjust theocclusion.

becomes inflamed making impressions difficult.Also some patients complain of a metallic taste.Although they are quick to make, they are lesssatisfactory than adjustable chair-side plastic orrelined polycarbonate temporary posteriorcrowns. Aluminium and stainless-steel temporarycrowns are sometimes preferred when minimaltooth preparations have been carried out formetal crowns and temporary crowns made fromacrylic would be too thin to last.

Chair-side techniques for makingtemporary crowns

Injection or pouring techniques

This is the temporary restoration of choice as itis made specifically for the prepared tooth. Itproduces a more accurate fit than pre-formedcrowns. Modern materials are strong in thinsection and can be added to with composite, theyhave a good appearance and a fast setting time.The mould used may be a preformed celluloidcrown form, a thin PVC slip vacuum formed onthe patient’s study cast (or a modified study cast),or a silicone putty or an impression taken priorto starting the preparation. Figure 6.17 illustratestypical techniques using an alginate impressionand a PVC slip with two different higher acrylics.Figure 6.18 shows a similar technique with a puttyimpression of a modified study cast and a higheracrylic in an automix gun.

Moulding techniques

Some of the higher acrylics go through a doughstage when they can be moulded rather like putty.In this consistency they can be formed into tempor-ary crowns simply by moulding over the preparedtooth with the fingers and the patient biting into itto establish the occlusion. Gross excesses will bepresent, but these can be removed by roughcarving in the mouth and then with an acrylic burin a straight handpiece once the crown has beenremoved and has become hard (Figure 6.19). Thisis a useful technique, particularly for posteriorteeth where the shape of the tooth to be prepared(often a core) is to be changed and so is notsuitable for copying in a chair-side impression.

A temporary crown made by this mouldingtechnique will have better contact points, occlusalcontact and marginal adaptation than analuminium crown form. There is no need tomodify the study model or make a vacuum-formed PVC slip and so it is an effective andefficient technique.

However the temporary crowns are made it isimportant that they should not be overbuilt at themargins so they will not interfere with oralhygiene. The temporary crown needs to have agood contact point with the adjacent teeth andthe occluding surface needs to be adjusted tomaintain contact with the opposing tooth.

Temporary post crown techniques

Some manufacturers supply temporary posts withtheir kits. An example of an aluminium temporarypost is shown in Figure 6.15. For tapered postholes, temporary posts may be made from wiremodified with acrylic before the temporary crownis added to the wire by one of the techniquesdescribed in the previous section (Figure 6.20).

Differences between temporaryand provisional crowns

It is useful to make a distinction between ‘tem-porary’ and ‘provisional’ crowns (and bridges).Temporary restorations are made to last for ashort while to protect the prepared dentine, tomaintain the appearance, and to prevent tilting orover-eruption of the prepared tooth by maintain-ing contact points and occlusion. Because they aretemporary, they are usually made by one of therelatively simple chair-side techniques describedabove and are cemented with a temporary crownand bridge cement.

Provisional restorations also have all thesefunctions, but are made to last for a longer periodwhile other treatment is being provided beforethe permanent restorations can be made or whena period of assessment is necessary. For example,if the patient has periodontal disease associatedwith poor margins on existing restorations, provi-sional restorations may be made with well-adapted margins and left for some time until the


treatment of the periodontal disease iscompleted. Similarly, when the occlusion is beingmodified, for example by increasing the OVD,provisional restorations will be left in place forsome months to assess the patient’s tolerance ofthis change before the new occlusion is finallyestablished by permanent restorations. Duringthis time, the occlusion can be modified by

occlusal adjustment or by additions to therestorations.

Long-term provisional restorations are alsoused when significant changes are being made toanterior teeth. They allow for a period of accom-modation to the change to assess the responseto appearance, function and speech. The provi-sional restorations can be altered by addition or


Figure 6.20

Temporary-post crowns.

a The same preparation as shown in Figure 6.14a after initial gingivalhealing.

b The thickest possible length of serrated German silver wire is tried in the root canal,coated with a higher acrylic and inserted into the post hole. When nearly set it iswithdrawn and reseated a number of times to prevent the possibility of the postjamming and not coming out. After excess material has been trimmed, the coronal partof the temporary crown is added using one of the techniques described earlier. In thiscase the polycarbonate temporary crown is illustrated in Figure 6.16.

grinding until the patient is happy with the resultand then a copy model made to help the labora-tory with the appearance of the final crown.

Laboratory-made provisionalrestorations

Many of the temporary crown and bridge materi-als are capable of lasting in the mouth longenough to function as provisional restorations.They can therefore be made at the chair side bythe same techniques as have been described fortemporary restorations.

Alternatively, the teeth are prepared, anaccurate impression is taken and temporaryrestorations are made at the chair side. Theimpression is then used to make provisionalrestorations in the laboratory, or sometimes asimple casting is made to which acrylic orcomposite is added (Figure 11.8). The expense ofthese is seldom justified for individual crowns,particularly as reinforced composite techniquesare now available which produce very satisfactoryand less expensive long-term provisional restora-tions but metal/composite provisional bridgesmay be needed for long span bridges.

Cementation of temporary andprovisional crowns

The retention of the temporary or provisionalrestoration and the likely dislodging forces shouldbe assessed and a cement of appropriate strengthselected. The following list of temporary cementsand their appropriate use is arranged in ascend-ing order of strength:

• A non-setting mixture of petroleum jelly andzinc oxide powder – used for short periodsbetween appointments, for example, forcementing temporary crowns when teeth areprepared and impressions taken in the morningand laboratory-processed provisional crownsfitted with a stronger cement in the afternoon.

• Temporary crown and bridge cement with ahigh proportion of modifier to reduce thestrength – used when several temporarycrowns are joined together, giving considerable

overall retention; this may be done eventhough the permanent crowns will be separate(Figure 6.17).

• Unmodified temporary crown and bridgecement – used for individual complete crownsthat will have to stay in place for periods of upto 2–3 weeks. Most materials contain eugenolwhich will be obtundent to the tooth followingpreparation. If adhesive bonding techniques areto be used for the final crown a non-eugenolcement should be used.

• Polycarboxylate and zinc phosphate cements –used with poorly fitting temporary crowns, forexample aluminium temporary crowns, orwhere the temporary crown has to last for anextended period, for example laboratory-madeprovisional crowns fitted for periods oforthodontic or periodontal treatment.

After temporary crowns have been cemented, itis important that surplus cement is removed,otherwise irritation of the gingival margin andplaque retention will produce gingival inflamma-tion.

The working impression

The working impression is the very accurateimpression from which a cast with removable diesis made. The crown is made on the removabledie of the prepared tooth. The impression shouldinclude not only an accurate impression of theprepared tooth but also the adjacent teeth so thatthe contact points and occlusal surfaces of thecrown may be contoured. It should also includethe remaining teeth in the arch so that theworking cast can be articulated against the oppos-ing cast. This usually means that it should be a fullarch impression.

Impression materials (Figure 6.21)

There are two groups of materials used forcrown and bridge impressions: elastomericmaterials (silicone or polyether (Figure 6.21a, band c)) and reversible hydrocolloid (Figure 6.21d).The elastomeric materials set by a chemicalreaction when two materials, usually two pastes,



Figure 6.21

Impressions for crowns and bridges in various materials.

a The impression of the patient shown in Figure 6.20in addition-curing silicone: light- and heavy-bodytechnique. For the impression of the post, the light-body material is spun down the post hole with a spiralroot canal paste filler. This is rotated during removal.A thin reinforcing wire is inserted to stiffen the impres-sion and prevent it bending when the die is cast.

b A different brand of addition-curing silicone showingthe impressions for the patient in Figure 6.10.

c An impression in polyether in a stock tray.

d Reversible hydrocolloid in a water-cooled tray.


e An automix gun with a standard nozzle adapted witha fine curved tip for direct use in the mouth.

f The automix gun being used in the mouth.

g A mixing machine for polyether impression material.

are mixed together. The reversible hydrocolloidis based on agar. It is melted in a water bath andsets on cooling. The teeth must be dry forelastomeric impressions, but may be wet withreversible hydrocolloid.

Elastomeric materials are very accurate and canusually be re-poured if necessary. They areconvenient to use and so have almost entirelyreplaced the reversible hydrocolloids.

Silicone impression materials

Most manufacturers supply addition-curingsilicones in a range of viscosities including putty,heavy-body, regular, light-body and wash. Thismeans that a whole range of techniques is poss-ible using combinations of these materials with orwithout special trays. Light-body material isusually inserted onto the preparation from an


h Material being delivered directly into a stock tray.

i A disposable plastic syringe being used to placeimpression material.

automix gun or syringe (Figure 6.21e, f and i) andthe medium or heavy-body either mixed in asecond gun or on a pad and placed in the tray.Putty is kneaded by hand.

The automix gun used with an extra fine nozzlehas several advantages in placing the light-bodymaterial directly around the preparations (Figure6.21f and g). The material is thoroughly mixedwithout air bubbles, and the mix is very freshwhen it is applied to the tooth preparations. Withlight- and heavy-body materials mixed on pads,the dental nurse usually mixes one material andthe dentist the other. Timing of the two mixes,loading the syringe and then drying and isolatingthe preparations requires very good coordination,and the mixed, light-body material may start toset before it is properly in place. With theautomix system the dentist maintains theprepared teeth in a dry and isolated state andstarts to inject the light-body material at the pointwhere the nurse is loading the tray. However,some operators find that the automix gun is toolarge and clumsy and they prefer to use a smallerdisposable syringe (Figure 6.21i).

Silicone does not wet tooth preparations well.In compensation, it is very clean to use. Toxic andallergic reactions have not been reported.

Hydrophilic impression materials are availablewhich are not so susceptible to the presence ofmoisture.

Polyether impression material

Polyether is convenient since the same materialmay be used in the syringe and the tray, only onemix being required, although light and heavyviscosities are also available. It is also best usedin thick sections, and so should be used in stocktrays; or if a special tray is used, it should bemade with extra thick space between the studycast and the tray.

An automatic mixing machine is available forpolyether (Figure 6.21g and h).

Reversible hydrocolloid

This was available long before the elastomericmaterials were developed, and it largely fell intodisuse with their introduction. However, therehave been sporadic revivals of interest in it. It hasthe advantage of being usable in a wet environ-ment.


Figure 6.22

Elastomeric impressions should not be relined with afurther mix of material once they are set unless all theundercut areas are cut away.

a A preliminary impression taken without a spacer.

b If this impression is relined with a second mix ofmaterial, distortion will occur: the second layer ofmaterial in the undercut areas of the unprepared teethwill distort the original material while it is in the mouth.When the impression is removed this will return to itsoriginal shape, distorting the impression of theprepared tooth. For a two-stage technique, a spacer ofpolythene or similar material should be used.Alternatively the primary impression should be cutback with a scalpel or burs, and in particular all theinterdental areas and impression of any undercutsurfaces removed.

Impression techniques

Single material technique (e.g. polyether)

When a single viscosity material is used, thematerial is mixed, and part of it placed in animpression syringe and the remainder in theimpression tray, usually a stock tray. The materialis syringed over the dry tooth preparation andthe tray is immediately seated in place. With astock tray that has no occlusal stops, it is impor-tant to localize the tray carefully and avoid seatingit too far so that it does not contact the preparedtooth.

Two viscosity material technique (e.g.light- and heavy-body silicone)

Two sets of material are mixed: a low viscositymaterial that is syringed around the preparations,and a heavier viscosity material used in theimpression tray and seated in the mouth beforethe light-body material has set. The light-bodymaterial is thus forced into intimate contact withthe preparation and gingival crevice.

A special tray with occlusal stops is preferablyused with the elastomeric materials.

Putty and wash (e.g. silicone)

This is a modification of the two viscositytechnique with a low or medium viscositymaterial used in the automix gun and a puttymaterial in the tray. Because of the viscosity ofthe putty material a large amount is needed andso a stock tray is usually used. This technique istherefore popular because the cost of a specialtray is saved, but it is difficult to achieve resultsof the same quality as other techniques becausethe pressure needed to seat the tray can distortit (Figure 6.22).

Polymer materials and gloves

One disadvantage of putty materials is that someof the gloves worn by dentists react with thematerial and prevent it setting. It is thereforeoften necessary for the dentist or nurse who will

be mixing the putty material to remove theirgloves and wash their hands before mixing it. Themost convenient method is to use one of thepolymer materials available in an automix gun.

Gingival retraction

It is essential that the impression has a clearrecord of the entire preparation margin and justinto the crevice beyond the margin so that thedie can be accurately trimmed and the techniciancan see the exact extent of the margin.

The ideal is to start with gingival health andsupragingival crown margins. Gingival retraction isnot then needed, impression taking is easier andmore reliable, but most importantly, gingivalhealth is easy for the patient to maintain.However, it is often necessary to retract thegingival tissues in order to obtain an impressionof the tooth surface beneath the gingival margin.This will always be necessary if the preparationmargin is subgingival. It will also be desirable if itis close to or at the gingival margin. This isbecause the crown contour at the peripheryshould be in line with the tooth surface to avoida plaque-retentive crevice at the margin. This canonly be achieved if an impression of the toothsurface is obtained for some distance beyond thepreparation margin.

There are five ways of retracting the gingivalmargins (in ascending order of potential damage):

• Blowing the impression material into the gingi-val crevice with vigorous blasts of air.

• Syringing kaolin-based materials into thecrevice prior to taking the impression. Theseabsorb moisture and cause retraction of thegingiva.

• Temporarily retracting the gingival margin withcord.

• Using cords impregnated with chemicals.• Electrosurgery.

Compressed air (Figure 6.23a and b)

With a healthy gingival margin undamaged by thepreparation it is often sufficient to blow theimpression material into the crevice with air. Thistechnique works best with light-body and wash



Figure 6.23

Gingival retraction.

a A crown preparation with the mesial margin levelwith the gingival margin.

b The mesial–gingival margin being retracted solely byblowing air into the gingival crevice. With light-bodiedelastomeric impression materials this is often all theretraction that is needed.

c A kaolin-based gingival retraction paste is syringedaround a prepared tooth.


d Following thorough washing of the preparation thegingival tissue is not bleeding and has retracted a littlefrom the tooth, allowing access for impression material.This will not be adequate for deep preparations.

f Gingival retraction with adrenaline-impregnatedbraided cord. These are the preparations shown inFigure 6.10f. The palatal margins are supragingival, andgingival retraction is only necessary on the labial andproximal surfaces.

e A clear impression has resulted.


g An instrument designed to insert gingival retractioncord.

h The instrument in use.

i Double cord for an impression for veneers. The gingi-val tissue has retracted and it is time to pull out thetop cord.

j Double cord retraction for multiple preparations.

silicones and with polyether. The viscosity andwetting ability of the material are critical.

Kaolin pastes (Figure 6.23c, d and e)

Minor degrees of gingival bleeding or access tomargins only just subgingival can be made clearand accessible by syringing the paste into thecrevice and leaving it undisturbed for 4–5minutes. The kaolin absorbs any fluid and soexpands pushing back the gingiva. After washingout the paste, the crevice is opened for theimpression material.

Cord and impregnated cord (Figure 6.23d)

If cord is to be used, it is usually impregnatedeither with adrenaline, which acts as a vasocon-strictor assisting in gingival retraction and inarresting any minor gingival haemorrhage, or withan astringent material such as aluminium trichlor-ide, which functions in a similar way. However,some operators prefer plain unimpregnated cord.

Cords are available in various thicknesses, bothtwisted and braided. Braided cords are preferredsince they do not unravel while they are beinginserted and newer cords have fine copper wirewithin them to keep their shape and stay put inthe crevice.

A cord of appropriate diameter is pressedlightly into the gingival crevice with a suitableinstrument, for example a flat plastic type or oneof the special instruments designed for thepurpose (Figure 6.23g and h). It may not benecessary to retract the gum all the way roundthe tooth if part of the preparation margin is suffi-ciently supragingival. The cord is left in place for2 or 3 minutes and then removed before theimpression is taken. If it is left for too short atime, gingival retraction is inadequate; if it is leftfor too long, the chemicals diffuse and becomeinactive. Too much force should not be used, orpermanent damage to the gingival tissues mayresult.

Another technique is to use a very thin cordpressed into the base of the gingival crevice anda thicker cord placed on top of it. Usually onlythe thick cord is removed before the impressionis taken (Figure 6.23i and j).

Electrosurgery (Figures 6.14a and 6.20a)

Electrosurgery can be used to arrest gingivalhaemorrhage before impression taking and toestablish a distinct gingival crevice, exposing asubgingival preparation margin. This techniqueshould be reserved for unusual situations, forexample where a tooth has been fractured withthe fracture line extending subgingivally and animpression is required in order to make a post,core and diaphragm. Further gingival re-contour-ing may be carried out surgically once the crownis fitted if necessary.

Occlusal records

An occlusal record is not always necessary (seeChapter 4). In some cases an intercuspal position(ICP) record is all that is required. In others acombination of retruded contact position (RCP)and left, right and protrusive excursion records,together with a facebow, may be needed.

The intra-occlusal records may be taken in asilicone material, wax, or wax and temporarycement.

Silicone occlusal records

These have become the most popular recordbecause of their accuracy and convenience.These advantages outweigh the additional cost.The materials and techniques were discussed inChapter 4. The material can be delivered by anautomix gun with a shaped nozzle so that a flat,broad band of material is laid directly from thegun over the occlusal surfaces of the lowerteeth.

It is important that the record is trimmed,removing most of the buccal and lingual excessand sometimes the interdental areas so that themodels can be seen to fit closely into the record(see Chapter 4).

Wax occlusal records

Ordinary pink wax can be used, but a betterchoice is wax made for the purpose which is soft


just above mouth temperature but becomes muchharder when cooled. The wax is softened in aflame or in hot water and shaped to the approx-imate size of the study cast. It is laid on the lowerteeth and the jaw is closed into the requiredposition. The wax is allowed to cool or is chilledwith water and then removed.

Wax records are liable to distort and may needto be readapted. This may be done by thoroughcooling outside the mouth and relining with atemporary crown and bridge cement (Figure6.24a and b).

Removing temporary crowns

At the next appointment the temporary crownneeds to be removed from the prepared tooth.With a weak cement this should be easy withgentle pressure applied to the margin with anexcavator or similar instrument. Thicker, well

fitting crowns may be better sectioned throughrather than risk damaging the margin. It issometimes possible to carry this out without theneed for a local anaesthetic.

Before removing the temporary crown thepermanent crown should be inspected on themodel to confirm it is as prescribed.

Trying in the permanent crown

Safety precautions

Small slippery objects like crowns tend to slip outof gloved fingers, especially when wet. They areeven more prone to slip out of tweezers, whichshould never be used.

The dangers of dropping a crown down thepatient’s throat are obvious. If it is inhaled, this isa serious medical emergency and the patientshould be rapidly inverted and encouraged to


Figure 6.24

Occlusal records.

a A full arch wax occlusal record modified by theaddition of a rapidly setting temporary crown andbridge cement to the upper and lower surfaces. Notethat the wax extends across the palate, supporting thetwo sides.

b Excess cement has been trimmed away from thelower side of the record with a scalpel so that whenthe casts are seated, very precise location of the lowercast within the record can be seen. Note that thispatient has an anterior open bite, making location ofthe casts without an occlusal record difficult.

cough. If this is not successful, the patient shouldbe immediately taken to hospital for the crownto be removed.

If the crown is swallowed, this is less danger-ous – and also less dangerous than swallowing asharp instrument such as an endodontic file.However, radiographs should usually be taken andif possible the crown recovered by the patientwhen it is passed to reassure the patient it haspassed safely. The patient should be advised touse a sieve and running water to find the crownin the faeces. Figure 6.25 shows an abdominalradiograph with a crown in the colon.

Various precautions are possible:

• With practice and experience it is possible tocontrol even small inlays and crowns bykeeping the gloves dry and the tooth wellisolated and dry. One finger should be keptbehind the crown at all times. A competentdental nurse with a wide-bore high-volumeaspirator should be at the ready.

• Gauze or sponge packs may be placed behindthe area where the crown is being tried in.These are theoretically a good idea, but withsome patients the irritation at the back of themouth makes them consciously suppress thecough reflex so that if a foreign object dropsbehind the pack, the risk of it being inhaledrather than swallowed may be increased.

• The patient may have treatment in an uprightposition and be told to lean forward, if thecrown drops, and cough it out.

• In some cases it is advisable to try in crownsunder rubber dam, but it is difficult to assessthe margins if clamps are used, and impossibleto judge the gingival relationship or occlusion.

The checking procedure

As pointed out in Chapter 5, a gold crown is triedin, adjusted if necessary and then cemented.

The metal part of a metal–ceramic crown maybe tried in before the porcelain is added and thenreturned to the laboratory and retried with theporcelain before being finally cemented.

At the try-in stage the following checks shouldbe made, together with any necessary adjust-ments.

Checking and adjusting the fit

The marginal fit is checked by eye and with asharp probe. Gaps, overhanging margins (positiveledges) and deficiencies (negative ledges) may bepresent (Figure 6.26).

A uniform gap all the way round indicates thatthe crown is not fully seated. Having checked forretained temporary cement or trapped gingivaltissue, a firmer seating force should be applied,and if the gap persists, the contact points shouldbe checked with dental floss. If, after any neces-sary adjustment to these, the crown still does not


Figure 6.25

A crown that has been swallowed at the try-in stage.It is now at the top of the descending colon, and waspassed 24 hours after this radiograph was taken. It wasrecovered, sterilized and cemented.

seat, it should be removed and the fit surfaceinspected. If it is metal, burnish marks on the axialwalls may show where the crown is binding.These are ground lightly with a bur or stone andthe crown is retried.

If there is some improvement but not completeseating, the fit surface should be lightly grit-blasted and reseated. If no burnish marks appear,it is likely that the margins or occlusal surface arepreventing complete seating. A common cause isa slightly over-trimmed die (Figure 6.27). If theinside of occlusal surface is suspected, disclosingwax (a very soft wax) is melted into the crown,which is seated before the wax sets. When thecrown is removed, high spots will show as perfo-rations of the opaque wax.

Fine powder suspensions in aerosol sprays orpainted colloidal graphite are also used to showwhere tight crowns are binding, but they are veryuntidy materials.

If there is a gap around only one part of thecrown, it may be seating unevenly because of atight contact point; otherwise the impression ordie may have been distorted.

A positive ledge should be adjusted until theprobe passes smoothly from tooth to crownwithout a catch. A negative ledge is a biggerproblem and often means that the crown has tobe remade.

Checking retention

A crown should not feel tight. A crown for a longpreparation with optimum taper, which will haveexcellent retention when cemented, may simplydrop off the preparation when tried in. A feelingof tightness is the result of unnecessary rough-ness of the preparation or a casting that has beendistorted. Tightness of fit is not a reliable test ofretention, and tight crowns may be more difficultto cement, resulting in an open margin.

The crown should be tested for a tendency totilt or pivot when rocked from side to side. Smallpivoting movements show that the crown is notfully seated and is rocking about the contactpoints or on high spots on the fit surface – inwhich case the margins should be checked again.


Figure 6.26

a A crown with a large positive ledge or overhang.This should not be cemented in this condition. Thedistal margin is a better fit, but the surface is bulbousand overcontoured, encroaching on the embrasurespace. Compare the contour of the distal surface of thecrown with the mesial surface of the tooth behind.

b A negative ledge or short crown margin. There is nogap. All the other restoration margins on this radio-graph are also overhanging or defective in some way.

Alternatively, there is too much space betweenthe crown and the tooth. This may be due to theexcessive use of die relief (a varnish spacerpainted on to the die, avoiding the margins), anover-expanded casting or one that has had its fitsurface ground.

Checking and adjusting contact points andaxial contours

Dental floss should be used to check that thecontact points are neither too tight nor too slack.Tight contacts can be lightly ground a little at atime and polished; any deficiency in porcelainshould have more porcelain added.

Buccal and lingual contours should not be toobulbous, the marginal area should be in line withthe tooth surface to reduce plaque retention andthe surface should look natural. Again, adjust-ments may be made by removing or addingmaterials and re-polishing or glazing.

Checking and adjusting the shade

Shades that are slightly too light (the chroma toolow) can be darkened by adding stain of appro-priate colour and re-firing. Stain can also be usedto add missing characteristics such as crack linesor mottled areas. However, if the basic hue iswrong or the chroma too dark, or the fault liesin the colour of the opaque core material or the‘dentine’ porcelain, it is often not possible tochange the shade sufficiently. The crown has to

be remade if it is a ceramic crown or the porce-lain removed and replaced if it is a metal–ceramiccrown.

Checking and adjusting the occlusion

See Chapter 4 for details of occlusal adjustments.If reduction is necessary, the thickness of theocclusal surface should be checked with magnify-ing calipers (Figure 6.28).


Figure 6.27

A common cause of crowns not seating. The impres-sion clearly shows the margin of the preparation butdoes not extend very far beyond it. It is often moredifficult to distinguish the margin on the stone die thanon the impression, particularly when dust from thetrimming obscures vision. This leads to an over-trimmed die. To be on the ‘safe side’, the technicianextends the crown beyond the margin, producing abevel. This fits the die well, but when tried in themouth the tiny bevel perches on top of the preparedmargin, preventing the crown from fully seating. It mayappear to fit in the over-trimmed area, but a gap willbe present elsewhere.

Cementation

When all the checks and adjustments arecomplete, the crown is permanently cemented. Itis possible to provisionally cement a permanentcrown to allow the patient to assess it for appear-ance and function. If this is done the temporarycement should be chosen so that the crown canbe easily removed without damaging it. Usually aheavily modified temporary cement is used andthe patient only left with this for a short time toreduce the risk of the crown falling off.

Choice of cements

Cements used for permanent cementationinclude:

• Zinc phosphate cement• Resin-based and adhesive cements• Glass ionomer cement.

Zinc phosphate cement

Zinc phosphate has been in use as a luting cementfor much longer than all the others. Although itsacidity has been assumed to be irritant to thepulp, literally millions of crowns have beencemented with it, with a very low proportion ofclinically detectable ill effects. Patients sometimescomplain of transient discomfort when thecement is setting if a local anaesthetic is not used,

but most patients need a local anaesthetic forcrown cementation anyway and so this is not amajor problem. However, the potentially irritantnature of the cement remains an anxiety. Thepulps of some teeth fitted with crowns dobecome inflamed and eventually necrotic. Thisalso happens occasionally with other cements,and it is difficult to identify the cause of pulpdeath. Was it the cement, the effects of prepar-ing the tooth, or the original condition for whicha crown was necessary?

Zinc phosphate has two major advantageswhich probably account for its continued popular-ity. It has a long, controllable working time and itproduces the thinnest cement film, which can beas little as 10 micrometres. Of course, this is stillten times the diameter of the micro-organismsthat lodge at the periphery of the cement film toform plaque.

Resin-based and adhesive cements

A variety of resin-based luting materials are avail-able. Although their use is increasing they are stillnot commonly used with conventional crowns fora number of reasons. First the truly adhesivecements containing either 4-META or a phospho-nate derivative are inhibited from setting by thepresence of oxygen. The margins of the restora-tion therefore have to be coated with an aqueousjelly material until the cement sets, and thencleaning up the surplus cement is difficult. Asecond reason is that, although they are initiallymore adhesive than the established luting


Figure 6.28

A pair of callipers which magnify 1:10 being used tomeasure the thickness of the occlusal surface of ametal–ceramic crown. It is about 1 mm thick at thispoint.

cements, they have not been used for longenough for dentists to be sure of their long-termsuccess.

Therefore the resin-based and adhesivecements are used for luting porcelain veneers andminimum-preparation bridges (see Chapter 8),but are not yet commonly used for cementingconventional crowns. These materials are beingdeveloped rapidly and so their use may increasesignificantly in the future.

Glass ionomer cements

Glass ionomer luting cement is popular withsome dentists It adheres to dentine and enamel,it leaches fluoride and is relatively non-irritant tothe pulp. However, it has a higher solubility thanother cements. It is available in capsulated formso the correct consistency of mix is alwaysachieved.

Cementation technique

Preparing the crown

The crown should be completely cleaned of alltraces of polish, disclosing wax, saliva and so on.This is best done in an ultrasonic cleaning bath,or if this is not available by scrubbing with atoothbrush and detergent. The crown should bethoroughly dried with blasts of air.

Preparing the tooth

The tooth should be thoroughly washed withwater spray and gently dried with air; it shouldnot be over-dried, since this may damage thepulp by desiccation. The washing and dryingshould be left until the last minute to avoidcontamination of the surface by saliva or gingivalexudate.

Mixing and applying the cement

The cement should be mixed according to themanufacturer’s instructions. With zinc phosphatecement, slow mixing of small increments of

powder on a cool glass slab, over a wide area,will increase the working and setting time. Thiswill also allow the pH to rise a little before thecement is applied to the tooth.

The cement is applied to the hollow part. Witha complete crown this is the fit surface of thecrown, while with a post it is the post hole in thetooth. When the opposite member is insertedinto the hollow, the cement coats it and isextruded from the margins. If the other surfaceis coated with cement, for example the toothpreparation for a complete crown, it may bescraped off the surface when the crown is seated,and part of the surface left bare of cement.

The walls of a post hole may be coated usinga rotary paste filler or file.

Nothing is gained by coating both surfaces.Time is lost, so that the cement becomes moreviscous by the time the crown is seated, result-ing in a thicker cement layer. The entire surfaceshould be coated quickly with plenty of surpluscement. Any benefit that might be gained byapplying a thin, even coat of cement is lostthrough the extra time taken to achieve this, withthe cement beginning to set and getting thicker.

Inserting the crown

The crown should be seated quickly and pressedhome with firm, continuous force to extrude allthe excess cement from the margins. Thepressure may be applied by the operator or bythe patient biting on a suitable prop, such as acotton wool roll. Pressure should be maintainedand the area kept dry with cotton rolls orabsorbent pads and aspiration until the cementhas set. Excess cement is also left until the set iscomplete and it is then removed.

Oral hygiene instruction andmaintenance by the patient

A final and important stage is to teach the patienthow to clean and maintain the crown, and inparticular how to clean the marginal area. Dentalfloss and an appropriate toothbrush techniqueshould be advised.

Some patients already have excellent oralhygiene, and too much emphasis on the


importance of cleaning around the crownedtooth may result in over-enthusiastic cleaning,causing damage to the gingival tissues or to thetooth.

Recall, assessment, maintenanceand repair

Assessment

A systematic assessment of all crowns should bemade at each recall examination. This shouldinclude evaluation of the following factors.

Oral hygiene

Plaque levels and gingival inflammation around thecrown should be compared with similar teethelsewhere in the mouth. If the crowned tooth isworse, the reason should be investigated anddealt with. In any case, when it is present,periodontal disease should be treated.

Margins

The crown margins should be examined forpositive and negative ledges and gaps, and prep-aration margins should be examined forsecondary caries and signs of abrasive wear.

Structure of the crown

This should be examined for fractures and wear,including occlusal perforations.

Appearance

The appearance of the crown or the adjacentteeth may have altered since it was fitted. Anychange should be assessed as acceptable orunacceptable. In the latter case the crown willusually have to be replaced: apart from grinding,little can be done to alter the crown’s appearanceonce it is cemented.

Radiographs

Periapical radiographs need only be taken if thereare signs of failure or symptoms from the tooth.Routine screening radiographs for caries shouldbe taken for the crown and for rest of the denti-tion at a frequency determined by the usualassessment of the patient’s caries risk level.

Adjustments and repairs to crowns insitu

These are dealt with in Chapter 14, together withbridges.


Part 2 Bridges

The number of bridges made in the UK NationalHealth Service (NHS) increased nearly 20-fold inthe decade between 1980 and 1990. Since thenthe numbers have fallen although the averagedentist working in the NHS still makes 34 bridgesa year. Evidence from Commercial DentalLaboratories shows that there is considerablegrowth in the number of bridges being madeprivately, although detailed national figures arenot available. It is probable that the overallnumber of bridges being made is still rising. Largeincreases are also reported in many othercountries.

Clinical experience suggests that patients who,in the past, would have been prepared to wear asmall partial denture are now reluctant to do soand ask for a bridge or implant-supported prosthe-sis. Implants are not available under the GeneralDental Services of the NHS and are generally moreexpensive than bridges. The result is that atpresent in the UK many more bridges are beingmade than implant-supported prostheses.

General terminology

The terminology used for bridges is sometimesrather loosely applied, and in different parts of theworld the same terms are used to describe differ-

ent things. The word ‘bridge’ itself is used in theUK to describe a fixed appliance only, whereas inparts of the world it also includes certain tooth-borne removable appliances.

The following names will be used for thevarious appliances. The terms in parentheses arethose commonly used in the USA, althoughAmerican terminology is rather variable.

• A conventional bridge (fixed partialdenture) is an appliance replacing one or moreteeth that cannot be removed by the patient(Figure 7.1a). The general term ‘fixed bridge’ isavoided since it implies one of the specificdesigns of bridge (see Chapter 8). Substantialtooth preparation is necessary for a conven-tional bridge. The bridge usually occupies nomore space than the original dentition.

• A minimum-preparation bridge (resin-bonded bridge, adhesive bridge,Maryland bridge) is attached to the surfaceof minimally prepared (or unprepared) naturalteeth and therefore occupies more space thanthe original dentition (Figure 7.1b).

• A removable bridge is very much the sameas a bridge in that it is retained by crowns, isentirely tooth-supported, does not replace softtissue and, unless it is examined closely,appears to be the same as a bridge. However,it can be removed by the patient (Figure 7.2).

177

7 Indications forbridges comparedwith partialdentures andimplant-retainedprostheses

178 Indications for bridges compared with partial dentures and implant-retained prostheses

Figure 7.1

Bridges.

a A conventional bridge replacing the upper rightlateral incisor with a single artificial premolar toothfilling the space between the canine and first molarteeth. The bridge has just been cemented. The gingivalcondition around the molar abutment is good, butaround the canine it is inflamed buccally as a result ofirritation from a broken temporary bridge in this area.

b and c A cantilever minimum-preparation bridgereplacing a central incisor tooth.

d and e Two separate minimum-preparation cantileverbridges replacing two lower incisor teeth. One issupported by the canine and the other by the tworemaining incisors splinted together. This design islikely to have a much better prognosis than afixed–fixed design which would not allow anymovement of the abutment teeth. The bridges hadbeen in place for 9 years when this photograph wastaken.

Indications for bridges compared with partial dentures and implant-retained prostheses 179

Figure 7.2

A removable bridge.

a Cast copings permanently cemented to the remain-ing teeth. The external surfaces of these are milledparallel to each other in the laboratory.

b The removable bridge, which the patient can takeout himself.

Figure 7.3

Partial dentures.

a A precision-attachment retained partial denture. Inthis case the two premolar teeth on the right of thepicture are splinted together and an intra-coronalprecision attachment is incorporated into the distalsurface of the second premolar. The first molar on theleft is an artificial tooth – part of a bridge – and it toocontains a precision attachment. The partial dentureretained by these two attachments can be removed bythe patient.

b A conventional cobalt-chromium partial upperdenture that is tooth-supported with rests, clasps anda major palatal connector. None of the metal work isvisible from the front of the mouth.


Figure 7.4

a A cross-section through a typical implant. Systems vary and so thisone will not be described in detail but it is typical in having fourelements: from the top down the coarsely threaded screw is theimplant which is screwed into a tapped hole in the bone and thencovered to osseo-integrate; the transmucosal abutment is smooth-sided and retained into the osseo-integrated fixture by the middle-sized screw; the small screw at the bottom holds the restoration (theprosthetic elements) to the implant. There is now a wide range ofprosthetic elements which will not be described here.

b, c and d A single tooth implant replacing the upperright lateral incisor.

b Shows the stage after the second surgical procedureto expose the fixture and to place the healingabutment. The healing abutment is in place and theincision line mesial and distal to it can still be seen.

c The healing abutment has been replaced by the trans-mucosal abutment (TMA).


d A crown has been fitted to the TMA.

e Four fixtures in the lower jaw.

f A three-unit ‘bridge’ has been attached to the threeimplants on the left and on the right, a three-unitcantilever bridge retained by the natural canine toothand the implant has been extended with a cantileverpremolar pontic.

g A three unit bridge retained by two fixtures showingthe screws which can be undone if necessary by thedentist. These are covered with composite until it isnecessary to gain access to the screws.

• A partial denture may be rested entirely onteeth, or be supported by the soft tissues, orby a combination of both. Rest seats arecommonly used, but otherwise it is usually notnecessary to prepare the natural teeth exten-sively. Partial dentures are retained by clasps,by adhesion to the soft tissues, or by dental orsoft tissue undercuts (Figure 7.3b).

• A precision-attachment partial denture isretained by proprietary attachments and isremovable by the patient. Soft tissue elementsare replaced and the appliance usually hasstructures that pass across the oral tissues, forexample across the palate or around the lingualalveolus. Natural teeth have to be preparedand crowns or other restorations made forthem, incorporating part of the precisionattachment (Figure 7.3a).

• An implant-retained prosthesis is oneretained by osseo-integrated implants (see Figure7.4a). A single implant may support a single toothprosthesis (Figure 7.4b, c and d) or a series ofimplants may support a prosthesis replacing anumber of teeth. This is usually known as animplant-supported bridge. The patient cannotremove it, but in some cases the dentist can, byundoing the screws holding the prosthesis to theimplants (Figure 7.4e, f and g). Implant-supportedbridges may be small, replacing only one or twoteeth, or may be larger, including replacing all theteeth in one arch. Implants may also be used tosupport a bar (or other attachments) on towhich a removable complete overdenture can beclipped. Overdentures are beyond the scope ofthis book.

The term ‘fixture’ is sometimes used to describethe osseointegrated part of the implant, but isalso sometimes used to describe the wholeimplant assembly. It is helpful to use the follow-ing terms:

• Implant to describe the part that osseointe-grates and that is buried beneath the gingivaltissues (in most systems) for a period ofmonths before exposing it and inserting the

• Transmucosal abutment (TMA), which isthe part of the implant that attaches to thefixture and passes through the gingival tissuesto the mouth. To this, is attached the

• Restoration which replaces the missing toothor teeth.

General advantages anddisadvantages of replacing missingteeth

It is not always necessary to replace missingteeth, and in some cases there are positive disad-vantages in doing so. At one time there was arather naive, simplistic view that the mouth wasa ‘functioning machine’ and that if part of it wasmissing, it was rather like a tooth or teeth missingfrom a cogwheel in a piece of machinery such asa car gearbox. This is not the case because thehuman body is much more adaptable and flexiblethan machinery engineered by man. In fact thereis reasonably good evidence that, with a moderndiet, it is perfectly possible to function with nomolar teeth at all provided that the first andsecond premolar teeth and incisors are allpresent in the upper and lower jaws and are inocclusion. This has become known as the ‘short-ened dental arch’. Some patients cope very wellwith this but others feel that they cannot eatproperly or are concerned about the appearanceof the space behind the premolar teeth. With ashortened dental arch it is important that all theremaining teeth have good occlusal contact, orover-eruption and tilting will occur.

Despite this evidence, many patients wouldprefer to have at least some of their missing teethreplaced. It is the dentist’s role, as a professionaladviser, to advise the patient whether or not it isreally in their best interest to have a tooth or teethreplaced. In some cases it is wise for a dentist torefuse to replace missing teeth, particularly bymeans that are likely to give rise to problemselsewhere in the mouth, or if the prosthesis has apoor prognosis, even if the patient attempts toinsist that a replacement should be made. This isprimarily for the patient’s benefit but also for thedentist’s. There have been a number of dento-legalcases in which dentists have been successfully suedfor making prostheses, particularly bridges andimplant-retained prostheses, when it was againstthe dentist’s better judgement and the prosthesishas subsequently failed.

The first big decision that therefore must bemade jointly by the dentist and patient is ‘shouldthe missing tooth/teeth be replaced or not?’

It is necessary for both the dentist and thepatient – and in some cases a third party finan-cially involved with the transaction – to be


convinced that the replacement will producesignificantly more benefit than harm. The follow-ing questions must be asked:

• How will the patient’s general or dental well-being be improved by the replacement?– What disadvantages will the replacement

bring with it?– What is the ratio of these advantages and

disadvantages?• If the balance is strongly in favour of replace-

ment, should the replacement be by means of:– A bridge– A partial denture– A removable bridge– An implant-retained prosthesis.(Of these, a bridge or a partial denture are byfar the most common.)

Advantages of replacing missing teeth

Appearance

For many patients with teeth missing in theanterior part of the mouth, appearance is anoverriding consideration. For them a replacementis certainly necessary. Just as with crowns, it isalso necessary to judge the appearance of gapsfurther back in the mouth, taking account of theanatomy and movement of the patient’s mouth.

Ability to eat

Many patients manage to eat quite successfullywith large numbers of teeth missing. Patients withno lower molar teeth who are fitted with well-designed and well-constructed partial lowerdentures frequently leave them out because theyclaim that it is easier to eat without them. Somepatients, though, have a genuine and persistentfeeling of awkwardness if they are deprived ofeven one posterior tooth. As with appearance,the patient’s concept of the problem is as impor-tant in deciding on a replacement as the problemitself. Generally though, the more teeth that aremissing, the more important is a replacement. Ifa posterior tooth is to be lost it is generally sens-ible for the patient to live with the space forseveral months following extraction to determine

how essential it was before a decision is maderegarding its replacement.

Occlusal stability

This was discussed in Chapter 4, where it wasalso made clear that in many cases – althoughocclusal stability is lost initially when teeth areextracted – tilting and over-eruption usuallyeventually lead to an occlusal relationship that,although it may not be ideal and may containocclusal interferences, is nevertheless stable. Ifthe missing teeth can be replaced before thetooth movements occur and when toothmovements are likely, this may well be sufficientjustification for the replacement. In many cases,however, the patient is first seen some years afterthe extraction and has a new stable relationship.Replacement of the missing teeth would thereforenot improve the stability and so is not justified(see Figure 7.14). Special occlusal considerationsare discussed under orthodontic retention andalterations to the OVD (see later).

Other advantages

The three advantages listed above are by far themost common indications for replacing missingteeth. The following, though less common, can beextremely important for individual patients:

Speech Patients concerned about the quality oftheir speech are usually also concerned abouttheir appearance. The upper incisor teeth are themost important in modifying speech, and so whenthey are missing they will usually be replaced toimprove both speech and appearance.

Periodontal splinting Following the successfultreatment of advanced periodontal disease, it maybe necessary to splint uncomfortably mobileteeth. In order to produce a cross-arch splintingeffect it is necessary to bridge any gaps to providea continuous splint – whether or not there areany other indications for replacing the missingteeth (Figure 7.5).

A feeling of ‘completeness’ Some patientsbelieve, or have been told, that there is a major


disadvantage to having teeth missing, even whenthey have no problems with appearance, eating orocclusal stability. These patients appear to receiveconsiderable comfort from a bridge – less from aremovable appliance. This feeling should not bediscounted if it is held with conviction, eventhough the dentist may not be equally convincedof the benefits of a bridge. However, suchattitudes should not be encouraged.

Orthodontic retention Most orthodontictreatment is stable, but it is occasionally neces-sary to provide a bridge partly to maintain anorthodontic result. A common example is incases where the lateral incisors are congenitallymissing and the upper canines have beenretracted to recreate space for them. The mainreason for replacing the missing lateral incisors is,of course, appearance, but a second reason is toprevent the canine teeth relapsing forwards again,and so the bridges must be designed to serve thispurpose. The resulting appearance is usuallybetter than attempts at converting the appear-ance of the canines to lateral incisors.

Another example is in patients with cleftpalates who have been treated orthodontically aswell as surgically (Figure 7.6).

Orthodontic retention is a special example ofan indication for tooth replacement for reasonsof occlusal stability. In almost all patients whohave taken the trouble to have orthodontic treat-ment, appearance will also be important.

Restoring occlusal vertical dimension(OVD) Occlusal collapse with excessive wear ordrifting of the incisor teeth sometimes follows theloss of a number of posterior teeth. This is a diffi-cult problem to treat, but in some cases theposterior teeth are replaced by bridges, remov-able dentures or implants that not only replacethe missing teeth but restore the lost OVD,creating space for the upper incisors to beretracted or crowned as necessary.

Wind-instrument players Players of brass orreed instruments contract the oral musculatureto form what is known as the embouchure. Thisallows for the proper supply of air to the instru-ment. Even minor variations in the shape of theteeth can affect the embouchure, and missingteeth can have a disastrous effect on the musicproduced by some players.

With brass instruments the mouthpiece issupported indirectly by the teeth, via pressure on


Figure 7.5

a A 12-unit bridge supported by 6 teeth with consid-erably reduced periodontal support. Several teeth wereuncomfortably mobile before the provisional bridgewas fitted. The patient was able to maintain good oralhygiene following periodontal therapy, and the provi-sional bridge was replaced after a year by this perma-nent bridge. The patient understood the reasons forthe visible supragingival margins and accepted them.The bridge lasted just over 20 years before it had tobe removed and the abutment teeth extracted.

b Radiographs of the three abutment teeth on the left-hand side for the patient shown in a at the time thebridge was fitted.

the lip. Clearly with these patients not only is thereplacement of any missing teeth essential but abridge or an implant-supported restoration willusually be necessary. This must be designed verycarefully to reproduce as much of the originalcontours of the missing teeth as possible.

With reed instruments the mouthpiece isinside the musician’s mouth and any pressurefrom it is to the palatal side of the upper incisors.

Disadvantages of replacing missingteeth

Damage to tooth and pulp

In preparing teeth for conventional bridges orprecision-attachment partial dentures, it may benecessary to remove substantial amounts ofhealthy tooth tissue. This damage, although it maybe justified if the indications are powerful enough,should not be undertaken lightly. The problem isless serious if the teeth to be used to support thebridge are already heavily restored or crowned.

Whenever a tooth is prepared, there is adanger to the pulp, even if proper precautionssuch as cooling the bur are followed. There issometimes an additional threat to the pulp whenteeth are prepared for bridges. With somedesigns, preparations for two or more teeth haveto be made parallel to each other, and if the teethare slightly out of alignment, the attempt to make

the preparations parallel may involve more reduc-tion in one part of the tooth than if it were fora crown and so endanger the pulp.

With the falling incidence of caries in manycountries, and a more conservative approach torestorative dentistry, situations arise more andmore commonly in which the logical abutmentteeth for a bridge are sound and unrestored orhave minimal restorations. To prepare theseteeth would be very destructive, and this is onereason why the minimum-preparation bridge andimplant-retained prostheses are becoming sopopular.

Secondary caries

As with all restorations, bridges carry the risk ofmicroleakage and caries. This risk is more signif-icant (particularly dento-legally) if the restorationis an elective one rather than the result of caries.

Failures

Chapter 14 contains a black museum of failuresamong crowns, bridges and implant-retainedprostheses. Provided that the bridge is wellplanned and executed and the patient is taughtproper maintenance and is conscientious, thechances of failure are small. However, there isalways an element of risk, and this must beexplained to the patient.


Figure 7.6

A surgically repaired cleft lip and palate with missinglateral incisor to be replaced by a bridge. The palatalgingival inflammation is exacerbated by the temporarydenture – an additional indication for a bridge.

How long do bridges last?

This is an impossible question to answer for theindividual patient and bridge. Some start with abetter chance of survival than others. Chapter 14gives a more extensive discussion of restorationfailure.

Effects on the periodontium

There is ample evidence that subgingival crownmargins, whether or not as part of a bridge,increase the likelihood of gingival inflammation.Although there is less evidence that thisprogresses to destructive periodontal disease, itis an unwelcome factor. Even poorly maintainedsupragingival crown margins can produceperiodontal effects, as can restrictions of embra-sure spaces and reducing access by the presenceof a bridge. The average partial denture is capableof causing even more significant periodontaldamage.

Some patients, given extensive courses oftreatment, even if without special oral hygieneinstruction, still improve their plaque control.Presumably their increased dental awarenessimproves their motivation. Although this changeseems to have long-lasting effects, it is obviouslynot a justification on its own for making bridges.

Cost and discomfort

Many patients regard these as the most impor-tant disadvantages in any tooth replacement.Bridges, dentures and implant-retained prosthesescannot be made other than by the individualattention of the dentist and the technician.Personal service of this type will always be expen-sive. The cost of partial dentures does not signif-icantly rise in proportion to the number ofmissing teeth, whereas the cost of a bridge tendsto do so.

Implant treatment is usually charged perfixture, irrespective of the number of artificialteeth supported by each implant. Traditionally,the charges were significantly higher than thosefor bridges, which are in turn higher than thosefor partial dentures. However, this has changedover the years, partly due to increasing costs for

conventional bridges and increasing use ofimplants. The reason for this ratio is more to dowith the perception by both patient and dentistof the skill and training required of the dentistand technician for each of the procedures,together with a concept that the more ‘sophisti-cated’ procedures must cost more. This approachis based on historical and traditional views and isnot particularly logical. There is nothing intrinsi-cally more difficult or demanding about any of thetechniques as compared with the others. Ifanything, bridges are more demanding because,once they are permanently cemented, it is diffi-cult to make any significant modifications.However, removable dentures and implant-retained prostheses that can be unscrewed andremoved by the dentist give more potential formodifications after they are fitted.

Comparing typical fees and calculating approx-imate fees per hour for the three treatmentmethods of partial dentures, bridges and implant-retained prostheses, and taking the cost forreplacing one tooth with a partial denture as thebaseline, replacing it with a bridge would cost twoto three times as much and with an implant-retained prosthesis three to four times as much,even allowing for the additional items that mustbe purchased for the implants.

Dental treatment is much less painful thansome patients imagine, but there is nearly alwayssome discomfort, such as holding the mouth openfor long periods and difficulty in controlling fluidsin the mouth. If the treatment is carried outunder sedation, there are the disadvantages of thepatient needing to be accompanied, being unableto drive after the appointment, and so on.

The choice between fixed andremovable prostheses

General considerations

Patient attitude

Patients show different degrees of enthusiasm forfixed and removable prostheses. Unless the patientis particularly anxious to have a bridge or implant-retained prosthesis and fully understands the impli-cations, it is often better, particularly when anumber of teeth are missing, to make a partial


denture first to see how the patient responds. Itmay be that the denture is satisfactory, bothaesthetically and functionally. If so, the destructiveand irreversible tooth preparations that may benecessary for a bridge or surgical procedures forimplants can be avoided, or at least deferred.

Alternatively, if the patient is unhappy with thepartial denture, he or she will enter into thearrangements for making a bridge or implant-retained prostheses with greater enthusiasm andcommitment. They will have had experience ofthe alternative to a bridge or implant-retained


Figure 7.7

a Original study cast and diagnostic wax-ups for apatient who lost the upper right central incisor in anaccident in his early teens. Rather than maintain thespace, the lateral incisor was moved into the positionof the central incisor and the canine tooth into theposition of the lateral incisor. On the patient’s left thefirst premolar was extracted since the teeth werecrowded, and so the midline was maintained. This studycast was made when the patient was 16 and wasbecoming very concerned about his appearance. Theshape of the lateral incisor had been modified withcomposite, but this and the canine tooth were stillunattractive. The appearance of simply crowning thelateral incisor to make it resemble a central incisor isshown in the first diagnostic wax-up. This is also notsatisfactory, and so the second diagnostic wax-upshows the effects of extracting the upper right firstpremolar, retracting the canine tooth and the lateralincisor orthodontically and making a bridge to replacethe central incisor tooth. However, the patient was notprepared to have further orthodontic treatment toachieve this ideal result, and so the bottom diagnosticwax-up shows the appearance that would be achievedby extracting the lateral incisor and making a conven-tional three-unit fixed–fixed bridge. This is a verydestructive approach, but was justified in this case inview of the patient’s considerable anxiety over hisappearance.

b The bridge in place. The patient is happy with thisappearance and considers the treatment to be success-ful.

prosthesis and therefore have better informationupon which to base their consent to a moreinterventive and expensive procedure.

Patients should never be persuaded to havebridges or implants against their wishes, and theymust give fully informed consent, includingenough time to reflect. Some patients take longerto make their minds up, or to plan the financialimplications. It is prudent to put the summary oftreatment proposed with the alternatives alongwith the full costs in simple, short but not patron-izing written form to allow the patient time toconsider the plan before starting treatment.

Age and sex

Similar arguments apply to bridges as to crowns(see Chapter 3). However, whereas there may beno satisfactory alternative to a crown for an oldor young patient, a partial denture may make avery satisfactory alternative to a bridge or implant-retained prostheses. This is particularly true forvery young patients, who may not fully appreciatethe lifelong implications of bridges or implants. Itis better to make a minimum-preparation bridgeor partial denture until the patient is matureenough to assess the relative merits of the alter-natives. But a teenager with a missing incisor whocannot be fitted with a minimum-preparationbridge may be desperately unhappy about wearinga partial denture. In this case, the provision of aconventional bridge or single tooth implant as earlyas possible may make a remarkable psychologicaldifference (Figures 7.4b, c and d and Figure 7.7).

At the other end of the age scale, no patient,however old, should be written off as being pasthaving a bridge. Figure 9.4c shows an immediateinsertion provisional bridge made for a spritely 76-year-old who would have been appalled at the ideaof wearing a partial denture. Many patients 10 ormore years older than this would have the sameattitude. Figure 9.4c was published in the first editionof this textbook and the patient is still well andhappily wearing the permanent bridge made whenthe two lower incisor sockets had fully healed. Thebridge has so far lasted for more than 17 years and,like the patient (now 93), is still going strong.

Confidence

Many patients feel more confident with a bridgethan with any form of removable appliance.

However retentive a partial denture, somepatients never lose the anxiety that it will becomedislodged during speaking or eating. Others arenot prepared to remove partial dentures at night.

Many patients do tolerate partial dentures verywell, however, and it is often difficult to tellbeforehand what the response will be to eitherform of treatment. The majority of patients whohave had both partial dentures and bridges preferthe latter.

Occupation

Sports players and wind-instrument players havebeen referred to earlier (see Chapter 3). Althoughsports players should be provided with crownswhen necessary, it may be better to defer makingan anterior bridge or implant-retained prosthesesuntil the patient gives up the more violent sports,and meanwhile to provide a partial denture.

Although wind-instrument players usually needa bridge replacement for their missing anteriorteeth, there are some who find that air escapesbeneath and between the teeth of a bridge. Theyare better able to maintain a seal with a partialdenture carrying a buccal flange.

Public speakers and singers who make moreextreme movements of the mouth often need theconfidence that comes from wearing a bridge.

General health

Both bridges and partial dentures are electiveforms of treatment, and need not be provided forpeople who are ill. When tooth replacement isnecessary for someone who will have difficultytolerating it because of poor physical or mentalhealth, or when there are medical complicationssuch as with patients who require antibiotic coverfor every appointment, it is better to consider thesimpler, less time-consuming form of treatmentfirst. Having missing anterior teeth replaced,though, can boost the morale of patients recov-ering from long illnesses or facial trauma.

Appearance

When a tooth is lost, alveolar bone and gingivalcontour are also lost, and it is never possible todisguise this fact entirely (Figures 7.1a and 9.9a).Thus no artificial replacements ever look exactlylike the natural teeth, although some may be


sufficiently realistic to deceive all except thedentist with their bright light and mouth mirror.In some cases dentures with flanges achieve thisobject better than bridges; in others bridges havethe better appearance. When a substantialamount of alveolar bone is lost, the combinationof a bridge with a separate removable buccalflange sometimes gives the best appearance(Figure 7.8).

When the loss of alveolar bone is significantand the lipline is such that it shows and is diffi-cult to disguise easily, the ridge may beaugmented surgically and the tooth or teethreplaced by a bridge or implant. The preferredmaterial is autogenous bone – usually taken fromsomewhere within the patient’s mouth, often thechin, or the maxillary tuberosity or the ramus –but freeze-dried bone or other artificial materialsare available (Figure 7.9).

General dental considerations

Questions of oral hygiene and periodontal healthwere dealt with in relation to crowns in Chapter3 and similar considerations apply to bridges.However, when there are strong indications forreplacing missing teeth in a case where there hasbeen periodontal disease and alveolar bone loss,

provided that the periodontal disease is undercontrol it is preferable to provide a bridgewhenever possible rather than a partial denture.This is because a number of abutment teethsplinted together as part of a bridge have a betterprognosis than individual teeth with reducedalveolar support, which may be mobile, used asdenture abutments.

When only one or two teeth are missing in thearch, a bridge or implant restoration is usuallyconsidered the better solution. When largenumbers of teeth are missing, particularly whenthere are free-end saddles, partial dentures orimplant-retained prostheses are a more logicalchoice. In some cases the preferred treatment isto replace one or two missing anterior teeth witha bridge and the posterior teeth with a partialdenture. This has the advantage that the patientis not embarrassed to leave the denture out atnight and is more confident when wearing itduring the day

Occlusal problems may indicate a bridge ratherthan a partial denture. For example, a missingupper incisor in an Angles Class II Division Imalocclusion with the lower incisors occludingagainst the palate would be difficult to replace bymeans of a partial denture without increasing theocclusal vertical dimension (OVD) or providingorthodontic treatment; a bridge would be morestraightforward (Figure 7.10).


Figure 7.8

a An unsightly appearance following gingival recessionand surgery. The teeth are splinted together.

b A removable acrylic gingival prosthesis in place.


Figure 7.9

Surgical ridge augmentation.

a A minimum preparation bridge with a very unsightlyand unsatisfactory flange in pink porcelain which doesnot match the patient’s gingival pigmentation. It is alsounhygienic.

b After removing the bridge the thin receded ridge canbe seen.

c The alveolar ridge is exposed.

d After augmentation and suturing.

Local dental considerations

The condition of the teeth adjacent to and oppos-ing the missing teeth may help to determinewhether a fixed or removable prosthesis isindicated. When the prognosis of teeth adjacentto the space is doubtful it may be better toprovide a partial denture – at least in the short

term until the prognosis is clearer. The doubtfultooth could then either be used as a support fora bridge or extracted and a larger bridge ordenture constructed. If the angulation or size ofthe teeth adjacent to the space make themunsuitable to support a bridge, it may be betterto provide a partial denture rather than design anunnecessarily elaborate and complex bridge.


e The ridge healed.

f A new minimum-preparation bridge with animproved appearance of the pontics and the augmentedridge. It is also more cleansable.

Figure 7.10

Sectioned study casts of a patient with a completeoverbite, with the lower incisors occluding against thepalate. A denture would not be possible without alter-ing the occlusion, shortening the lower teeth orproviding orthodontic treatment.


Figure 7.11

Cases illustrating indications for bridges.

a A small but visible space in an otherwise healthy andunrestored arch. The indication here is to improve thepatient’s appearance and prevent any further mesialmovement of the posterior teeth, which have alreadyreduced this two-unit space to less than a single toothwidth. Orthodontic treatment to complete the closurewas not possible. A minimum-preparation bridge wasmade.

b The patient, a professional singer, had tried a succes-sion of partial dentures of various designs. She did notlike the appearance of any of them and did not feelsecure about their retention when singing. The bridge,which is shown in Figure 9.4d (page 217), has now beenpresent for more than 30 years and has solved herproblems.

c Substantial alveolar bone loss together with the lossof several teeth following a motorcycle accident in an18-year-old girl. The soft tissue support is not ideallysuited to dentures. The patient was very depressedabout her other facial injuries and felt unable to copewith dentures. The preparations for the lower bridgeare shown in Figure 3.17h (page 72) and the finishedbridge, together with the preparations for the upperbridge, in Figure 8.1a (page 198).

d A patient with developmentally missing upper lateralincisor teeth replaced with minimum-preparationbridges.


e Originally both of fixed/fixed design they haverepeatedly debonded and one has been converted to acantilever design. The patient requests alternativetreatment.

f Long cone periapical radiographs show root conver-gence that does not allow space for implants.Orthodontic treatment to open the spaces wasdeclined by the patient.

g The central incisor teeth are prepared for conven-tional metal–ceramic cantilever bridges.

h Completed bridges.


Figure 7.12

Indications for partial dentures.

a A heavily worn dentition with short clinical crownsand no posterior teeth on one side. A partial dentureis the obvious choice here.

b Several teeth missing as a result of hypodontia in a19-year-old patient. In view of the recent extraction ofthe deciduous teeth together with the apparently highcaries incidence judged from the number and size ofamalgam restorations, and the small size of the anteriorabutment teeth, a partial denture was provided. Anadditional problem in providing a bridge would havebeen the lingual inclination of the first lower molarteeth, making parallel preparation difficult. Implantswould have been a possibility, probably with ridgeaugmentation, if funding was available.

In both a and b the indications for replacing the missingteeth are both aesthetic and functional.

Figure 7.13

Indications for implants.

a and b The upper left lateral incisor and canine teethhave been lost through trauma. The mouth is veryclean and well cared for with no caries or restorations.The ridge is substantial (confirmed by appropriateimaging).

Examples of specific indications forbridges, dentures and implantbridges

Figure 7.11 shows four cases in which bridgeswere preferable, and Figure 7.12 one where apartial denture was the chosen treatment and onewhere it might have been. Figure 7.13 showspatients for whom implant-retained bridges areindicated. Figure 7.14 shows a case where itwould be better to leave the patient with noprosthesis.

Scope of this book

This chapter has dealt with the decisions to bemade about whether the tooth or teeth shouldbe replaced and, if so, whether the replacementshould be by a partial denture, bridge or implant-retained prosthesis. All these alternatives need tobe considered equally at the planning stage andso are included here in some detail.


c The occlusion of the patient shown in a and b. It isunfavourable for a minimum-preparation bridge.Therefore all the indications in this case are for animplant-retained prosthesis.

d In this case only one central incisor tooth is missingbut the gap is much greater than the other centralincisor. The patient had previously had a midlinediastema with which he was content. The mouth is wellcared for with few restorations and a single toothimplant is indicated.

Figure 7.14

This patient had had these spaces for many years and,despite some mesial drift of the lower molar tooth andconsiderable mesial movement of the upper molarteeth – so that space for both premolar teeth was nowless than half a unit – she was not concerned aboutthe appearance, had no difficulty eating, and even theseextensive tooth movements had not produced occlusalinterferences. There is therefore insufficient justifica-tion to replace any of the missing teeth.

The appliances used to replace missing teethwere defined in Chapter 7. Some of the termsused for bridges are also used in relation topartial dentures.

• The saddle is the edentulous alveolar ridgewhere a tooth/teeth are to be replaced.

• An abutment is a tooth to which a bridge (orpartial denture) is attached.

• A retainer is a crown or other restorationthat is cemented to the abutment. The terms‘retainer’ and ‘abutment’ should not beconfused or used interchangeably.

• A wing is a term sometimes used for aminimum preparation retainer.

• A pontic is an artificial tooth as part of abridge.

• A span is the space between natural teeth thatis to be filled by the bridge.

• A pier is an abutment tooth standing betweenand supporting two pontics, each pontic beingattached to a further abutment tooth.

• A unit, when applied to bridgework, meanseither a retainer or a pontic. A bridge with tworetainers and one pontic would therefore be athree-unit bridge.

• A connector (or joint) connects a pontic toa retainer, or two retainers to each other.Connectors may either be fixed or allow somemovement between the components that theyjoin.

Conventional and minimum-preparation bridges

Conventional bridges involve removing toothtissue, or a previous restoration, and replacing itwith a retainer. This may be destructive of toothtissue and will certainly be time-consuming andexpensive. The alternative, minimum-preparationbridge involves attaching pontics via a metal wingto the unprepared (or minimally prepared) lingualand proximal surfaces of adjacent teeth. The

attachment is made by an adhesive resin material,retained by the acid-etch technique to the enameland by other means to the metal. These bridgescan be used only when the abutment teeth havesufficient intact enamel.

Basic designs, combinations andvariations

There are four basic designs of bridge, the differ-ence being the type of support provided at eachends of the pontic. The same name is given tothe design, however many pontics there are inthe span and abutment teeth splinted at one endof the span (Figure 8.1).

The four basic designs are the same whetherthe bridge is a conventional or a minimum-prep-aration type. It is possible to combine two ormore of the four basic designs and to combineconventional and minimum-preparation retainersin the same bridge (the hybrid bridge – see page204).

Of the four basic designs, the first three maybe either conventional or minimum-preparationtypes. Minimum-preparation versions of thespring cantilever bridge are rarely made andconventional spring cantilever bridges are onlymade very rarely. There are better alternatives.However, some patients still have springcantilever bridges which need to be maintained,so dentists need to recognise them.

The four basic designs (Figure 8.1)

Only the first three designs are still made.

Fixed–fixed bridge

A fixed–fixed bridge (Figure 8.1a) has a rigidconnector at both ends of the pontic. The

8 Types of bridge

197

198 Types of bridge

Figure 8.1 Four basic designs of conventional bridges.a Fixed–fixed: both upper and lower bridges will befixed–fixed, the lower retained by complete crowns onthe canine tooth and central incisor (see Figures 7.11cand 3.17h for the preoperative condition and thepreparations). The upper bridge will be retained by thecanine teeth only (see page 227 for the rationale forthis design). The bridge was made before implants weregenerally available but if they had been, a considerableamount of bone grafting would have been necessary.

e Spring cantilever: the first molar tooth is theabutment tooth. There is a midline diastema, and adiastema between the lateral incisor and canine on theside of the missing central incisor. Any other bridgedesign would have involved closing one or both ofthese spaces. Today a single-tooth implant would bethe preferred solution to the problem.

b Fixed–movable: a DO inlay in the lower secondpremolar and full crown on the molar tooth. Thisbridge has been present for 20 years – in fact so longthat the occlusal surface of the crown has wornthrough (see Chapter 14). The movable joint can beseen between the pontic and the minor retainer. Itwould not normally be as obvious as this.

c Cantilever: these are the two metal–ceramic bridgesmade for the case shown in Figure 7.11d–h.

d Occlusal view of the bridges in place.

abutment teeth are therefore rigidly splintedtogether, and for a conventional bridge must beprepared parallel to each other so that the bridge,which is a minimum of three units, can becemented in one piece. The retainers should haveapproximately the same retention as each otherto reduce the risk that forces applied to thebridge will dislodge one retainer from itsabutment, leaving the bridge suspended from theother abutment.

To minimize this risk, it is also important forthe entire occluding surface of all the abutmentteeth for a conventional bridge to be covered bythe retainers. The opposing teeth cannot thencontact the surface of an abutment tooth, depressit in its socket and break the cement lute. If thisshould happen, the retainer will not appear loosesince it will still be held in place by the rest ofthe bridge.

However, oral fluids will enter the spacebetween the retainer and the abutment prepara-tion, and caries will rapidly develop (Figure 8.2).For example, linking a full coverage crown to asimple DO inlay would quickly result in dislodge-ment of the inlay with consequent caries devel-oping underneath.

This rule does not apply so much to minimum-preparation bridges in which the bond betweenthe retainer and the abutment tooth is muchstronger. However, it is sometimes not strongenough, and debonding occurs as a result of amechanism similar to that shown in Figure 8.2.

The fixed–fixed design should be avoided if atall possible for minimum-preparation bridges. Anumber of success – and failure – surveys haveshown that the cantilever design of minimum-preparation bridges with one abutment tooth ismore successful with anterior bridges and a formof fixed–movable design is preferred for posteriorbridges (see later).

At one time it was thought that the supportfor the abutment teeth at each end of afixed–fixed conventional bridge should be similar.In other words, the root surface area of theabutments should be approximately the same.Today this is not considered necessary (seeChapter 10).

Fixed–movable bridge

A fixed–movable conventional bridge (Figure8.1b) has a rigid connector, usually at the distalend of the pontic, and a movable connector thatallows some vertical movement of the mesialabutment tooth. The movable connector shouldresist both separation of the pontic from theretainer and lateral movement of the pontic(Figure 8.3a and b).

Occasionally the fixed and movable connectorsare reversed, but this has a number of disadvan-tages. The retainer with the movable connector(the minor retainer) is smaller and less visible andso is better in the more anterior abutment tooth.

Types of bridge 199

Figure 8.2

An unsatisfactory design for a fixed–fixed bridge.

A conventional fixed–fixed bridge should have all theoccluding surfaces of the abutment teeth protected bythe retainers. Otherwise an occlusal force directed atthe unprotected area will depress the abutment toothin its socket while the retainer is held by the bridgeand the other abutment tooth. This will break downthe cement lute, causing leakage. The retainer is heldin place by the bridge, and so secondary caries devel-ops rapidly (see Figure 14.8).

Posterior teeth commonly tilt mesially, and thistends to unseat distal movable connectors, but isresisted by mesial ones.

The movable connector can be separatedbefore the bridge is cemented, and so the twoparts of the bridge can be cemented separately.The abutment teeth do not therefore have to beprepared parallel to each other and the retentionfor the minor retainer does not need to be asextensive as for the major retainer. Neither does

it need full occlusal protection. Occlusal forcesapplied to the tooth surface not covered by theretainer will depress the tooth in its socket, andthere will be movement at the movable jointrather than rupturing of the cement lute (Figure8.2).

A fixed–movable minimum-preparation bridgecannot have a large, dovetailed shaped movableconnector (Figure 8.3a and b) without over-preparing the minor abutment tooth. Instead the

200 Types of bridge

Figure 8.3

Fixed–movable bridges.

a A conventional bridge with an MOD gold inlay as theminor retainer and a full gold crown as the majorretainer.

b Acrylic burn out patterns for movable connectors.The blue is very tapered, the red more parallel-sided.

c and d A minimum-preparation fixed–movable bridge.

c Shows the minor retainer in place with a depression,rather than a slot, in the distal rest. The preparationsfor both the major and minor retainers are withinenamel.

d Shows the finger from the pontic resting on theminor retainer. This will resist axial forces on thepontic which would tend to tilt the molar abutmenttooth forwards. It does not resist lateral forces butthese can be made insignificant by contouring theocclusion of the pontic.

Types of bridge 201

Figure 8.4

a All-ceramic cantilever bridges. The lateral incisorteeth were congenitally missing and the deciduouscanine teeth remained with good long roots. Theywere therefore used as abutments for these cantileverbridges until such time as the deciduous canine teethare lost. The patient is in her early twenties and thismay not be for a decade or two when implants willprobably be the treatment of choice but will have tolast for less time than if they were placed now.

b and c A minimum-preparation cantilever bridgereplacing the lateral incisor retained by the caninetooth.

d A similar cantilever bridge but this time retained bythe central incisor tooth.

design, which is shown in Figure 8.3c and d, is toallow some movement of both abutment teeth,thereby reducing the risk of debonding, and toresist axial forces on the pontic. The design doesnot resist lateral forces on the pontic or forcestending to distract the pontic from the minorretainer. It should therefore only be used forrelatively short spans in the posterior part of themouth. However, in these circumstances it is nowthe preferred option.

Cantilever bridge

A cantilever bridge (Figure 8.1c and d) providessupport for the pontic at one end only. Thepontic may be attached to a single retainer or totwo or more retainers splinted together, but hasno connection at the other end of the pontic. Theabutment tooth or teeth for a cantilever bridgemay be either mesial or distal to the span, but forsmall bridges they are usually distal.

202 Types of bridge

Figure 8.5

A large splint/bridge with cantilevered pontics.

a The working dies.

b The metal framework, showing two cantileveredpontics on the right of the picture.

c The completed restoration in the mouth(photographed in a mirror so that the cantileveredpontics are shown on the left).

Two conventional cantilever bridges are shownin Figures 8.1c and d. The central incisors neededto be crowned and so a convential rather than aminimum-preparation design was used. Also seeFigure 8.4.

Spring cantilever bridge

Spring cantilever bridges (Figure 8.1e) wererestricted to the replacement of upper incisorteeth. Only one pontic could be supported by aspring cantilever bridge. This was attached to theend of a long metal arm running high into thepalate and then sweeping down to a rigid connec-tor on the palatal side of a single retainer or apair of splinted retainers. The arm was made longand fairly thin so that it was springy, but not sothin that it would deform permanently withnormal occlusal forces (i.e. exceed the elasticlimit). Forces applied to the pontic were absorbedby the springiness of the arm and by displacement

of the soft tissues of the palate so that excessiveleverage forces did not disturb the abutmentteeth. They were made when the teeth eitherside of the gap were sound.

Spring cantilever bridges are no longer madeand have been superseded by minimum-prepar-ation bridges and single-tooth implants. However,a number of patients still have spring cantileverbridges and so dentists should be able to recog-nise them.

Combination designs

The four basic designs can be combined in avariety of ways. In particular, the fixed–fixed andcantilever designs are often combined. In largerbridges additional cantilever pontics may besuspended from the end of a large fixed–fixedsection (Figure 7.5). Similarly, it is possible tocombine fixed–fixed and fixed–movable designs.

Types of bridge 203

Figure 8.6

A hybrid bridge with a conventional retainer (an inlay in the premo-lar tooth carrying a movable connector for a fixed–movable bridge).The other retainer on the canine is of the minimum-preparation type.Hybrid bridges should only be made fixed–movable and with themovable joint in the conventional retainer.

It is possible to combine a bridge or splint witha removable buccal flange that replaces lost alveo-lar tissue (Figure 7.8).

Hybrid design

This term refers to a bridge with a combinationof conventional and minimum-preparation retain-ers. There are three different hybrid designs:

• Fixed–movable with the minimum-preparationretainer carrying the movable connector.

• Fixed–movable with the conventional retainercarrying the movable connector.

• Fixed–fixed with one conventional and oneminimum-preparation retainer.

The third design should not be used and thesecond only rarely. In either case, if the minimum-preparation retainer becomes debonded then itwill not be possible to re-cement it withoutremoving the conventional retainer, which maywell involve destroying the bridge.

The first design is one of choice given circum-stances in which one of the abutment teethalready has a restoration that could be replacedby means of an inlay or other conventionalretainer and the other abutment tooth isunrestored or the restoration does not involvethe surfaces to be covered by the minimum-preparation retainer (Figure 8.6). These circum-stances occur surprisingly often, and so thisdesign of bridge is now common.

Variations

Removable bridges

All the designs described so far are permanentlycemented in the patient’s mouth. With largebridges there are disadvantages in permanentcementation in that the maintenance and furtherendodontic or periodontal treatment ofabutment teeth is difficult, and if something goeswrong with one part of the bridge or with oneof the abutment teeth, usually the whole bridgehas to be sacrificed. For this reason, largerbridges, including full arch bridges, are sometimes

made so that they can be removed by thepatient. The advantage of this is that cleaningaround the abutment teeth and under the ponticsis much easier. The bridge has to withstandhandling by the patient, and so it is usually madewith acrylic facings (Figure 7.2). The acrylicfacings are less liable to chip if the bridge isdropped. They can also be replaced without therisk of distorting the framework as would be thecase with porcelain.

Advantages and disadvantages of thefour basic designs

A comparison of conventional fixed–fixed,fixed–movable and cantilever bridges is shown onpage 206. Spring cantilever bridges are notincluded.

A comparison of minimum-preparationfixed–fixed, fixed–movable and cantilever designsis shown on page 207.

Choice of materials

Metal only

Many posterior bridges, both conventional andminimum-preparation, can be made entirely ofcast metal, whether they are fixed–fixed,fixed–movable or cantilever. If the retainers orpontics do not show when the patient smiles andspeaks then an all-metal bridge is the best choicewith conventional bridges – the material necessi-tates the least destruction of tooth tissue andmay be the least costly. The margins are alsoeasier to adapt to the preparations.

Metal–ceramic

When the strength of metal is required togetherwith a tooth-coloured retainer or pontic,metal–ceramic is the best material.

A range of composite crown and bridge facingmaterials is now available, but it is too early tosay whether these have an advantage overmetal–ceramic materials for permanent bridges.

204 Types of bridge

Ceramic only

The traditional all-ceramic bridge was limited byits relatively poor strength to two-unit cantileverbridges or three-unit fixed–fixed bridges but theywere popular at one time and some patients stillhave them. With improvements in metal–ceramicmaterials, these all-ceramic bridges fell intodisuse. However, with the development ofstronger ceramic materials (see Chapter 1 andFigure 8.4), there is a resurgence of interest inall-ceramic bridges..

One advantage of the all-ceramic bridge is the‘fuse-box’ principle (see Chapter 2). All-ceramicbridges, if properly designed and constructed,have sufficient strength to survive normalfunctional forces, but will break if subjected toexcessive forces. This potential for fracture maysave the roots of the abutment teeth from

fracturing if the bridge receives a blow. It is notuncommon for patients who lose a tooth as aresult of an accident to have a further accident,either because of their occupation or sport orbecause, with a Class II Division I incisor relation-ship, their upper incisors are vulnerable totrauma. A broken bridge is better for the patientthan broken roots.

Combinations of materials

Many combinations are possible, but two deservespecial mention:

• A metal–ceramic retainer and pontic with amovable connector to a gold inlay or otherminor retainer.

Types of bridge 205

Figure 8.7

Typical temperature ranges for the metal–ceramicprocess. These vary according to the metal, porcelainand solder used, and with the type of furnace, in partic-ular its rate of temperature rise.

206 Types of bridge

ADVANTAGES

Fixed–fixed

– Robust design with maximum retentionand strength

– Abutment teeth are splinted together; thismay be an advantage, particularly whenteeth are uncomfortably mobile followingbone loss through periodontal disease

– The design is the most practical for largerbridges, particularly when there has beenperiodontal disease

– The construction is relatively straightfor-ward in the laboratory because there areno movable joints to make

Fixed–movable

– Preparations do not need to be parallel toeach other, so divergent abutment teethcan be used

– Because preparations do not need to beparallel, each preparation can be designedto be retentive independently of the otherpreparation(s)

– More conservative of tooth tissuebecause preparations for minor retainersare less destructive than preparations formajor retainers

– Allows minor movements of teeth – Parts can be cemented separately, so

cementation is easy

Cantilever

– The most conservative design when onlyone abutment tooth is needed

– If one abutment tooth is used, there is noneed to make preparations parallel to eachother; if two or more abutment teeth areused, they are adjacent to each other, so itis easier to make the preparations parallel

– Construction in the laboratory isrelatively straightforward

– Most suitable in replacing anterior teethwhere, if the occlusion is favourable, thereis little risk of the abutment tooth tilting.

– Easier maintenance and cleaning

DISADVANTAGES

Fixed–fixed

– Requires preparations to be parallel, andthis may mean more tooth reduction thannormal, endangering the pulp and reduc-ing retention; the strength of theprepared tooth may also be reduced

– Preparations are difficult to carry out,particularly if several widely separatedteeth are involved; the preparation is slowand the parallelism has to be constantlychecked, or alternatively (and wrongly)the preparations are over-tapered toensure that there are no undercuts andso retention is lost

– All the retainers are major retainers andrequire extensive, destructive prepara-tions of the abutment teeth

– Has to be cemented in one piece, socementation is difficult

Fixed–movable

– Length of span limited, particularly withmobile abutment teeth

– More complicated to construct in thelaboratory than fixed–fixed

– Difficult to make temporary bridges– The major retainer needs to be able to

provide the support for pontics withlonger spans

Cantilever

– With small bridges the length of span islimited to one pontic because of the lever-age forces on the abutment teeth; if moreteeth are to be replaced with a cantileverbridge, a large number of abutments widelyspaced round the arch must be used

– The construction of the bridge must berigid to avoid distortion

– Occlusal forces on the pontic of smallposterior bridges encourage tilting of theabutment tooth, particularly if theabutment tooth is distal to the pontic andis already predisposed to tilting mesially.

COMPARISON OF CONVENTIONAL BRIDGE DESIGN

Types of bridge 207

ADVANTAGES

Fixed–fixed– A large retentive surface area– A single casting and so relatively simple in

the laboratory– Can be used to retain post-orthodontic

cases as well as replacing teeth

Fixed–movable– Independent tooth movement is possible,

particularly for the minor abutment tooth(with the movable joint). The majorretainer can be designed for optimumretention, sometimes incorporating intra-coronal as well as extra-coronal elementsreplacing restorations

– The retention of the minor retainer neednot be substantial, particularly if the movablejoint consists only of a rest seated in a seaton the minor retainer. In this case there arefew displacing forces on the minor retainer

– The retention of the two retainers can bevery different, usually with the major retainerdistally and the smaller, minor retainerattached to a premolar tooth. The retainercan be made very small, and its appearanceis similar to a small amalgam restoration

– Prevents a posterior abutment toothtilting as is sometimes the case with acantilever bridge. The movable jointmerely acts to prevent this rather than toprovide any retention for the bridge

Cantilever– The most conservative of all designs,

usually only involving a single minimal-preparation retainer

– Ideal for replacing upper lateral incisors,using the canine tooth as the abutment,provided that the occlusion is favourable

– Suitable posteriorly when the span is short– Easy for the patient to clean with floss

passed through the contact point betweenthe pontic and the unrestored adjacenttooth

– No need to align preparations– Easy laboratory construction

DISADVANTAGES

Fixed–fixed– Because part of the occlusal surfaces of

both abutment teeth are usually opposedby teeth in the opposing jaw, there is atendency for them to be dislodged fromthe retainer, thus debonding the bridge.As the bridge will be retained there is arisk of caries developing under the failedwing

– With tilted abutments it is sometimes diffi-cult to achieve an adequate retentive sur-face without substantial tooth preparation

– The retention of both retainers should beapproximately equal. This is difficult toachieve when one retainer is a molartooth and the other a premolar

Fixed–movable– Not suitable for anterior bridges– More difficult to make in the laboratory,

requiring two separate castings– Not suitable for longer-span bridges,

where a conventional fixed–movablebridge would be satisfactory. This isbecause the movable joint is seldom largeenough to resist lateral forces on thepontic, but will only resist axial forces bymeans of the rest on the minor retainer

Cantilever– Relatively small retentive area, and

vulnerable to debonding through torquingforces

– Seating of the bridge may be difficult toensure on cementation as there may belittle guidance from the tooth preparation

COMPARISON OF MINIMAL-PREPARATION BRIDGE DESIGNS

• An all-metal retainer (a crown or minimum-preparation retainer) towards the posterior endof the bridge with anterior metal–ceramic units.

An example is where a complete all-metalretainer would be ideal for a posterior abutmentto conserve tooth tissue but the pontics will bevisible and need to be of metal–ceramic material.If the design is fixed–moveable with the moveablejoint mesially then the all-metal molar retainerwill need to be soldered or laser welded to themetal ceramic pontics. Preparing the molar

abutment for a metal–ceramic crown would besimple and would not require soldering but itwould be more destructive. Alternatively, the all-metal retainer can be cast in metal–ceramicmaterial so the whole casting can be in one piece.

The solder joint is made in a low-fusing solderafter the porcelain has been added, and the bridgecannot be returned to the furnace for furtheradjustments to the porcelain after it has beenmade. Figure 8.7 shows the range of tempera-tures of the various components in themetal–ceramic system.

208 Types of bridge

Figure 8.8

a This patient presented with periodontal disease andgross calculus. As an initial phase in his treatment,following removal of the calculus, the mobile lowerincisor was splinted to the adjacent teeth with acid-etch retained composite. However:

b it was decided that the prognosis was hopeless andthe root was resected. This simple and very cost-effec-tive treatment kept the appearance reasonable whileperiodontal treatment continued and the sockethealed. It also kept the longer-term replacementoptions open.

Figure 8.9

A Rochette bridge replacing one central incisor. Theporcelain or composite facing is yet to be added to thepontic. The palatal spur is a handle which will beremoved in due course. Conventional composite isoften used as a luting cement as it is stiff enough to fillthe holes. This design is still used for provisional splintsand bridges because it is less traumatic to remove. Thecomposite is drilled out of the holes to release theretention.

Types of minimum-preparationbridge

• Direct bridges (Figure 8.8). These may bemade using the crown of the patient’s own

tooth. This can often be done as a simple andrapid way of replacing a tooth lost throughinjury (which cannot be reimplanted) or whichhas to be extracted urgently. Sometimes metalmesh or wire is added to the lingual surface to

Types of bridge 209

Figure 8.10

a A scanning electron micrograph (SEM) of a castnickel–chromium metal surface grit-blasted with 50 mmaluminium oxide particles. The surface has no physicalundercuts but is irregular. This is the recommendedmetal finish for the chemically adhesive cementingresins.

b An SEM of a cast nickel–chromium metal surfaceetched in the laboratory (the Maryland technique). Thesurface is very retentive but delicate.

increase strength, but this is not always neces-sary. If the natural crown of the tooth is notavailable or is not suitable, an acrylic denturetooth can be used in the same way.

• Macro-mechanically retentive bridges.The earliest version of the minimum-prepara-tion bridge was known as the ‘Rochette bridge’after its developer who, in fact, developed thetechnique for periodontal splinting. They arestill sometimes used for this purpose becausethey are less retentive than newer types ofminimum-preparation bridge and so can beremoved more readily. In other words they areused for provisional restorations and no longermade for permanent use. They have largeundercut perforations through the cast metalplate, through which the luting compositeflows. These holes are cut in the wax or acrylicpattern with a bur and are then countersunk(Figure 8.9).

From the time of Rochette to the presentday there have been a series of techniquesdeveloped. The best known of these was the‘Maryland’ technique (developed at theUniversity of Maryland) involving etching thenon-precious metal surface either electronicallyor with hydrofluoric acid. These techniqueshave now been superseded. The term‘Maryland bridge’ should therefore not be usedas it is inaccurate for the modern type ofminimum-preparation bridge.

• Chemically adhesive (resin-retained)bridges. With current techniques the non-precious metal surface is simply grit-blasted(sandblasted), preferably immediately beforebonding (Figure 8.10a). A comparison with theetched (Maryland) metal surface, at the samemagnification is shown in Figure 8.10b. Thechemically active resin cement chemicallybonds to the metal and the resin componentmicro-mechanically bonds to the roughenedmetal and the etched enamel surface.

Advantages of minimum-preparationbridges in general

• The pulp is not put at risk from the preparation.• The technique is mostly reversible and if the

bridge eventually fails it is possible to proceedwith alternative ways of replacing the missingtooth.

• Temporary restorations are not required.• The technique is generally less expensive than

conventional bridges.

Disadvantages of minimum-preparation bridges in general

• As the metal plate is added to the surface ofthe tooth or only replaces part of it, the thick-ness of the tooth is increased, and may (forexample in a normal Class I incisor relation-ship) interfere with the occlusion unless spaceis created orthodontically or by grinding theopposing teeth (see Chapter 11).

• The margin of the retainer inevitably producesa ledge where plaque can collect. This is aproblem, especially in the replacement of lowerincisors. Here plaque and calculus deposits arecommon on the lingual surface towards thegingival margin, and the presence of such aledge can only make it more difficult for thepatient to clean in this area.

• It is difficult to predict the longevity of mini-mum preparation bridges and clinical studiesgenerally agree that although success rates areimproving, in some patients their outcome ispoor. Patients can be provided with sparedentures in case the bridge should becomedislodged. Minimum preparation bridges canusually be re-cemented if they de-bond butpatients will understandably lose faith in thebridge if this should happen regularly and alter-native treatment should be considered.

210 Types of bridge

Each part of the bridge should be designedindividually, but within the context of the overalldesign. This chapter should therefore be read inconjunction with the next, since in practice thetwo processes – designing the bridge and itscomponents – are done together, although it isclearer to describe them separately.

Retainers

Major or minor

As described in Chapter 8, all fixed–fixed andcantilever bridges have only major retainers.Fixed–movable bridges have a major retainer atone end of the pontic and a minor retainer(carrying the movable joint) at the other.

Major retainer preparations must be retentiveand, with conventional bridges, must cover thewhole occluding surface of the tooth. It is impor-tant to recognise the difference between theoccluding and the occlusal surface.

A major retainer for a conventional posteriorbridge should not be less than an MOD inlay withfull occlusal protection. For incisor teeth it isusually a complete crown.

Minor retainers do not need full occlusalprotection: a minor retainer may be a completeor partial crown, or a two- or three-surface inlaywithout full occlusal protection (Figures 8.1b and8.3). Minimum-preparation minor retainers arealso used for minimum-preparation bridges

where the occlusion is favourable (Figure 8.3cand d).

Complete crown (full crown), partialcrown, intracoronal or minimum-preparation retainers?

The choice between complete and partial crownretainers for posterior conventional bridges shouldalways be considered and will depend upon a fullassessment of all the circumstances of the case. Itshould not be made from habit. It will be found thateven after a full assessment, 80–90% of conven-tional bridge retainers will be full crowns, but forthe remaining 10–20% there are sound reasons forchoosing a partial crown. (See Chapter 2 for acomparison of complete and partial crowns.)

Intracoronal retainers are used only as minorretainers except for very retentive MODprotected cusp inlays/onlays.

With the reduction in caries and with a moreconservative approach to cavity preparation, anincreasing number of potential abutment teethhave sufficient enamel available for minimum-preparation retainers to be considered. Whenthis is so, they are usually the retainers of choice,provided that the other conditions for their useare met (see later). This is because they are themost conservative retainers, and it is wise topreserve as much natural tooth tissue as possible.

When an anterior tooth is intact a minimum-preparation retainer is more conservative oftooth tissue than a complete crown, and so is thepreferred choice whenever possible.

211

9 Components ofbridges: retainers,pontics andconnectors

Materials

Minimum-preparation retainers are usually madein base metal alloys because they are strong inthin sections and they bond well to adhesiveluting resins when they are freshly grit-blasted. Ofthe conventional retainers, an all-metal retainer isthe most conservative of tooth tissue, and thesimplest and usually the least expensive toproduce. When appearance permits, this shouldbe used in the posterior part of the mouth. In theanterior part of the mouth metal–ceramic is themost suitable material.

Criteria for choosing a suitableretainer

In some cases the type of retainer will be obvious.For example, if a root-filled tooth that already hasa post crown is to be used as a bridge abutment,there is little choice but to use another post-retained crown, whether as a major or minorretainer. In other cases, the full range of choiceis available, and the decision on the type ofretainer cannot be divorced from the decisionson the overall design and which abutment teethto use. These three sets of considerations aredealt with separately (in Chapters 8, 10 and here),but in reality the decision-making process is notso clear-cut, and thoughts on possible abutmentteeth, retainers and the overall design interminglein the operator’s mind and influence each otheruntil a final decision on all three emerges.

The criteria for selecting a particular retainerwill include:

• Appearance• Condition of abutment teeth• Conservation of tooth • Alignment of abutment teeth and retention

tissue• Occlusion• Cost

Appearance

In some cases a complete crown will have a betterappearance than a minimum-preparation retainer.Sometimes neither will be completely satisfactory.

Figure 9.1b shows an example of where a distal-palatal inlay would produce the best appearance.Figure 9.1c also shows an example of ‘metal shine-through’, which sometimes occurs with minimum-preparation retainers.

When several teeth are to be crowned orreplaced as pontics, there is an aesthetic advan-tage to the bridge retainers and pontics beingmade in the same material (usually metal–ceramic), giving consistency of appearance.

The condition of the abutment tooth

Frequently a minimum-preparation or partialcrown retainer cannot be used because of thepresence of caries or large restorations involvingthe buccal surface, or because of the loss of thebuccal surface from trauma or other cause. Inthese cases a complete crown retainer is chosen.

Conservation of tooth tissue

There is a natural reluctance to remove soundbuccal enamel and dentine from a healthy intacttooth. This weakens the tooth, destroys itsnatural appearance and sometimes endangers thepulp. Therefore minimum-preparation retainersshould be used whenever possible. However, ifthere are sound indications for a complete crown,the operator should not allow his or her clinicaljudgement to be influenced by an overprotectiveattitude to dental enamel.

Alignment of abutment teeth andretention

When the abutment teeth are more or less paral-lel to each other and a fixed–fixed conventionalbridge is being considered, complete crownretainers can be made. If the abutment teeth arenot parallel (Figure 9.2), complete crown retain-ers with a common path of insertion are notfeasible. They could not be made independentlyretentive without one or other of the teeth beingdevitalized.

The solution will usually be to employ aminimum-preparation bridge or a design otherthan fixed–fixed so that the teeth do not have tobe prepared parallel to each other.

212 Components of bridges: retainers, pontics and connectors

Components of bridges: retainers, pontics and connectors 213

Figure 9.1

The appearance of retainers.

a The canine tooth has a partial crown retainer that isbarely visible from the front. The bridge has beenpresent for many years.

b The upper canine tooth has an extensive incisal wearfacet and pronounced buccal striae. The buccal surfacewould be difficult to reproduce in porcelain if acomplete crown retainer were used and the occlusionwas unfavourable for a minimum-preparation bridge.The design of the bridge in this case was thereforefixed–movable with a distal-palatal gold inlay in thecanine tooth. An implant could also be considered.

c The upper central incisors both have minimum-preparation retainers. Blue ‘metal shine-through’ can beseen. The incisal edge of the upper left central incisorhas been restored with composite, which is beginningto lose its polish. ‘Metal shine-through’ can be reducedby finishing the retainer short of the incisal edge, butthis also reduces its retention. The problem can beminimized by using opaque luting cements.

Figure 9.2

Non-parallel abutment teeth. It would not be possibleto make a conventional fixed–fixed bridge withcomplete crowns on the central incisor and canine,although it would be possible to make a minimum-preparation bridge. As the canine and central incisorsare all intact the design of choice would be twoseparate minimum-preparation cantilever bridges; oneretained by the canine and the other retained by thetwo central incisors splinted together, or perhaps justone central incisor depending on the occlusion andperiodontal health. This would be comparable to thebridges shown in Figure 7.1c and d.

It is impossible to give in absolute terms theamount of retention necessary for any oneretainer. It is reasonable to assume that the reten-tion for a bridge retainer should be at least as greatas for a similar restoration made as a single unit.

Occlusion

In some cases the abutment teeth are sound butthere is insufficient space for a minimum-prep-aration retainer. The choice therefore is betweencreating space by reducing the opposing teeth,preparing part way through the enamel of theabutment teeth, moving the abutment teethorthodontically or a combination of theseapproaches. Often the best way to achieve a smallamount of axial tooth movement is to use a fixedDahl appliance (Figure 4.6). If none of thesemethods are acceptable then a conventionalretainer will be necessary.

Cost

Complete metal crowns and partial crowns maybe less expensive than metal–ceramic crowns(see Chapter 2), and minimum-preparation retain-ers are the least expensive. When there are noother overriding factors affecting the choice, thisis obviously important.

Pontics

Principles of design

Pontics are designed to serve the three mainfunctions of a bridge:

• To restore the appearance• To stabilize the occlusion• To improve masticatory function.

In different areas of the mouth the relative impor-tance of these will alter. The principles guiding thedesign of the pontic are:

• Cleansability• Appearance• Strength.

The compromise often necessary between cleans-ability and appearance will also vary in differentparts of the mouth.

Cleansability

All surfaces of the pontic, especially the surfaceadjacent to the saddle, should be made as cleans-able as possible. This means that they must besmooth and highly polished or glazed, and shouldnot contain any junctions between differentmaterials. In a metal–ceramic pontic the junctionbetween the two materials should be well awayfrom the ridge surface of the pontic.

It is important too that the embrasure spacesand connectors should be smooth and cleansable.They should also be as easy to clean as possible.Access to them and the patient’s dexterity shouldbe taken into account in designing pontics.

When a conflict exists between cleansabilityand appearance, priority should be given tocleansability.

Appearance

Where the full length of the pontic is visible, itmust look as tooth-like as possible. However, inthe premolar and first molar region it is oftenpossible to strike a happy compromise betweena reasonable appearance for those parts of thepontic that are visible and good access for clean-ing towards the ridge.

Strength

All pontics should be designed to withstandocclusal forces, but porcelain pontics in theanterior part of the mouth may not of course beexpected to withstand accidental traumaticforces.

The longer the span, the greater the occlusalgingival thickness of the pontic should be.Metal–ceramic pontics are stiffer and withstandocclusal forces better if they are made fairly thickand if the porcelain is carried right round themfrom the occlusal to the ridge surface, leavingonly a line of metal visible on the lingual surfaceor none at all (Figure 9.3a and b).


The surfaces of a pontic

A pontic has five surfaces:

• The ridge• The occlusal• The approximal• The buccal or labial• The lingual or palatal.

Some of these will be similar to the natural toothbeing replaced; others will be very different.

The ridge surface

This surface of the pontic is the most difficult toclean, and yet it also has a considerable influenceon appearance. There are four basic designs ofridge surface (Figures 9.4 and 9.5).

Wash-through (Figure 9.4a)

Other terms used for this type of pontic arehygienic and sanitary, but the term wash-throughis more descriptive and less suggestive of vitre-ous china bathroom fittings. The wash-throughpontic makes no contact with the soft tissues andso is the easiest to clean. It is used where a pontic

is required for functional purposes rather thanappearance and is most useful in the lower molarregion.

Dome-shaped (Figure 9.4b and c)

This is the next easiest to clean and is used wherethe occlusal two-thirds or so of the buccal surfaceof the pontic show, but not the gingival third. Itis commonly used in the lower incisor andpremolar regions and sometimes in the uppermolar region.

This has also been described as torpedo-shapedor bullet-shaped, but the less aggressive term,dome-shaped, is preferred.

Ridge-lap and modified ridge-lap (Figure9.4d, e and f)

The principles of this design are that the buccalsurface should look as much like a tooth as poss-ible right up to the ridge, but the lingual surfaceshould be cut away to provide access for clean-ing. Ideally the pontic should have a completelyconvex lingual surface, making only a line contactalong the buccal side of the ridge. However, thisis often impractical because of the shape of theridge, and so the modified ridge-lap pontic, whichhas minimum contact with the ridge from the


Figure 9.3

The strength of metal–ceramic pontics.

a The central incisor pontics in this case have no metalvisible on the palatal surface.

b When the palatal reduction of the abutment teeth isonly sufficient for a layer of metal, this is often carriedalong the pontics as well, leaving an occluding surfaceentirely in metal. The porcelain, however, is carriedright under the pontics so that only porcelain contactsthe ridge.


Figure 9.4

The four designs of pontic ridge surface.

a A wash-through pontic with a concave mesio-distalcontour.

b and c Dome-shaped pontics. b A molar dome-shapedpontic with the male part of a movable connector anda lingual handle which will be removed after the bridgeis tried in. c An acrylic provisional bridge fitted as animmediate replacement for the lower left central andlateral incisors, showing an application of the dome-shaped pontic.


d and e Ridge-lap pontics. d This bridge has been satis-factory, but the patient complains of food impactionunder the single lateral incisor pontic. She can sweepthe other side clean with her tongue since the span islonger. e A modified ridge-lap pontic with contact overthe buccal half of the ridge but cut away lingually. Thebridge has failed because of a fractured solder joint (seeChapter 14).

f Typical ridge-lap pontics on another failed bridge.This time the failure was due to loss of retention.Despite the design and the smooth porcelain surface,the ridge beneath these pontics was moderatelyinflamed.

point of contact on the buccal side up the crest,is often used (Figure 9.4e).

These designs, particularly if the pontic is fairlynarrow mesio-distally, as in the case of an incisoror premolar pontic, are sometimes unpopularwith patients because they find that food impactsinto the space on the lingual side and cannot bereadily removed with the tongue (Figure 9.4d).Besides, considerable manual dexterity is neededto manoeuvre dental floss, tape or other cleaningaid, holding it first against the pontic and then ina secondary cleaning movement against the ridge(Figure 9.5).

These pontics were designed at a time whenthere was a lot of concern about the effect ofpontics on the soft tissues but before the signifi-cance and nature of plaque were as well under-stood as they are today. They are still commonlyused, perhaps through habit and convention.Other designs should also be considered and areoften better.

Saddle

The saddle pontic is so named because of itsshape. It has by far the largest area of surfacecontact with soft tissue, and so, although it waspopular in the early days of bridgework, itbecame much less so as dentists became moreconcerned about the effects of pontics on ridges.Now that it is recognised that plaque can causeinflammation however small the surface area of

contact and must be removed in all cases, theemphasis in pontic design has shifted. Accessibilityfor cleaning and patient comfort and convenienceare the important criteria, rather than the size ofarea of contact. Many patients prefer the saddle-shaped pontic since the lingual surface feels morelike a tooth than any other design. With moderncleaning aids, such as superfloss, the ridge surfaceof properly designed and constructed saddlepontics is relatively easy to clean. This alsorequires less manual dexterity by the patient thanridge-lap pontics (Figure 9.5d).

A saddle pontic should closely follow thecontour of the ridge but should be smooth onthe under surface. It should not displace the softtissues or cause blanching when it is inserted, butshould make snug contact.

The effects of pontics on the ridge

Sometimes when bridges are removed the areaof the ridge that was in contact with the pontichas a red appearance. Biopsy studies have shownthat there are always some chronic inflammatorycells in this region, but the main explanation forthe redness is probably the reduction inkeratinization. The surface does not have thenormal stimulation from food and the tongue thatstimulates keratinization elsewhere. Unless clearlyinflamed or ulcerated, the redness is of little clini-cal consequence (Figure 9.6).


g Saddle-shaped pontics with well-contoured, cleans-able connectors


a b

c d

Figure 9.5

Four sectioned casts of the same patient, showing the profile of the midpoint of a lower molar edentulous areawhere a bridge is to be made. The profiles of four pontics are shown.

a A wash-through pontic with no contact with the ridge – very cleansable but poor appearance.

b A dome pontic making point contact on the tip of the ridge – still cleansable and with a better appearancebuccally towards the occlusal surface.

c A partly modified ridge-lap pontic with a buccal surface resembling a natural tooth but with minimum ridgecontact. The difficulty of cleaning the lingual aspect near the ridge is obvious.

d A full saddle pontic that, if well polished or glazed on the gingival surface, would be cleansable with superfloss.This design has the best appearance and feels more natural to the tongue.


Figure 9.6

a Mucous membrane reactions under pontics. This areaof reduced keratinization under a pontic produced nosymptoms. There was no ulceration or bleeding on floss-ing under the pontic. Although inflammation requiringtreatment at the gingival margin of the abutment teethis present, it is doubtful whether the changes in theremainder of the ridge have had any real significance.

b A much more serious case, with ulceration and avery inflamed mass of granulation tissue. This mustclearly be treated in the first place by removal of thebridge. In fact no further treatment was necessary. Theinflammation resolved over a 3-week period.

Figure 9.7

Well-contoured open embrasure spaces.

Figure 9.8

Sections through both the lateral incisor areas of thesame patient. Left: the lateral incisor is present. Right:it is missing and the alveolus has resorbed. The profileof the resorbed side has been superimposed on theother to show the extent of the resorption and threeways in which a pontic might be modified to overcomethis problem.

The occlusal surface

The occlusal surface of the pontic should resem-ble the occlusal surface of the tooth it replaces.Otherwise it will not serve the same occlusalfunctions and may not provide sufficient contactsto stabilize the occlusal relationships of itsopponents.

In some cases, when occlusal stability is lessimportant (for example when the pontic isopposed by another bridge), the pontic may bemade narrower bucco-lingually to improve accessfor cleaning. Other arguments for narrowingpontics are less convincing (see Chapter 10).

The approximal surfaces

The shape of the mesial and distal surfaces of thepontic will depend upon the design. Withfixed–fixed bridges the approximal surface willconsist partly of a fixed connector. It is impor-tant that the embrasure space between the

connector and the gingival tissue be as open aspossible to ensure that there is good access forcleaning, particularly if the pontic is a ridge-lap orsaddle pontic (Figure 9.7). The gingival side of amovable joint is more difficult to leave entirelysmooth, and so it is again important that thereshould be good access for cleaning. A balance hasto be achieved to ensure that there is adequatemetal present to provide sufficient strength andrigidity for the connector as well as allowing openembrasures for cleaning.

The approximal surface of a cantilever bridgeon its free side will simply make normal contactwith the adjacent tooth, or in some cases theremay be a diastema with no contact. Occasionally,where the span is very short, a cantilever ponticmay be made to overlap the adjacent tooth toimprove its appearance. In this case the ponticsurface in contact with the natural tooth shouldbe as smooth as possible, although it may beslightly concave. If the patient is taught to cleanwith dental floss, the natural tooth surface shouldnot be any more susceptible to caries than witha normal contact point.


Figure 9.9

Buccal pontic–ridge relationships.

a A pontic replacing an upper canine, where the neckof the pontic has been curved inwards to meet theresorbed alveolar ridge at the correct vertical position.The incisal two-thirds of the buccal surface have beencontoured in line with the adjacent teeth so that all thecompensation for the missing alveolar bone is in thegingival buccal third. (Note the excessive amount ofgold shown by these two partial crowns, in contrastwith those in Figure 9.7.)

b The same compromise has not been made with thislower premolar pontic, which instead looks too long.This would also create difficulty in cleaning under thepontic.

The buccal and lingual surfaces

The buccal surface of a wash-through or dome-shaped pontic does not resemble the shape of anatural buccal surface, particularly gingivally. Withridge-lap and saddle pontics the buccal surface isintended to look as much like a tooth as possiblefor its entire length. The problem is that when atooth is missing, so also is some of the alveolarbone that supported it. This means that the alveo-lar contour where the pontic touches the ridgenever looks entirely natural, and the pontic mustalso be shaped unnaturally to meet the resorbedridge. Figure 9.8 shows, by means of sectionsthrough a study cast, how the ridge contour in aresorbed saddle area necessitates a compromisepontic appearance. Figure 9.9a shows an exampleof this where an upper canine is missing. Figure9.9b also shows an example of a case in whichthis compromise has not been made. Theaesthetic result is not good and there is greaterdifficulty than necessary in cleaning.

No ridge–pontic relationship can ever appearentirely natural, even when the ridge has notresorbed significantly or where it has beenaugmented – see Figure 7.9. But at the normaldistance from which teeth are seen, the illusionthat the tooth emerges from the gum can be suffi-ciently convincing: which are the pontics andwhich are the retainers in Figure 9.10?

The lingual surface of a pontic will be designedas a result of deciding the ridge surface. Withridge-lap pontics, the lingual surface should besmooth and convex.

Materials

The choice for pontics is the same as for retain-ers. At one time there was also the choice of a

number of proprietary pontic facings. These havenot been used for many years but some patients,with long-standing bridges, still have proprietarypontic facings. They are easy to recognise incontrast to metal–ceramic pontics.

Connectors

Fixed connectors

There are three types of fixed connector:

• Cast• Soldered or laser welded• Porcelain.

Cast connectors are made by wax patterns ofthe retainers and pontics connected by wax beingproduced so that the bridge is cast in a singlepiece. This has the advantage that a secondsoldering operation is not required. But the moreunits there are in the bridge, the more accuratethe casting must be. Minor discrepancies in thecompensation for the contraction of moltenmetal that may be acceptable for single-unitcasting become unacceptable when magnifiedseveral times.

Cast connectors are stronger than solderedconnectors, and also it is sometimes possible todisguise their appearance more effectively. Forthese reasons, multiple-unit bridges are often castin several sections of three of four units dividedthrough the middle of a pontic. The split ponticsare then soldered with high-fusing solder beforethe porcelain is added, so that all the connectorsare cast. The solder joint produced in this way isstrong both because it has a larger surface areathan if it were at the connector and because it is


Figure 9.10

An acceptable appearance for a bridge – or is theremore than one bridge?

covered by porcelain, stiffening it. Laser weldingis an increasingly popular way of linking compo-nents of bridges together. The welds achieved arestronger than soldering and the technique is quickand simple, although the equipment required iscomplex and expensive.

Soldered or laser welded connectors areused if the pontics and retainers have to be made

separately. This is necessary when they are madeof different materials, for example a completegold crown retainer with a metal–ceramic pontic.

Porcelain connectors are used only in conjunc-tion with all-porcelain bridges. The details of theirconstruction are beyond the scope of this book,but the same principles of accessibility and cleans-ability still apply.


Figure 9.11

Creating a fixed–fixed design with non-parallelabutments.

a The central incisor could not be retracted sufficientlyto be parallel to the other abutment teeth even ifdevitalized and with a post and core fitted. It wouldinterfere with the occlusion.

b and c The bridge is made in two parts with separatepaths of insertion, and the divided pontic connected inthe mouth by cement and a screw attachment.

Movable connectors

Movable connectors are always designed so thatthe pontic cannot be depressed by occlusalforces. This means that the groove or depressionin the minor retainer must always have a goodbase against which the male part of the attach-ment can seat. Sometimes, with small pontics andshort spans, this is the only force that needs tobe resisted, and therefore the female part of theattachment, in the minor retainer, need only bea shallow depression (Figure 8.3c). This is thecommonest design for fixed–movable minimum-preparation bridges.

However, with longer-span bridges themovable joint must also resist lateral forcesapplied to the pontic and (assuming the movablejoint is mesial) distal forces on the pontic, whichwould separate the components of the movableconnector. In these circumstances the connectoris designed as a tapered dovetail-shaped slot sothat the pin can move up and down a little andyet seat firmly against the base of the slot, but itcannot move laterally and the connector cannotseparate. Examples of this type of connector areshown in Figure 8.3.

There are different ways of producing movableconnectors. In the freehand method a waxpattern is produced for the minor retainer witha shallow depression or tapered groove prepared

in the wax, the retainer is cast and with a groove,the shape is refined with a tapered bur. Thepontic is then waxed-up with a finger or ridge tofit into the depression or groove. This is cast andthe two parts of the movable joint are fittedtogether before the bridge is taken to the chairside for trying in (Figure 8.3a).

In some cases a depression or groove may beprepared in an existing cast restoration in themouth and an impression taken of it togetherwith the other prepared abutment tooth orteeth.

Acrylic burn out patterns are available that maybe incorporated into the pontic and minorretainer so that the whole bridge can be waxed-up in one operation and the minor retainer andremainder of the bridge invested and castseparately (Figure 8.3b, page 200).

Proprietary groove-and-ridge precision attach-ments in metal may also be used as movableconnectors, but are generally too retentive andthere is the risk that they will not permit suffi-cient movement. When precision attachments areused, the minor retainer should have more reten-tion to its abutment than would be necessary if aless retentive connector were used.

Screw precision attachment connectors may beused to produce a fixed–fixed bridge by connect-ing two retainers that cannot be prepared paral-lel to each other (Figure 9.11).


Criteria for selecting a bridgedesign

No firm rules can be given for selecting anyparticular design. Bridge design is complex, poorlyresearched and dominated by personal opinionderived from clinical experience, or lack of it.Many of the ground rules of bridge design werelaid down in the early part of the last century byteachers who were trying to rescue the subjectfrom the purely empirical approach used until thattime. Although they were a major advance onwhat had gone before, these ground rules werenot scientifically investigated. Yet they becameaccepted as irrefutable and remained relativelyunaltered for over 50 years, despite a growingunderstanding in that time of related subjectssuch as the supporting structures of teeth inhealth and disease, and of occlusion and jawfunction. In the last two decades, great develop-ments have been made in restorative materialsand techniques, so that bridges now fit better,look better and are stronger. This increasedunderstanding and technical development shouldaffect traditional ideas of design to a considerabledegree.

Clinical (and some research) evidence suggeststhat many of the early rules of bridge designshould no longer be applied. However, thisevidence is not yet sufficiently clear-cut for new,firm rules to be established, leaving today’sdentists, including the authors, still having to makeclinical decisions about bridge designs based onan incomplete ‘evidence base’.

A number of criteria may nevertheless be usedin choosing a design, although the weight given toeach will vary with the circumstances and theopinions held by the operator. It is to be hopedthat with further clinical research (which isurgently needed) the relative importance of thesecriteria will become clearer.

Support

One of the best known rules for bridge designwas devised by Ante and described by him in1926. He suggested that each pontic should besupported by the equivalent of an abutment toothwith at least the same root surface area coveredby bone as would have supported the missingtooth; that is, a given area of periodontalmembrane could support up to twice its normalocclusal load. The root surface area of anabutment tooth covered by bone is of coursereduced following destructive periodontal disease.This was a guiding principle for many years but isnow regarded as little more than a guiding prin-ciple – if that.

This arbitrary, mechanical rule is similar to theengineering principles used for designing bridgesacross rivers. There are many reports in the liter-ature of experiments (usually carried out in thelaboratory on models or in computer simulations)relating occlusal forces to reactions in thesupporting structures of teeth. The results ofthese experiments have tended to reinforce thesemechanical ideas of how bridges should bedesigned.

The evidence now suggests that these prin-ciples are wrong, or at least do not tell the wholestory. Provided that any periodontal disease istreated and periodontal health is maintained, andprovided that the occlusal forces are evenlydistributed, bridges can be successful with as littleas one-quarter of the support advocated by Ante.Such bridges have been successful for many years.

The assumptions made by the engineeringschool of thought ignore the fact that the occlusalload on a bridge is determined not by extrane-ous influences, such as lorries driving across roadbridges, but by the muscles of mastication. Theseare under the control of the neuromuscularmechanism, itself influenced by proprioception

225

10 Designing andplanning bridges

from receptors in the periodontal membrane ofthe teeth supporting the bridge. Comparisonswith road bridges are therefore meaningless.

There is plenty of evidence that occlusalloading is modified by the presence or absence ofnatural teeth and by their condition. For example,patients can generate 10 times as much forcebetween upper and lower natural teeth as theycan between upper and lower complete dentures,where the force is resisted by mucous membrane.It is false logic to assume that increasing the

occlusal area of a tooth by adding a pontic to itwill inevitably increase the occlusal loading on thattooth. However, forces in an ‘unnatural’ direc-tion, for example rotational or leverage forces,may not be resisted so well. There is not thesame inbuilt mechanism to perceive and controlthese forces (Figure 10.1).

Teeth will also adapt to increasing load physio-logically with changes to the periodontal ligament,which thickens to cope with the extra load. Thebone support for a tooth will be stable with

226 Designing and planning bridges

Figure 10.1

Occlusal loading of abutment teeth.

a In an intact dentition an occlusal force, X, is resistedby an equal and opposite force generated within thesupporting structures of the tooth.

b When a tooth is extracted and replaced by meansof a bridge, engineering principles suggest that the sameforce, X, delivered to each of the three occlusalsurfaces would require the generation of 1.5X in thesupporting structures of the two remaining teeth. Thisprinciple is no doubt true for inanimate objects butassumes that the occlusal force is constant.

c The occlusal force is of course generated by musclesof mastication, which are under physiological controland do not function independently. Therefore, if thesupporting structures of the two remaining teeth are

only capable of generating a resisting force of X, and ifthey retain a full periodontal sensory mechanism, onceforce X is exceeded, the proprioceptive mechanism willsuppress the contractions of the muscles of mastica-tion so that the force delivered to the three occlusalsurfaces totals 2X.

This is an oversimplified version of what happens inreal life. Sometimes the sensory mechanism is notintact owing to periodontal disease and alveolar boneloss. The proprioceptive mechanism may be overrid-den by stimulae from higher centres, producingbruxism or other parafunctional activity. The descrip-tion also ignores the effects of lateral forces, which aremore complex. However, the illustration serves toshow that bridges should not be designed simply byusing engineering principles: the biological implicationsmust be taken into account.

increasing function and loading as long as thepatient maintains oral hygiene and no inflamma-tion is present.

These considerations are often less importantin designing small bridges than they are with largebridges. Figure 10.2a shows a case where the spanis so small that any of the available abutmentteeth would meet all the traditional criteria forsupport; while in the case of Figure 10.2b a bridgecould not be provided if Ante’s law were to beobserved. Figure 10.2c shows the bridge for the

same patient. It has been successful for manyyears.

An example of a bridge design that issometimes unnecessarily destructive because itrelies in part on Ante’s law for its justification isthe replacement of four upper incisor teeth whenthe canines and first premolar teeth on both sidesare used as abutments. Not only is this destruc-tive, it also creates embrasure spaces betweenthe splinted abutment teeth, which are difficult toclean. The premolars are less satisfactory

Designing and planning bridges 227

Figure 10.2

Abutment support and length of span.

a A tiny pontic is needed here, and any of the avail-able abutment teeth, which have no alveolar bone loss,would provide more than enough support.

b Radiographs of the six abutment teeth supporting the10-unit bridge in c. All the abutment teeth have lessthan half their original bone support. For the remain-ing lower teeth this is also much reduced, but theperiodontal treatment has been successful and therehas been no increase in bone loss or further mobility.

c The bridge has been satisfactory, with no furtherbone loss, but the terminal abutment on the left of thepicture has had to be root-treated through theretainer. The bridge, which was 9 years old when thisphotograph was taken, is rather bulbous.

abutments than the canines, and add little to thisdesign. It has been said that occlusal pressure onthe pontics, which are in front of a straight linebetween canine abutments, would produce atilting force on the canines. However, in a canine-guided occlusion these same teeth will withstandthe entire force of lateral excursions and yetoften remain the firmest teeth in the arch. Figures8.1a and 11.6a show two cases where the caninesalone have been used very satisfactorily asabutments. This design, using the two canineteeth as the only abutments, can now be regardedas the normal design for a bridge to replace thefour incisor teeth in either the upper or lowerjaw. It is not necessary and is indeed counter-productive to include the first premolar teeth.

The best guidance that can be given for thepresent is that abutment teeth with healthyperiodontal tissues are well able to support a(theoretical) increase in loading in an axial direc-tion, because the load is limited by the naturalbody defence mechanism.

However, they are not so well able towithstand twisting or levering forces. This meansthat large bridges of fixed–fixed design can bemade with very limited numbers of abutmentteeth. The curvature of the bridge around thearch reduces the leverage and twisting forces sothat all forces are in the long axis of the abutmentteeth (Figure 10.2c). This is the principle of ‘cross-arch splinting’, and it may be extended so that inideal circumstances long cantilever extensions ofseveral units may be carried by such bridges(Figure 8.5). However, these long cantileverscannot be supported by individual abutment teeth.They would produce a leverage or twisting forceon the abutment tooth causing movement of thetooth in the same way as an orthodontic appli-ance, or they would loosen the tooth.

The application of these principles of supportis illustrated in a series of examples at the end ofthis chapter, and more practical advice on select-ing abutment teeth is also given later in thechapter.

Conservation of tooth tissue

The most conservative design is a minimum-preparation bridge. This is therefore the design ofchoice whenever possible.

All conventional bridges are potentiallydestructive, and some are immediately so. Forexample Figure 10.3 shows a case in which abridge was made before the introduction ofminimum-preparation bridges. The bridge hasremained stable and satisfactory. A conventionalbridge necessitated extensive preparation ofsound tooth tissue of the lower left canine. Acantilever design with only one abutment toothwas a conservative approach at the time ratherthan fixed–fixed or fixed–movable designs thatwould have involved more abutment teeth. Todaythe design would only be used if the canineneeded a crown in any case or was unsuitable fora minimum-preparation retainer.

Cleansability

This is a critical consideration. Some bridges failin the short term because of poor retention orthe appearance is unsatisfactory; however, morebridges fail in the medium term because the designdoes not take sufficient account of accessibility fororal hygiene procedures. Not only should accessbe possible, it should be easy – to help theaverage, rather than the highly committed patient.

Abutment teeth towards the front of themouth are easier for patients to clean than thosefurther back, partly because of access and partlybecause the bucco-lingual width of the contactareas is greater with posterior teeth. Chapter 14includes advice on cleaning bridges.

Figure 9.7 shows well designed, accessibleembrasure spaces which are easy to clean.

Appearance

This is the criterion which is often the one ofgreatest concern to the patient. Some patientshave unrealistic expectations and it is sometimesnecessary to persuade the patient that a compro-mise in favour of cleansability over appearance isimportant for the survival of the bridge. Figure 9.7illustrates this.

Figure 9.1b illustrates the way in which theappearance of the bridge is often one of thefactors in determining its design. If a conventionalfixed–fixed or distal cantilever design had been


used it would have been necessary to make acomplete crown for the upper canine. This wouldprobably not have been as attractive as thenatural tooth. A fixed–movable design with adistal-palatal inlay in the canine carrying a slot forthe movable connector as the minor retainerwould mean that the appearance of the buccalsurface of the canine would be left undisturbed.

The ideal would be a minimum-preparationretainer on the canine supporting a distalcantilever pontic, but the occlusion wasunfavourable for this. Another possibility wouldbe a single tooth implant which would not involvethe canine at all.

Thus consideration of support, a conservativeapproach to tooth preparation, cleansability andappearance led to a decision in the case illustratedin Figure 9.1b to make the bridge fixed–movablerather than fixed–fixed or cantilever.

Planning bridges

Collecting information about thepatient

Chapter 5 includes a detailed review of thehistory and examination of a patient for whomcrowns are being considered. The same approach

should be taken with a patient for a bridge. Thereare, however, a number of additional considera-tions relating to bridges. These are listed belowand should be read in conjunction with therelevant paragraphs in Chapter 5.

Consideration of the whole patient

With crowns, the choice may be between crown-ing a tooth or extracting it, and the decision maywell be to make a crown even though manyfactors, for example, the patient’s age, attitude totreatment or oral hygiene are less than ideal.With bridges, there is often the alternative of apartial denture, a minimum-preparation bridge ora conventional bridge or an implant, and so it maynot be necessary to make so many compromises.If there is any doubt, it is better to make a partialdenture first.

Clinical examination

Assessing abutment teeth

Abutment teeth for minimum-preparation bridgesshould have sufficient enamel for the retainer,


Figure 10.3

a and b A cantilever bridge with a single lower canineabutment tooth and two incisor pontics (only onetooth is missing). One reason for this design was toavoid preparing the lower incisors. This bridge wasmade before the days of minimum-preparation bridgesor implants, which might now solve this problem. Ithas, however, been successful for many years, with norotation, mobility or bone loss.

although small composite restorations can beincluded in the area covered by the retainer.Composite restorations should not be partlycovered.

For posterior teeth the retainer may bedesigned to replace small restorations (Figure10.4). This increases the enamel bonding surfacearea intracoronally as well as extracoronally.

For anterior abutment teeth there must eitherbe sufficient clearance for the minimum prepara-tion retainer between upper and lower teeth orit must be possible to create space, preferably bya type of Dahl approach or, more destructively,by adjusting the opposing tooth or preparing theabutment tooth within enamel.

Using the Dahl principle it can be acceptable tocement a conventional or minimum-preparationbridge high and allow for it to create toothmovement. With a minimum-preparation bridgethe contact should be onto the retainer not thepontic so that the bridge is not dislodged beforethe occlusion stabilizes. This is not the same as a‘high’ filling, as the retainer is cemented onto anintact external surface and the patient will notexperience pulpitic symptoms.

Very thin, translucent incisors are not alwayssuitable as abutment teeth for minimum-prepara-

tion bridges because the metal ‘shines through’spoiling the appearance (see Figure 9.1c).

Preferably none of the metal of the minimum-preparation retainer should show but this shouldbe assessed and if metal will show, for examplean occlusal rest on a lower premolar tooth, thenthe patient should be warned about this inadvance. Short incisor spans are almost alwaystreated by cantilever bridges, preferably of theminimum-preparation design. This is thepreferred option and should be considered firstand only if there are problems with it shouldalternative designs be considered.

With short posterior spans an option is toconsider a cantilever design with a single abutment,again preferably of a minimum-preparation design.However, the occlusion must be carefully assessed.Occlusal forces on a cantilever pontic will produceleverage forces on the abutment tooth and tendto tilt it. This is not the case with anteriorabutments where the forces are not axial orsubstantial. The limit is normally a premolar widthpontic supported by a molar tooth with afavourable occlusion or perhaps a ponticcantilevered distally from a premolar abutment forreasons of appearance when the pontic will beunopposed or opposed by a denture.


Figure 10.4

Minimum-preparation retainer incorporating an intra-coronal element replacing an MO amalgam. This is avery strong and retentive retainer.

When an anterior span is longer it is prefer-able to use separate cantilever bridges rather thana fixed–fixed design for reasons described earlier(Figures 7.1d and e).

It is not uncommon for all four incisor teethto be missing either in the upper or lower jaws.This may be due to trauma or multiple failedrestoration in a caries-prone patient or periodon-tal disease. The canine teeth are often in reason-able condition in these cases and so the preferredoption is frequently a fixed–fixed design with thecanine teeth as the only abutments.

With larger posterior spans the preferredoption is a fixed–movable design, either minimumpreparation or conventional and with properlydesigned movable joints (see Chapter 9) longspan fixed–movable bridges are very successful.There is little reliable survey evidence of therelative success rates of long span fixed–movableand fixed–fixed posterior bridges. But what thereis, together with clinical evidence suggests that,with appropriate abutment teeth, the fixed–movable design is preferred unless there is aneed to splint the teeth (see later). With longerspans the potential advantages of implant-supported restorations increase as the complex-ity and risks of bridges increase. However, manypatients cannot pay for multiple implant restora-tions and if this is the case and a bridge wouldhave a poor prognosis the only alternatives areto leave the space unrestored or provide apartial denture.

Any tooth that can be crowned can also beconsidered as an abutment tooth for a conven-tional bridge, but the abutment tooth may haveto withstand forces from different directions thanone crowned as an individual tooth (Figure 3.7).

Teeth with active periodontal disease shouldnot be used as abutment teeth until the diseasehas been successfully treated. However, manyteeth with reduced alveolar support followingsuccessful treatment of periodontal disease canbe used. They are commonly splinted to otherabutment teeth to give mutual support.

Some dentists prefer to avoid root-filled teethor teeth needing post crowns because of thechances of fracture of the roots. However, thisrisk exists whether or not the tooth is used asan abutment tooth. It may even be reduced if thetooth is used as one of a number of abutmentteeth in a larger bridge, so that the force of ablow to the tooth is shared by the other

abutments. Given the choice between a toothwith a post crown as an abutment and a perfectlysound tooth, it is more conservative of toothtissue to use the former. Although some surveys(mostly out of date and relating to obsolete postsystems) have shown a higher incidence of failurewith post crowns than other forms of retainer,these figures are similar to the failure rate forindividual post-retained crowns.

There may be no suitable alternative abutmentto a root-filled tooth, and the choice is thenbetween using the tooth or not making a bridge.

Many successful bridges have been made usingroot-filled abutment and post and core supportedretainers. Success will depend upon the amountof residual dentine present and the occlusal loadson the bridge as well as the stability of the posts.

Length of span

Any design of bridge may be used for short spansof one premolar or incisor width. Simpleminimum-preparation cantilever bridges might stillbe possible for longer upper anterior spans usingeither the canine or central incisor as theabutment tooth, but the palatal surface of thepontic should be designed so that it shares theguidance with the other teeth and does not carryit exclusively.

Occlusion

Not only should the occlusion of the remainingteeth be assessed, as described in Chapter 4, butthe potential occlusion of the pontic with theopposing teeth should also be assessed. In somecases the occlusal relationships of the potentialabutment teeth will help determine which shouldbe used and which design of bridge is suitable.Figure 10.5 shows two cases: one suitable andone unsuitable for a simple cantilever bridgereplacing the upper lateral incisor, with the canineas the only abutment tooth. The differencebetween them is the way the lower incisors relateto the space when the mandible is moved in theprotrusive lateral direction. In the second casetwo abutment teeth will be necessary: either thecanine and the first premolar with a cantileverdesign, or the canine and central incisor with afixed–fixed or fixed–movable design.


Shape of ridge

The contour of the saddle area will be taken intoaccount in determining whether a bridge with amovable buccal veneer or a partial denture shouldbe made (see Chapters 7 and 8), or whethersurgical ridge augmentation should be considered(Figure 7.9).

When a bridge is to be made, the shape of theridge will affect the appearance of the pontic, andif this is likely to be a critical factor, in other wordsif the neck of the pontic shows and the patient is

very concerned about their appearance, then oneof the procedures described below should befollowed to ensure an acceptable final result.

Predicting the final result

The final appearance of the bridge can bepredicted using the study casts, by various intra-oral trials or by means of a provisional bridge.Sometimes combinations of these methods are


Figure 10.5Occlusal assessment.a A missing lateral incisor with deep overbite. Thelower canine tooth touches the palate and the palatalsurface of the upper canine. There would be insufficientspace for a minimum-preparation bridge without minororthodontic treatment or some reduction of upper orlower canine teeth.

b The same patient in a right lateral excursion. Theocclusion is canine-guided and the lower teeth are clearof the upper lateral incisor space, so that a simplecantilever bridge with the canine as the sole abutmentwould not apply unfavourable forces to the pontic.Even with some tooth reduction, this would still be theleast destructive design. Alternatively the bridge couldbe made without any tooth reduction or toothmovement, leaving the retainer high in the occlusion –acting as a Dahl appliance. The occlusion of the patientshown in Figure 8.1c is similar.

c In this case the lower incisor passes through theupper lateral incisor space in right lateral excursion,such that group anterior guidance will be inevitable.

necessary. In straightforward cases the dentistand technician will have a good idea of what thefinal bridge will look like, but the patient will beless clear. The prediction is therefore for thepatient’s benefit. In other cases where there areunusual features, the dentist and technician maynot realize the full aesthetic implications ofattempting to make a bridge, or their under-standing may be different. In these cases thepatient is likely to be even more confused.

Many patients who complain about bridgesafter they are fitted are unhappy with theirappearance. Not only is it good planning topredict the final appearance of the bridge andseek the patient’s acceptance before starting, buta record of the predicted appearance may also beuseful from a dento-legal point of view should thepatient eventually complain.

As well as the appearance of the final bridge,potential difficulties in preparing the teeth shouldbe predicted when possible. These includeproblems with retention and path of insertion,and the possibility of endangering the pulps of theabutment teeth.

Study casts

A second study cast should be poured of the archin which the bridge is to be made. It is sufficientsimply to cast the alginate impression a secondtime, if it can be removed from the initial castintact. Alternatively, two impressions should betaken or the study cast duplicated in the labora-tory.

The second study cast may be used for trialpreparations to predict the problems outlinedabove as well as problems of the individualabutment preparation (Figure 5.3b, c and d). Theprepared study cast may be used to make a trialor diagnostic wax-up of the bridge to show tothe patient. This is particularly useful when theshape of the abutment teeth will be altered bythe retainer crowns or where orthodontic treat-ment is planned prior to bridge construction.

There are usually alternative means of replac-ing missing teeth and these change the appear-ance in different ways. Figure 5.3 shows modifiedstudy casts illustrating two ways of changing theappearance that would be difficult to describe tothe patient. Neither is ideal, and so the patientmust be warned that compromise is necessary.

The work should not be started until the patientunderstands and accepts this.

If the patient is to be shown the study cast, itis best to produce the wax-up in ivory-colouredwax, or to duplicate the waxed-up cast. Thepatient can thus look at a cast without thedistraction of the contrast between the artificialstone and coloured wax. Most find it easier tocompare the second, modified study cast with thefirst, rather than with themselves, partly becausestudy casts look so artificial to them that they arebetter comparing two similarly artificial objects,but also of course because they have difficulty inrelating a study cast, which is how others seethem, to a reversed, mirror image of themselves.Other precautions are detailed in Chapter 5. It ishowever, difficult for the patient to interpret thepotential clinical result from a diagnostic wax-upand ideally an intra-oral trial should be providedfor patients where extensive changes to theirteeth are planned.

Intra-oral trials

Partial dentures

Many patients who are to have anterior bridgesalready have a partial denture. If the appearanceof the artificial tooth on the denture is satisfac-tory and can be duplicated in a bridge, no furthertrial is necessary. An impression of the denturein the mouth should be taken to guide the techni-cian on the appearance required. However, if thedenture carries a buccal flange, it is wise to try adenture tooth or teeth in the mouth without abuccal flange, usually attached to a simple wax orshellac base, to show the patient the effect (Figure10.6). The change can be dramatic. This form ofintra-oral trial is suitable only when the shape ofabutment teeth is not to be changed.

Other reversible intra-oral modifications

The size and shape of potential abutment teethcan be increased by the temporary addition ofwax or composite. In some cases a denture toothrepresenting a pontic can also be temporarilyattached to adjacent teeth by means of com-posite. This is particularly useful when there is an


incisor space which is larger than the missingtooth. The design will usually be a cantilever fromone or other of the adjacent teeth leaving a resid-ual space. If the patient is not willing to haveorthodontic treatment to reduce the space to theappropriate size, then showing them the appear-ance of the diastema will give them the informa-tion to consent to having one. Again animpression should be taken of the temporarypontic to guide the technician. With extensiverestorations a matrix can be taken from adiagnostic wax-up and a labial slip made in themouth over the teeth using temporary crown andbridge materials to give the patient a better ideain the mouth of the anticipated appearance.

Temporary and provisional bridges

Once the preparations for a conventional bridgehave been made in the mouth, it is possible tomake a provisional bridge in the laboratory withacrylic or other materials. These provisionalbridges are rather more permanent than tem-porary bridges (usually made at the chair side),which are only intended to last for 2 or 3 weekswhile the permanent bridge is being made. This iscomparable to temporary and provisional crowns(see Chapter 6).

One of the purposes of a provisional bridge isto allow further modifications to the shape of thebridge for aesthetic reasons or as modificationsto the occlusion until both the dentist and patient

are satisfied with the result. These modificationsare then incorporated into the permanent bridge.The provisional bridge can also be removed andadjusted to allow periodontal or endodontictreatment as necessary.

A provisional bridge can also be used toconfirm that the selected abutment teeth areadequate to support the bridge. For example, ifthe abutment teeth demonstrate increasingmobility or the provisional bridge continuallydebonds it may indicate that the abutment teethare not suitable for long-term use and alternativeapproaches should be considered.

An example of the use of a provisional bridgeis as an immediate insertion replacement of atooth or teeth to be extracted. If the permanentbridge is to be a conventional design, theabutment teeth are prepared and the provisionalbridge made before the extraction – the prep-arations being protected by separate temporarycrowns.

If the permanent bridge is to be a minimum-preparation design, a Rochette design may beused to facilitate removal and modification as theextraction socket heals.

Practical steps in choosing a bridgedesign

So far, all the discussion of bridge design has beenrather theoretical and somewhat inconclusive.


Figure 10.6

An intra-oral trial.

a A partial denture replacing four incisor teeth, whichis to be replaced by a bridge. There is a buccal flangeand a midline diastema.

b Denture teeth set on a wax baseplate being tried into ensure that the patient is happy about the appear-ance of pontics without a buccal flange or the midlinediastema. Periodontal treatment will be providedbefore a bridge is made. The bridge will be six units,with the upper canines as the two abutment teeth.

This is inevitable, since designing bridges is stillrather more of an art than a science. It is basedpartly on the clinical experience of the dentist,which will vary from person to person, and onthe clinical condition of the patient, which againwill vary. However, the design process has tostart somewhere. Examples at the end of thischapter illustrate the logical steps in this process.

General approach

A list should be made of all the likely designs fora bridge in the case being considered. This shouldinclude the potential abutment teeth and theirretainers, together with the basic design of bridge(conventional, minimum-preparation, cantilever,fixed–movable, fixed–fixed and so on). In a simplecase when the dentist is experienced, the list canbe made mentally. For the less experienced and formore complex cases, it is helpful to write it down.

Every design should be considered in turn andadvantages and disadvantages listed. In somecases, the optimum design will be obvious fromthis procedure. In others, further investigationswith modified study casts, intra-oral trials orprovisional bridges may be required.

Details of stages in the designprocess

Selecting abutment teeth

After the general examination of the patient andwhole mouth, individual potential abutment teeth

should be examined and a note made of thepresence of caries or restorations and the extentand quality of any restoration present.

The periodontal state should be examined,including the presence of plaque and otherdeposits, gingival bleeding and periodontalpockets.

The vitality and mobility of the tooth should betested and a periapical radiograph obtained.

Usually any major problems with the individualtooth should be dealt with first by appropriatetreatment, but sometimes the more sensiblesolution is to extract the tooth and replace it asan additional pontic on the bridge rather thanretain a dubious tooth as an abutment when itspresence may well jeopardize the future of thewhole bridge. An example of this is where threelower incisor teeth are already missing and thefourth has very little bone support. The lowercanines are sound and will make good abutmentteeth. They will have to be used in any case tosupport the bridge. Including the remainingincisor will not add significantly to the support ofthe bridge and may well detract from its long-term prognosis.

A judgement must be made as to the progno-sis of all the teeth in the vicinity of the bridge andin the rest of the mouth to reduce the risk ofanother tooth having to be extracted shortly afterthe bridge is made.

Selecting the retainers

The list of potential alternative retainers willinclude minimum-preparation, complete andpartial crowns retainers. The choice of a crownis inevitable when the tooth is already heavily


Figure 10.7

The upper first molar has over-erupted. It shouldeither be intruded to the original occlusal planeorthodontically or ground level to the occlusal planeprior to a lower bridge being made. This is in order toavoid occlusal interferences in lateral excursions.


Figure 10.8Bridge designs for single missing incisors.a A missing upper lateral incisor with rotated canine and firstpremolar teeth. There is a Class II Division II incisor relation-ship with a deep overbite and minimum overjet. This means thatno space is available for a minimum-preparation bridge attachedto the central incisor without orthodontic (or Dahl) treatmentor some tooth reduction. The occlusion is satisfactory for asimple cantilever bridge using just the canine tooth as theabutment, and its rotation could be corrected with a completecrown and a conventional bridge. The first premolar has a failedamalgam restoration, which will be replaced separately with acomposite restoration to improve the appearance.b Another missing lateral incisor, this time with onediscoloured, non-vital, root-filled central incisor and a largemesial carious lesion in the other. The central incisors are alsomisaligned. The canine is sound. The choice here is between(in order of preference):• Making a cantilever minimum-preparation bridge retained by

the canine and restoring the two central incisors indepen-dently, possible by bleaching the root-filled central incisor.

• To restore the carious incisor and crown the root-filledincisor, bringing it into line with the other tooth and usingthe crown as a conventional retainer for a cantilever pontic.

• If it is considered that the root-filled central incisor wouldnot be an adequate abutment alone for a cantilever bridge,both central incisors could be crowned and splintedtogether to support the cantilever pontic.

c A missing upper canine tooth. These are difficult to replace bybridges when the occlusion will be guided by the pontic in lateralexcursions, and in these cases several abutment teeth may benecessary. By grinding the lower canine slightly and leaving thepontic slightly short, it was possible to maintain group functionin this patient rather than produce canine guidance by the pontic.The two premolar teeth were connected as abutments for athree-unit cantilever bridge. This design was chosen in prefer-ence to a fixed–fixed bridge so that preparing the sound, match-ing and well-aligned incisor teeth could be avoided.

A fixed–movable design with an inlay in the distal surface ofthe lateral incisor would not have been practicable, because theangulation of the lateral incisor would prevent a common pathof insertion between the first premolar and a groove in an inlayin the incisor. This design would have been possible (althoughnot desirable) if the lateral incisor had been more proclined.

A fixed–fixed minimum-preparation bridge could be consid-ered, but not a cantilever design as both adjacent teeth wouldbe poor abutments.d A complicated case. If the only tooth missing was the upperlateral incisor then the ideal design would probably be acantilever minimum-preparation bridge from the canine tooth.However, this tooth is needed to help retain a bridge replac-ing the two premolar teeth. Therefore the design to replacethe lateral incisor consisted of a minimum-preparation bridgecantilevered from the central incisor. The two premolar spaceswere both restored by minimum-preparation, fixed–fixedbridges with the canine and first molar teeth as abutments.

These were made in the days when fixed–fixed minimum-preparation designs were generally used. Now the designwould be fixed–movable on both sides with the caninessupporting the major retainers with fixed connectors andmovable joints distally. On the right-hand side of the picturethe molar tooth would have a minimum-preparation retainerwith a rest seat supporting a finger from the pontic, resistingaxial forces on the pontic. On the left the minor retainerwould be a gold inlay replacing the MO amalgam restoration– in other words a hybrid bridge.

d

c

b

a


Figure 10.9

Replacing more than one tooth.

a With an anterior open bite and normal lateralincisors, a four-unit fixed–fixed minimum-preparationbridge with the lateral incisors as the abutment teethwould be satisfactory. There is no need to include thecanine teeth. This design is also acceptable in somecases where there is normal occlusion between theanterior teeth, but the left and right lateral excursionsare canine-guided. In such cases the design may beminimum-preparation or conventional.

b Even with three incisors missing, with the occlusionbeing protected in right lateral excursion by a soundcanine (shown here), the lateral incisor may be used asthe only abutment on the right side for a five-unitfixed–fixed conventional bridge. The bridge could beextended to include the right canine, giving additionalsupport and a more symmetrical appearance, but thiswould make the embrasure space between the lateralincisor and canine difficult to clean and would beunnecessarily destructive.

c and d The central incisor space is now reduced toapproximately one-quarter of its proper width,although this is somewhat difficult to judge since theleft central incisor has an acrylic crown. The rightcanine also has an unsatisfactory acrylic crown. Trialwax-ups showed that a satisfactory appearance couldbe obtained by extracting the non-vital lateral incisorand making a four-unit fixed–fixed bridge. Both centralincisors and the lateral incisor pontic would be small,but would give a better appearance than two largecentral incisors with the midline offset even further.The alternative of orthodontic treatment prior tobridgework was offered to the patient and declined.This is a similar problem to that of another patient,shown in Figure 7.7.

d The upper lateral incisor has been extracted and thetwo abutment teeth prepared. It is now clear thatthere will be sufficient space for the planned treatment.A provisional bridge will be fitted and periodontaltreatment provided before impressions are taken forthe permanent bridge.

restored. The choice between a minimum-prep-aration retainer and a crown will depend uponwhether the abutment teeth have restorations inthem, the occlusal clearance and the appearanceof the abutment teeth. If the only difficulty withminimum-preparation retainers is the lack ofocclusal clearance, it may be possible to createsufficient clearance by the means described in thecaption to Figure 10.5.

Selecting the pontics and connectors

The design of pontics and connectors is theresponsibility of the dentist and not the techni-cian. Detailed instructions should be given to thetechnician, particularly on the contour of theridge surface of the pontic (see Chapter 9). Whenthe technician is unfamiliar with the dentist’s usualrequirements, the details of the design should be drawn and sent to the technician as part ofthe prescription for the bridge. Where ametal–ceramic pontic is to be made, the dentist

should indicate where the porcelain should befinished. In some cases an all-porcelain occlusalsurface is required; in others the porcelain coversonly the buccal surface and buccal cusp, leavingthe remainder of the occlusal surface in metal.Again, this should be specified.

Planning the occlusion

Details of this stage were given in Chapter 4. Thefirst decision to be made is whether to articulatestudy casts and, if so, whether it is necessary touse a simple hinge or semi-adjustable articulator.With small bridges it is helpful to mount casts onat least a simple hinge articulator. With mostlarge bridges a semi-adjustable or fully adjustablearticulator should be used.

The second decision is whether any occlusaladjustment is necessary prior to tooth prepara-tions for the bridge. With posterior bridgeworkit is often necessary to adjust an over-eruptedopposing tooth (Figure 10.7). The anticipated


e Existing crowns and a bridge following extensive,successful periodontal treatment and the extraction ofthe lateral incisors. None of the remaining teeth aresatisfactory abutments for simple small bridges, and apartial denture replacing the lateral incisors and thetotally unsatisfactory premolar pontic would be damag-ing to the periodontal tissues. A splint/bridge incorpo-rating the principle of cross-arch splinting is indicatedhere.

f Sound restored abutment teeth and a span that willaccommodate one premolar and one molar pontic. Thebuccal surface of the premolar is sound and of goodappearance, and so one design could be a fixed–movable conventional bridge with a complete crown onthe molar tooth and an MOD inlay (without cuspalcoverage to avoid showing gold) in the premolar. Themovable joint will be accommodated in the distal boxof the inlay. A hybrid bridge would not be suitable,since the restoration in the molar tooth is too largefor this tooth to have a minimum-preparation retainer.

occlusal relationship of the pontic with the oppos-ing teeth may influence the basic design of thebridge as well as the details of the occlusal surfaceof the pontic; although this step is listed as thefinal one in the sequence, and it is usually consid-ered last. If the bridge design is influenced by it,it will be necessary to introduce feedback loopsto earlier stages.

Examples of the bridge designprocess

Figures 10.8 and 10.9 give practical examples andthe reasons for the choice. Some alternativedesigns, and the reasons for rejecting them, arealso given.

Dentists will inevitably become biased in theirselection of bridge designs by their own experi-ence of clinical success and failure. Indeed, thebias of the authors may be detected, for example,

in Figures 10.8 and 10.9. Care should be taken toprevent this bias overriding more substantial clini-cal criteria.

As another example, the above list sets outalternative designs for the replacement of oneupper lateral incisor. Some of the suggestions onthis list are uncommon and would not be usedother than in exceptional circumstances. In somecases, however, the upper lateral incisor wouldbe replaced as part of a much larger bridge/splint.In that case all the remaining upper incisors andprobably teeth further back in the arch wouldalso be included. So the list could be extendedfurther to show an even greater variety of poten-tial abutment teeth, and extended further still toshow the choice of materials.

The important point is to show the large rangeof designs possible and the dangers inherent infalling into the habit of only using a limited rangeof designs. Each case has a unique combination offeatures and a specific design should be chosen toaddress these.


Alternative designs for a bridge to replace an upper lateral incisor

Design Abutment(s) Retainer(s)

1 Cantilever Canine Minimum-preparation2 Cantilever Canine Crown3 Cantilever Central incisor Minimum-preparation4 Cantilever Central incisor Crown5 Cantilever Both central incisors Minimum-preparation6 Cantilever Both central incisors Crowns7 Fixed–fixed Canine + central incisor Minimum-preparation8 Fixed–fixed Canine + central incisor Crowns9 Fixed–movable Canine + central incisor Minimum-preparation

10 Fixed–movable Canine + central incisor Crowns11 Fixed–movable Canine + central incisor Minimum-preparation + crown

The most common designs are at the top of the list but all the designs, and others, are used in appropriatecircumstances.

This chapter should be read in conjunction withChapter 6. Many of the techniques are identical,and so this chapter will deal only with those thatare peculiar to bridges or where a differentemphasis is necessary.

Preoperative procedures

All the planning stages described in Chapters 5, 6and 10 should be undertaken. In particular, theshade should be taken and an impression for theopposing cast made. The following additionalpreoperative procedures may also be required.

Occlusal adjustment

It is more often necessary to carry out an occlusaladjustment in preparation for a bridge than forcrowns. Indications for this are identified inChapter 4. A new impression must be taken forthe opposing jaw if this has been adjusted, sincethe study cast will obviously no longer beaccurate.

Additional space for anterior retainers can beproduced by using a Dahl appliance (see Chapter 6).

Preparations for a temporary bridge

For a conventional bridge it must be decidedwhether a temporary bridge will be made orwhether the patient will be left with individualtemporary restorations to protect the abutment

teeth. When the patient has a satisfactory tem-porary denture and especially if the design is acantilever or fixed–movable conventional bridgeor any design of minimum-preparation bridge, itis often better to make separate temporaryrestorations rather than a temporary bridge.When the design is fixed–movable and the pathsof insertion of the retainers will not be parallelto each other, it may be impractical to make atemporary bridge. Besides, when a minor retainersuch as a distal-occlusal inlay is to be made for afixed–movable bridge, the temporary bridge(which will be fixed–fixed) may loosen at theminor retainer.

However, in many cases, particularly for largerfixed–fixed conventional bridges, a temporarybridge is essential to protect the abutment teethand to retain their relationship with each otherand the opposing teeth. Temporary bridges maybe made in one of two ways: either by one of thechair-side techniques described in Chapter 6, orby making an acrylic temporary bridge on thestudy cast, using the trial preparations, and thenrelining and adjusting this at the chair side asnecessary.

If a chair-side technique is to be used, a trialwax-up on the study cast should be made andduplicated (by means of an alginate or elastomericimpression) to make a stone cast. A vacuum-formed PVC slip or a silicone putty matrix canthen be produced (Figure 6.17). Alternatively, asilicone impression of the waxed-up study castmay be used directly in the mouth to make thetemporary bridge.

Figure 11.1 shows a laboratory-made tem-porary bridge, constructed before the teeth areprepared so that it can be adapted and cementedat the tooth preparation visit. Techniques for

241

11 Clinical techniquesfor bridgeconstruction

constructing chair-side temporary bridges aredescribed later.

Preparing the abutment teeth

Preparations for minimum-preparation bridges

Since the introduction of minimum-preparationbridges there have been fluctuations in fashion asto the degree of tooth preparation that should becarried out. Initially very little preparation wasundertaken, and then over the next few yearsvarious authors recommended more and moreextensive preparation with finishing lines, seatinggrooves at right angles to the path of insertionand location grooves in the line of the path ofinsertion all being advocated. Some dentists evenwent as far as using operating microscopes andcomplicated paralleling devices.

There is very little evidence that any of theseproduce significant benefit. With this lack ofevidence the only sensible approach is to limittooth preparation to those aspects where thereis a logical reason to prepare the tooth (seelater).

There is no justification for producing a finish-ing line near to the gingival margin of eitheranterior or posterior teeth. Some dentists haveadvocated this in order to identify where thetechnician should complete the wax-up. It is notjustified to prepare enamel for this purpose whichcan be achieved equally well by drawing a line onthe study model with a pencil. Gingival marginpreparation is also advocated to reduce the thick-ness of metal at the surface and therefore the

discrepancy between the retainer surface and theenamel which might be plaque-retentive. Usuallythe enamel cannot be prepared to a sufficientdepth to allow a straight-line junction betweenthe retainer and the tooth surface withoutencroaching on dentine. It is better to make themargin of the retainer a millimetre or two awayfrom the gingival margin so that access for clean-ing is easy.

One danger of overpreparing teeth for thesetypes of retainer is that if the retainer becomesdebonded but the bridge is held in place by otherretainers, features such as grooves tend tobecome carious more rapidly than unpreparedenamel surfaces and, because the dentine is closerto the base of the groove, it too becomes cariouswith the result that a further minimum-prepara-tion retainer is not possible. There is also lessenamel to bond to and all resin cements bondbetter to enamel than to dentine. Therefore thepreparation should be within enamel and the useof a local anaesthetic is discouraged to allow thepatient to warn the dentist when the dentine isbeing approached.

However, some tooth preparation is usuallynecessary and the principles that should guide theoperator in deciding what this should be are:

• The maximum surface area of enamel shouldbe used for retention of major retainers, avoid-ing the incisal edges and potential ‘shinethrough’ of the metal framework (Figure 9.2c).

• The bridge should seat positively so that it canbe held firmly in place without movementagainst the resistance of rubber dam while thecement is setting. For anterior teeth this shouldbe a small horizontal notch with the base atright angles to the path of insertion of the

242 Clinical techniques for bridge construction

Figure 11.1

A laboratory-made temporary acrylic bridge (see alsoFigure 9.4c).

Clinical techniques for bridge construction 243

Figure 11.2

Preparations for minimum-preparation bridges.

a A shallow rest in the cingulum and minor prepara-tion to the mesial surface with a shallow slot. Thisdefines the extent of metal wrap-around possiblewithout metal showing labially.

b A mesial rest seat on a molar tooth with minorpreparation to the palatal surface to allow the metalframework to extend over the maximum surface area.

c Incorporating an MO preparation into a minimum-preparation retainer.

bridge. The notch should be entirely in enameland need only be 2 or 3 mm wide (Figure11.2a). With posterior retainers this purpose isusually served by preparing a shallow rest seatin enamel (Figure 11.2b) or alternatively byreplacing a small restoration (Figure 11.2c).

• Preparation may be necessary to allow anadequate thickness of retainer when the occlu-sion is unfavourable. It is sometimes possiblefor the framework of a minimum-preparationbridge to be cemented ‘high’ to achieve spacein the same way as a Dahl appliance works.

The maximum enamel surface area can often beachieved with anterior bridges without any toothpreparation other than a seating ledge (Figure11.3).

Figure 11.4 shows a section of an unpreparedmolar tooth (Figure 11.4a), together with apattern for a minimum preparation retainerwithout preparing the tooth (Figure 11.4b) andafter preparing the tooth (Figure 11.4c). In bothFigure 11.4b and c there is inevitably a change ofcontour at the margin of the retainer, which mustbe kept clean by the patient. In this, and manycases, there would be no advantage in preparingthis axial surface. It has been suggested that afinishing line indicates to the technician where the

retainer is to finish. This is a completely unjusti-fiable reason, because the same indication couldbe given by the dentist drawing the retaineroutline on the study cast.

Preparations for conventional bridgesParalleling techniques for conventionalpreparations

A fixed–fixed conventional bridge requires two ormore teeth to be prepared in a common path ofinsertion. Special techniques are used to ensurethat there are no undercuts and yet each individ-ual preparation is as retentive as possible. Theseare listed in increasing order of complexity.

Paralleling by eye

Two or three teeth close together can be madeparallel by eye. The clinician will become moreadept at doing this with experience, and shouldconcentrate on developing the skill, in the firstplace on study models. The inexperienced opera-tor should always practice on study models beforepreparing the teeth.


Figure 11.3

a The lingual view of an incompletely erupted uppercanine tooth to be used as an abutment for a minimum-preparation bridge.

b Crown lengthening has been carried out to give agreater surface area and a horizontal seating ledge hasbeen prepared at the cingulum to stabilize the retainerfirmly while the bridge is being bonded. This size ofledge is all that is necessary for the purpose.


Figure 11.4

Posterior minimum-preparation retainer design.

a A section through an extracted molar tooth.

b The tooth has not been prepared and the retainerwill be bonded directly to the enamel surface. Thejunction at the gingival margin should be well clear ofthe gingival tissue

c There has been some preparation, entirely withinenamel, and so the retainer is partly within and partlyoutside the original tooth contour.

In the anterior part of the mouth it is poss-ible to see along the long axis of the teeth bydirect vision. Only one eye should be used, sincebinocular vision can ‘see around’ undercuts.Figure 11.5a shows a dentist assessing the pathof insertion of the upper canine teeth, which arebeing prepared for a bridge replacing the four

incisor teeth. It helps to look from as far awayas possible. It may also be useful to make a smallpencil mark on the two surfaces that may stillbe undercut. An assessment of parallelism orundercut can then be made by closing one eyeand moving the head so that one of the pencilmarks just disappears and then continuing to


Figure 11.5

Clinical methods of assessing parallelism of bridgeabutment preparations.

a Using direct vision, from a distance with one eyeclosed when, for example, upper canine teeth are beingprepared for a fixed–fixed bridge.

b Using a full arch mirror for the same purpose in thelower jaw. Full arch mirrors are used in clinical photog-raphy and many of the photographs in this book weretaken with one of these large mirrors.

c Using a straight probe as an ‘intra-oral surveyor’. Theoperator must stay very still other than moving theprobe round the abutment teeth in a controlledfashion. This, on its own, may not be enough but givesa guide to the presence of undercuts. Operators whomake a significant number of bridges develop consid-erable skill in this technique.

move the head until the other pencil markappears.

In the lower jaw and in posterior parts of themouth large mirrors are useful to show all thepreparations in the same field. Parallelism cannotbe assessed satisfactorily when two or morepreparations can be seen only by moving thesmall mouth mirror. Many of the photographs inthis book have been taken in larger, front-surface-reflecting mirrors (Figure 11.5b).

It is also helpful to use a straight probe like alaboratory surveyor, but in the mouth. The probeis placed against one of the prepared toothsurfaces, and then, held rigidly, it is moved overto the other abutment tooth without its angula-tion being changed. This is not a completely

reliable guide of course, and will detect only fairlygross undercuts or over-taper, but many cliniciansdo find it useful (Figure 11.5c).

The risk of not striving for near parallelism isthat excessive taper between abutment teeth ismore likely to result in de-cementationsomewhere on the bridge.

Extra-oral survey

With larger bridges and when teeth are preparedon both sides of the arch, the simplest and mostreliable method of assessing parallelism is to takea simple impression (usually in alginate) once thebasic reduction has been carried out on all the


Figure 11.6

Surveying preparations.

a Initial preparations have been carried out on theupper canine teeth in the mouth and this cast has beenmade from an alginate impression. The two prepara-tions have been varnished. The bridge will be a six-unitimmediate-insertion provisional bridge, and the tworemaining incisors with extensive alveolar bone loss willbe extracted.

b The preparations are surveyed with a fine rod, and the cast istrimmed until they are parallel. Trimmed areas show up in contrastto the untouched varnished areas. Similar reduction is carried out inthe mouth. The process may need to be repeated.

abutment teeth. The impression is cast at thechair side in a fast-setting plaster, usually with anaccelerator such as alum added. The cast is thensurveyed at the chair side and further preparationcarried out as required (Figure 11.6). The proce-dure may be repeated several times with moredifficult cases or with large numbers of abutmentteeth. When the abutments are satisfactory forthe path of insertion, the final smoothing andfinishing of the preparations is carried out.

Paralleling devices for crown preparations

Many of the devices available are cumbersome,unreliable or extremely expensive. One of thesimpler ones consists of a stainless-steel mirrorwith vertical lines scribed on it. This is placedbuccally or lingually and used to assess the mesio-distal parallelism of abutment preparations. Itcannot be used for the buccal-lingual surfaces.Another device consists of a clear plastic discwith a pin passing through it. This is held againstthe occlusal surfaces and can be moved around,acting as a surveyor. These two devices may beuseful, but the inexperienced dentist is betteradvised to master the basic techniques of survey-ing by eye and extra-oral surveying

Making temporary and provisionalbridges

Temporary bridges are made at the chair side ormore rarely in the laboratory and are expectedto be used clinically for a few weeks while thepermanent restorations are made. Provisionalbridges are used when longer-term temporaryrestorations are required, usually while occlusalor appearance issues are resolved and time isneeded before the permanent restoration isprovided. They may also be used where endodon-tic or periodontal treatment needs time to beassessed or while implants are integrating. Theyare normally laboratory-made.

Choice of material

Chair-side temporary bridges are made with oneof the materials designed for the purpose.

Laboratory-made temporary bridges are madewith similar techniques to those described forcrowns in Chapter 3. However, long-term provi-sional bridges often need reinforcement tostrengthen the span and this can be achieved bythe technique described in Chapter 6 using light-cured composite and quartz-fibre reinforcement.Alternatively a simple metal casting may be madeusing an impression of the prepared teeth and thiscan be faced with composite or acrylic. However,the occlusal surface and margins should be acrylicor composite so that adjustments can be madeeither by removing or adding material if neces-sary. Modern temporary crown and bridgematerials are sufficiently resistant to wear for thisto be possible.

Choice of technique

Chair-side construction

The majority of temporary bridges can readily bemade at the chair side, often in less time than ittakes to modify a laboratory-made temporarybridge, and of course avoiding the additionallaboratory cost.

The chair-side technique illustrated in Figure11.7 is similar to the temporary crown techniquesshown in Figures 6.17 and 6.18. The mould maybe an impression of a study cast with the ponticmade from a denture tooth, or it may be avacuum-formed PVC slip. In many anteriorbridges, though, the patient is already wearing atemporary denture, and it is sufficient to take analginate or silicone impression of the arch withthe denture in place and use this to make thetemporary bridge. This is the technique illustratedin Figure 11.7. The excess material flowing intothe areas of the impression previously occupiedby the denture can be removed with an acrylicbur in a straight handpiece, once the plastic hasset and the temporary bridge has been removedfrom the mouth.

For posterior bridges, where there is often notemporary denture, but where the appearanceof the pontic is not important, an impressionmay be taken (with nothing in the saddle area)before the teeth are prepared, and used to makeindividual temporary crowns for the abutmentteeth.


Laboratory-made provisional bridges

If the provisional bridge is to last for more thana week or two, if it is large or if its appearanceis particularly important, then a laboratory-madeprovisional bridge is preferable to a bridge madeat the chair side.

One technique is to make preparations of theabutment teeth on a study cast so that when thefull-scale preparation is done in the mouth thetemporary bridge will be a loose fit. If the prep-arations on the study cast are completed to fulldepth, it will be impossible to duplicate themexactly in the mouth, and the provisional bridgewill not seat.

If full-scale trial preparations have been madeto assess parallelism, this cast may still be usedto make a provisional bridge, the prepared teethon the study cast are covered with a spacer oftin foil, or the fit surfaces of the bridge areenlarged with a bur in the laboratory before thebridge is returned to the chair side. The bridge

is waxed up and processed in the laboratory usingconventional acrylic techniques (Figure 11.1).

Once the abutment teeth have been preparedin the mouth, the provisional bridge is tried in andwill usually need to be relined with a higheracrylic.

The technique using an accurate workingimpression of the prepared teeth to make aprovisional bridge is more reliable, and oftenproduces a better marginal fit but takes an extraappointment. This is necessary, however, if ametal casting is to be incorporated for extrastrength (Figure 11.8).

Cast metal and composite provisional bridges(and crowns) have become popular with somedentists. They are strong, fit well and have a goodappearance. However, the laboratory costs arehigh, sometimes nearly as much as permanentrestorations. When they are used, all the surfaceswhich may need to be adjusted must be incomposite which can be ground and added towith more composite. This is sometimes difficult,


Figure 11.7

Chair-side temporary bridge construction.

a An alginate impression is taken before the prepara-tions are started, with the temporary denture in place.The alginate is removed from the buccal sulcus area tofacilitate reseating in the mouth before the temporarycrown and bridge material is put into the impression.

b The plastic temporary bridge removed from themouth. The material has flowed palatially into the spaceleft by the plate of the denture. This can now beremoved together with the thin flash over the adjacentunprepared teeth. The almost-transparent buccal–incisal surface of the upper right central incisor retainershows that more preparation of the abutment tooth isneeded here or the retainer will be too thick. To alesser extent the same is true at the tip of the upperleft lateral incisor. These modifications should be madeto the preparations before the impression is taken.

for example, when the palatal surface of an upperincisor abutment has been prepared with achamfer for a metal–ceramic retainer there willnot be enough room for metal–composite. Thismeans that the laboratory will make a metalpalatal surface which will be difficult to adjust andwhich negates the purpose of a provisionalrestoration. An all-acrylic or composite provi-sional restoration would be better. Before makinga metal–composite provisional restoration thereshould be a cost–benefit analysis compared withless expensive alternatives.

Cementing temporary and provisionalbridges

Temporary bridges should be sufficiently retentivenot to cause trouble between appointments, butit should be possible to remove them withoutexcessive force or damage. The temporary crownand bridge cementing materials are supplied witha modifying paste that may be combined in varyingproportions with the base and catalyst pastes toweaken the final mix. Modified cement is recom-mended with large or very retentive bridges.Experience will guide the operator as to thecorrect proportions for the particular bridge andthe particular cement; 50% or more of the totalmix may consist of the modifying paste.

Provisional bridges may also be cemented withtemporary crown and bridge cement, but usuallywithout modifying paste because they need to lastlonger. Temporary cements are easily fractured

with ‘sudden’ loading such as with a bridgeremover (see Chapter 14). The cement remain-ing on the preparation can easily be removed asit is not adhesive.

The working impression

Any of the impression materials or techniquesdescribed in Chapter 6 are suitable for bridges.With fixed–fixed bridges it is often an advantageto have two working casts, one with removabledies for making the individual retainers and onethat is not sectioned and therefore preserves thefull contour of the saddle area together with therelationship of the abutment teeth and adjacentteeth. With good die location and a small bridge,an unsectioned model is not necessary, but withlarger reconstructions where the dies have to beremoved and replaced often, some die locationsystems tend to wear, allowing movement of thedies. Then a solid model may be necessary. Byensuring the correct relationships betweenabutment teeth the solid model is used toconfirm the bridge will seat easily in the mouth,the correct contacts to the soft tissues and thecontact points with the adjacent teeth.

All bridges should be made with full archworking impressions for maximum stability of theocclusion.

For short span bridges a stock tray is normallyadequate but for extensive or complex bridges aspecial tray is better. Figure 6.4 shows theconstruction of special trays.


Figure 11.8

A long-term, immediate-insertion, metal/compositeprovisional bridge in heat-cured acrylic incorporating ametal casting.

Occlusal records

For the choice of appropriate occlusal records,see Chapter 4. The larger the bridge, the moretime-consuming is any occlusal adjustment at thechair side, so a semi-adjustable (or fully adjustablearticulator) should be used, together with theappropriate occlusal records.

Trying in the metal framework orseparate units

Metal–ceramic conventional bridges should betried in at the metal stage. Experienced operatorsmaking small bridges, who are familiar with theirtechnician’s work, sometimes omit this stage, butthis is inadvisable under other conditions.

Metal–ceramic bridges of up to six units areoften cast in one piece. When they are tried in,all the checks listed in Chapter 6 should be made,and if the framework is acceptable, it may be

returned to the laboratory for the porcelain tobe added.

If the framework does not seat, and onceobvious causes have been eliminated, such as tightcontact points or air blows on the fit surface ofthe casting, it must be assumed that the relation-ship between the abutment teeth is the problem.This may be wrong either because the abutmentteeth have moved since the impression was taken(perhaps because a temporary bridge was notprovided) or because the die location is at fault. Ifthis is suspected, the bridge should be divided andthe separate components tried in. It is better tosaw through the bridge with a fine fretsaw cuttingdiagonally through one of the pontics rather thanthrough a connector (Figure 11.9a). This gives alarger surface area for the bridge to be re-soldered, and the solder joint will be covered byporcelain, which will further strengthen it. If theseparate units fit, the bridge is relocated (seebelow) and soldered with a high temperaturesolder or laser welded before the porcelain isadded. It is advisable to retry the bridge again.


Figure 11.9

Localization

a The metal framework for a three-unit metal–ceramicbridge. This has been tried in the mouth and found notto fit. When sectioned diagonally through the pontic, theseparate retainers fitted, and so it was relocalized in themouth and soldered before the porcelain was added.

b A large bridge, cast in four sections, being localizedin the mouth with fast-setting acrylic. To stabilize it, abar (a long bur shank is the right stiffness) will beattached across the back with more acrylic.

If, once the bridge is sectioned, some of theretainers fit and others do not, a further impres-sion is needed. This will be of the unsatisfactoryabutments, with the satisfactory retainers and theattached parts of the pontic left in situ. They willbe a guide to the technician in waxing-up therepeated sections. A further retry and localizationin the mouth is necessary before soldering.

With larger bridges not cast in one piece, theseparate sections should be tried in before local-ization and soldering.

Bridges made in other materials are usuallycompleted and not tried in as separate units.Posterior all-metal bridges are necessarilyrelatively small, as are anterior all-porcelainbridges. Where metal units are to be soldered tometal–ceramic units, it is possible to try in theseparate retainers before the connectors aresoldered.

Localization techniques

Full arch impressions should be taken almostuniversally for bridges and so there is little needfor localization of individual retainers. However,problems still arise, as outlined in the previoussection, with the fit of one-piece castings. It mayalso be difficult to get a single impression of allthe teeth at once, especially when large bridgesare made in the lower arch. The tongue makes itdifficult to obtain a dry field on both sides of thearch at the same time. In these cases separateimpressions of groups of abutment teeth aretaken and related to each other with a localiza-tion technique.

An overall impression of the castings in placemay be used for localization. A rigid elastomericmaterial such as polyether must be used, sincesofter materials distort when the casting and diesare seated in the impression. Sometimes, it isnecessary to cement the retainers with a veryweak cement or petroleum jelly to keep them inposition while the localizing impression is taken.An alternative is to use acrylic with a paint-ontechnique. When adjacent retainers are to belocated or a cut pontic re-soldered, it is sufficientsimply to clean the surfaces, paint a fast-settingcold-cure acrylic over the surface and allow it toharden before withdrawing the bridge (Figure11.9b).

Try-in and trial cementation offinished bridges

The checking procedure is as described inChapter 6. In some cases the bridge does not fullyseat and the operator may suspect that the teethhave moved, particularly if a metal stage try-inwas satisfactory. Rather than sectioning the bridgeagain, it may be left in the mouth for a few hours,preferably with no cement, or with petroleumjelly and zinc oxide powder (which does not set)to prevent oral fluids from irritating the exposeddentine. If after a few hours the bridge has notseated, the next stage is to use a silicone ‘seating’cement which does not set fully and flows underpressure. These cements will retain the bridge for24 hours while full seating occurs. Once fullseating has occurred it can be cemented with avery weak temporary crown and bridge cementwith a large proportion of modifier. Bridgescemented in this way may be left for days or evenweeks to settle before being finally cemented.This should be done routinely with larger bridges.

The advantages of trial cementation are that, aswell as possible improvements in marginal fit, thepatient has a chance to become accustomed tothe appearance and feel of the bridge, which canstill be modified out of the mouth if necessary.Any problems with the occlusion are likely toshow themselves and can be dealt with before thebridge is permanently cemented.

Trial cementation should not be attemptedwith all-porcelain bridges or the minor retainersof fixed–movable bridges. Trial cementation is notpossible with minimum-preparation bridges.

Permanent cementation

Minimum-preparation bridges

This depends on the technique used to make thebridge and the luting cement. The commonesttypes are now those with grit-blasted fit surfacesluted with an adhesive resin, or Rochette (macro-mechanically retentive) bridges or splints cementedwith a conventional, chemically cured compositematerial. These are used when it is likely that theywill have to be removed atraumatically.

Figure 11.10 shows the luting process for agrit-blasted bridge.


Summary of clinical techniques forminimum-preparation bridges

The stages in the construction are usually asfollows (Figure 11.10):

First appointment• Thoroughly scale and polish the abutment teeth

(and the remainder of the mouth of course)• Carry out any necessary tooth preparation

• Take an accurate working impression in anelastomeric material and other records, forexample the shade and an impression of theopposing teeth – although these should havealready been done at the planning stage.

Laboratory stage• Make the metal framework and pontic – the

pontic is usually metal–ceramic. Grit-blast thefit surface of the retainer(s).


Figure 11.10

Clinical technique for a minimum-preparation bridge.

A fixed–fixed design was used as the canine pontic wasin occlusion in lateral excursion. It was decided that acantilever bridge supported by either abutment wouldnot be sufficiently robust to withstand the occlusalforces.

a The working impression.

b The metal framework and pontic on the model. Thisis tried in the mouth and adjusted before etching orsandblasting. It should not be retried after etching, orthe delicate etched surface will be damaged. The designis fixed–fixed because the tooth being replaced is anupper canine that will be in occlusion in lateral excur-sions. A cantilever bridge from the premolar would nothave been adequate with this occlusion, and there wasinsufficient space to make a movable connector in thelateral incisor without excessive tooth preparation.

c Polishing the abutment teeth with pumice and waterafter applying rubber dam.

Second appointment• Try in and if necessary adjust the bridge• Re-grit-blast the fit surface with a chair-side

grit-blaster, or if one is not available, it is worthreturning the bridge to the laboratory if the fitsurface has been contaminated with saliva orrubbed too hard against the tooth surface. Nottaking care over this stage is one of the common-est causes of failure of the bond

• Re-polish the enamel surfaces to which thebridge is to be cemented, apply rubber damand acid-etch the enamel surfaces (if the lutingcement used requires this)

• Cement the bridge with a bonding resin madespecially for the purpose, and remove excesscement from the margins

• The set of some bonding resins is inhibited byair and so a water-soluble gel is applied to themargins until the bonding resin is fully cured

• The manufacturer’s setting time must bestrictly adhered to and very firm, very stillpressure applied for the whole period. If yourfingers do not go white and hurt a bit you havenot done it properly. Slight movement of thebridge while the cement is curing is anothercommon cause of failure.


d Phosphoric acid gel applied carefully with a paint-brush or with a syringe to the areas to be covered bythe retainers.

e Luting the etched bridge with a chemically curedcomposite specially made for the purpose. It wouldhave been a good idea to place some floss between theabutment and the adjacent teeth before cementing thebridge. Pulling this through the contact points beforethe cement set would have helped to remove excesscement. If the bridge had been sandblasted and lutedwith an adhesive resin, the margins would be coatedwith gel to exclude air while the cement set.

f Immediately after removing the rubber dam.

Success with minimum preparationbridges

In the early days minimum-preparation bridgesfailed commonly and developed a poor reputationwith some dentists. This attitude has lingered onto some extent and some dentists still do notmake minimum-preparation bridges. However, allthe surveys of properly made bridges withmodern designs and materials now show goodsuccess rates and it is likely that these successrates will improve.

If you or one of your family needed a toothreplaced, would your first choice be a minimum-preparation bridge, a conventional bridge or animplant?

Minimum-preparation bridges only work if theyare done properly but then they work well. Bothauthors of this book have made minimum-prep-aration bridges which have been successful formore than 20 years.

The golden rules for success are:

• Proper control of moisture from saliva or thehumidity of the patient’s breath using rubberdam.

• A dry supply of air for the three-in-one syringe.Some air supplies are contaminated bymoisture. This can be checked by blowing airat a clean sheet of paper. If the air supply iscontaminated it should be repaired or an alter-native air supply used.

• The bridge should be tried in before the rubberdam is applied and then re-grit-blasted immedi-ately before bonding. If there is a laboratory onhand this is straightforward. If there is not thenit is worth investing in a chair-side grit-blaster.

• Framework of adequate rigidity so that noflexing occurs.

• Well planned design – cantilevers andfixed–movable designs work better thanfixed–fixed designs.

• Adequate bonding areas.• Proper adjustment of the occlusion.

The cementation of conventionalbridges

This differs from crowns only in that withfixed–fixed bridges the surface area of the

combined abutment preparations is larger than anindividual crown and so the hydrostatic pressureof the unset cement is greater. Greater forcetherefore has to be applied to seat the bridgefully.

Because of the difficulty of cementing largebridges and the need for a long working timebefore the cement starts to set, zinc phosphatecement is still the most popular for large bridges.Its working time can be extended considerably:the mixing slab is cooled, very small incrementsof powder are added at a time, and mixed for along period (approximately 90 seconds). Readyproportioned cement in a plastic syringe is alsoavailable and is mixed in a mechanical vibrator. Ifthe syringe is used straight from the refrigerator,a consistent, slow-setting, air bubble-free mix isobtained.

For preparations with nearly parallel walls thetechnician may use an additional layer of die-reliefvarnish on the axial walls. This increases thecement film thickness in this area without increas-ing it at the margins, and so reduces hydrostaticpressure during cementation. With multiplepreparations it is also important that a dry fieldis maintained across all teeth. The dentist shouldalways be in control of the seating of the bridgewith finger pressure, getting the patient to biteinto a cotton wool roll is not acceptable.

Oral hygiene instructions andmaintenance

This is particularly important with bridges, and insome cases the techniques will be entirely differ-ent from those the patient has been taught todate or for crowns. The areas where differentcleaning techniques may be needed are betweenthe pontic and the ridge and the gingival marginsof the abutment teeth beneath the connectors.The technique will depend upon the design of theridge surface of the pontic, the part of the mouthwhere the bridge is situated and the patient’smanual dexterity. With ridge-lap and saddlepontics, superfloss, dental floss or tape should bethreaded through an embrasure space and thenpassed under the pontic to clean it and the ridge.The furry section of superfloss makes cleaningunder pontics much easier (Figures 9.5 and11.11).


Wash-through and dome-shaped pontics areusually cleansable entirely with the toothbrush,although in some cases a bottle brush is better.

Oral hygiene instruction should be given at thesame appointment as the bridge is cemented. Thepatient should be seen again in 1 or 2 weeks toensure that the new cleaning techniques aresuccessful. At this stage it may be helpful to usedisclosing tablets or solutions. At the same

appointment the occlusion and the retainersshould be checked.

It is advisable to see the patient at regularintervals when the full range of checks of margins,gingival health, cleaning, occlusion and themechanical integrity of the bridge are made.Chapter 14 deals with repairs and modificationsto bridges where these checks reveal anyproblems.


Figure 11.11

a Cleaning aids for use with bridges. This is only asmall selection of products which are available. Newcleaning aids are being developed and introduced all thetime.

From the top:

• a soft toothbrush with two rows of bristles that canbe used around dome and ridge-lap pontics;

• two single tuft interspace brushes – these are oftentoo stiff except in very large open embrasure spaces;

• two ‘bottle’ brushes with multiple small lateral tuftsthat are useful for medium-sized embrasure spaces;

• a ‘bottle’ brush with a simple wire handle;• superfloss, the most useful of the bridge cleaning aids

– this has a stiffened end, right, and a furry sectionthat is very useful for cleaning under pontics, andespecially under smooth saddle pontics;

• regular floss, which can sometimes be passed throughembrasure spaces to clean under pontics; but whenthis is difficult it is used in conjunction with;

• a floss threader, a flexible nylon loop with a stiff endthat passes easily between tight embrasure spaces.

b Superfloss being used to clean beneath the upperpontic.


d The clinical use of some of the cleaning aids shownin a and b – they would not normally all be used atonce!

c Superfloss being used in an embrasure space togetherwith a different design of interspace brush (the TeePeebrush).

Part 3 Implants, splintsand maintenance

The purpose of this chapter

This chapter is not a comprehensive text on theuse of titanium osseo-integrated implants but isintended to give sufficient information to allow thereader to make reasoned decisions regarding thealternative techniques to replace a missing toothor teeth. A second purpose is to allow dentists toproperly inform patients regarding choices andmaintain patients who may have implants. Thechoice of an implant-supported restoration asopposed to a bridge, denture or nothing must bemade with the patient fully understanding theimplications including the risks involved and poss-ible complications. Implant dentistry is a specialistsubject and dentists should be properly trainedbefore offering them to patients. Dentists who arenot properly trained or have only been on a shortcourse of a few days should not place implantswithout supervision.

Basic principles

Pure titanium is highly bio-compatible and has anaffinity for bone. If placed in direct contact withundamaged bone cells, fresh bone will bedeposited on the titanium surface and this can bea stable, dynamic bond between bone andtitanium. Dental implants are normally titaniumcylinders with a coarse external thread and aninternal threaded space which allows for arestoration to be attached to the implant (Figures7.4a and 12.1). An implant restoration isproduced in three parts:

• The implant (sometimes called the fixture),which is placed in the residual alveolar bone.

• The abutment, which is attached to and remov-able from the implant, extending through thesoft tissues and giving support to the restora-tion.

• The restoration, which can be a single toothcrown, a bridge or a removable denture.

12 Dental implants

261

Figure 12.1

A section through an Astra implant, representative ofimplants with a conical or internal connection betweenimplant and abutment. The basic components aresimilar to those described in Figure 7.4a.

The implant is placed into the bone and isnormally left for 3 months to allow the bone togrow on the surface before the abutment isattached and the restoration placed so that it isstable enough to be able to resist occlusal loads.Treatment therefore normally extends overseveral months and involves surgical and restora-tive stages.

Osseo-integration

The process whereby the titanium implant isfused into the alveolar bone is termed osseo-integration. It is a process similar to ankylosiswith a direct physical attachment betweentitanium and bone. The surface finish of thetitanium surface is important in affecting thespeed and degree of bone apposition on thetitanium surface with slightly roughened surfaces(40–80 microns), similar to sandblasted or acid-etched surfaces working better than very smoothor very coarse surfaces. The following featuresaffect the integration process:

• The bone must be in very close contact withthe titanium on insertion, so the hole cut toreceive the implant must be an exact size sothe implant is a tight fit.

• The titanium surface must be uncontaminated.Commercially pure titanium rather than alloysor other metals integrates with bone best.

• The bone must not be damaged, particularly byheating while the hole is prepared, as this willdamage the osteoblasts and prevent osseo-integration. A rise of only 3–4°C above bodytemperature may be detrimental and socopious irrigation and the use of gentlepressure on sharp drills is essential when thebone is being drilled.

• The volume, density and vascularity of the boneare important. There needs to be adequatebone to surround the implant on all aspects.Very dense bone is easily heated on prepara-tion and may well be avascular and it can bedifficult to achieve integration. Equally veryatrophic bone with little density can also beavascular and affords little support for theimplant. Patients who smoke heavily often haveless vascular alveolar bone and have beenshown to have a higher rate of implant failure.

• During the osseo-integration process theimplant should be stable, as movement (partic-ularly if it is subject to occlusal loads) willprevent bone formation and lead to a fibrousunion between implant and bone. The externalsurface of the implant is therefore threaded tostabilize it immediately on insertion. Ideally theimplant should extend from the more denseouter cortical plate of bone to the opposingside, as the internal medullary bone is often ofpoor density and offers little support for thenewly placed implant.

General indications for implants

As part of any decision process, all possibleoptions for a situation must be considered. If atooth needs to be replaced it is sensible toconsider the simplest and least invasive optionfirst such as a minimum-preparation bridge or aremovable denture. If adjacent teeth are alreadycrowned or require restoration they maybenefit from the provision of crowns and there-fore a conventional bridge becomes the optionof choice. Implants become the prime optionwhen:

• With a spaced dentition where placement ofbridges is impossible without significantlychanging the other teeth (Figure 12.2a).

• Where key abutment teeth are missing, forexample if a canine tooth is missing, makingconventional or minimum-preparation bridgesdifficult (Figure 12.2b).

• There are no suitable abutment teeth, forexample with hypodontic patients where onlydeciduous teeth are present, or for a pos-terior distal free end saddle situation (Figure12.2c).

• With long spans where conventional bridgesmay not have adequate support.

• When disturbance of extensive existingrestorations becomes more complex or expen-sive than using an implant (Figure 12.2d).

• Where key potential abutment teeth arecompromised, such as teeth with posts andcores present (Figure 12.2e).

• The adjacent teeth are unrestored (Figure12.2f).

262 Dental implants

Dental implants 263

Figure 12.2

a A missing right central incisor tooth in a patient whohad spacing between their natural teeth. Replacementof the tooth with a bridge would necessitate alterationof the size of several teeth.

b Following trauma this young patient has lost upperright canine to left lateral incisor teeth. If a fixedrestoration is to be provided, implants will be neces-sary as there are insufficient suitable abutment teeth toretain a bridge.

c A panoramic radiograph of a patient with hereditary‘short root syndrome’. The upper left central incisortooth has to be lost due to failed endodontic treat-ment and as the adjacent teeth are inadequate tosupport a bridge an implant is indicated.

d The patient has lost the upper right central incisortooth but the remaining crowns and bridges are satis-factory. Replacing the missing tooth with a new bridgewould involve removal and replacement of so manyrestorations that it is financially better to place animplant and leave the existing restorations untouched.

General contraindications toimplants

As for any dental treatment patients can beunsuitable for treatment for medical, physical ormental reasons. Specific concerns for implanttreatment as opposed to other dental treatmentsinclude:

• Uncontrolled diabetic patients are more proneto surgical complications and carry a higher riskof long-term loss of implants.

• Following radiotherapy, patients may have lessvascular bone if the area was subject to radiation.

• Patients with uncontrolled periodontal diseasecarry a higher risk of complications withimplants.

• Heavy smoking. Over 20 cigarettes a day isconsidered high risk and the more years thepatient has smoked compounds the problem.Cessation of smoking for a short period (a fewweeks) prior to and after the surgical place-ment of the implant may help but a long-termheavy smoker who has stopped smoking forseveral years still carries a higher risk of failurethan a non-smoker.

• Young patients should only be consideredwhen they have stopped growing. This isnormally by the age of 18 for women and upto 21 for men. There is no upper age limit forimplant use.

• Patients with a strong bruxing habit may havemore complications with dental implants butalso with other forms of tooth replacement.

264 Dental implants

e A bridge would seem to be the obvious choice toreplace this missing upper right first premolar tooth;however, the tooth behind is a stable post crown andthe canine tooth in front is unrestored. The patientshould be given a clear choice with advantages anddisadvantages explained to allow them to decidebetween a conventional bridge, minimum-preparationbridge or a single tooth implant.

f A completely healthy unrestored dentition with atraumatically lost central incisor tooth. A conventionalbridge would be considered too destructive. Aminimum-preparation bridge could be highly successful.Many patients would wish for a single tooth implantdespite the extra treatment and cost as the end resultwill be long-lasting and independent of the other teeth.

Many patients have unrealistic expectations forthe outcome of their treatment and this can beheightened by the complexity and expense ofimplant treatment. It is essential that a verythorough planning process is undertaken andpatients’ expectations are understood so thatmistakes are avoided. A removable denture maywell restore a patient’s appearance better than acomplex fixed implant-supported restoration

Choosing between implants andbridges

There are situations where the choice of restora-tion is clear-cut as the alternative options are notpossible, for example if a central incisor tooth islost in a patient with a spaced anterior dentition(Figure 12.2a). However, in most situations thereare choices to be made and it is essential that thepatient is fully advised about these options so thatthey can make an informed choice and giveinformed consent. This will include likely lifespansof the restoration, the implications for failure, thelength of time the treatment will take and its cost.It should never be assumed that a patient cannotafford an implant and if the dentist does notprovide implant treatment it should still beoffered to the patient and an appropriate referralmade.

Designs of dental implants

Many different manufacturers produce implantsand claim advantages for certain design features.Care should be taken to use an implant systemthat is well researched with published studies andhas a good infrastructure of training and support.Many patients have implants placed that havebecome difficult to maintain over the years asimplant manufacturers withdraw products orchange designs.

Implants are produced in a variety of diametersranging from 3 mm to 6 mm with the average sizebeing 3.75–4 mm in diameter. The narrow diame-ter implants are used for smaller spaces but aremechanically weaker so cannot be used in highstress areas. Wider implants have greatermechanical strength and a larger surface area for

osseo-integration so are indicated for high stressareas where adequate bone volumes allow.

Implants can also vary in length from 6 mm to20 mm, the most commonly used sizes being inthe 9–13 mm length. The longest implants are notnecessarily the best as they may be harder toplace and cooling the burs during preparation isdifficult. Implants less than 10 mm in length needto be splinted to other implants to survive.

Variations in the external surface have alreadybeen covered and the difference between systemsmay well be over-emphasized by manufacturers.

The fixation of the abutment to the implant alsovaries between manufacturers. Some have a flattop with an anti-rotation projection that theabutment fits over (see Figure 7.4a). An abutmentscrew then clamps the two parts together.Alternatively, the implant can have a recess(normally a cone shape) into which the abutmentseats (Figures 12.1a and 12.3). Once again anabutment screw holds the two parts togetheralthough the mechanical interlock of the abutmentto the implant places less reliance on the screw.

Most implants are designed to be entirelyembedded into bone and the part that extendsthrough the mucosa is the detachable abutment,known as the transmucosal abutment (TMA).These implants are either left buried under themucosa and later accessed with the TMAattached at a second stage of surgery or they areplaced with the TMA already attached, i.e. a one-stage surgical approach (see later). An alternativeimplant design is for the transmucosal portion tobe part of the implant, so this is already presentwhen the implant is placed and these implants aretherefore always used as a one-stage surgicalapproach (Figure 12.3).

Abutments

The restoration is attached to the implant by theabutment and these can be designed so therestoration is screw- or cement-retained and thiswill have a direct effect on the ease with whichthe restoration can be retrieved, as screwrestorations are completely predictable, whereascemented restorations may be difficult to removefrom the mouth if the cement has not failed. Theabutment can either be prefabricated by themanufacturer (Figure 12.4) or an abutment can be

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266 Dental implants

Figure 12.3

a A Straumann implant which is a one-piece threadedimplant with polished transmucosal portion. Made oftitanium, the roughened part is placed in the bone andthe polished part will extend through the gingiva. Anabutment will internally attach to the implant to retainthe restoration in a similar way to the previous designsof implant.

b A Straumann implant used to replace an upper firstpremolar tooth. A titanium abutment has beenattached.

c and d The completed crown is screw-retained forease of removal if required in the future. The screwhole visible on the occlusal surface will be restoredwith composite filling material.

b

c

d

a

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Figure 12.4

a Missing central incisor teeth replaced temporarilywith an acrylic partial denture whilst implants areintegrating.

b The healing abutments are just visible.

c Screw-retained impression copings are attached tothe implants.

e A periapical radiograph showing the completedrestoration.

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e An elastomeric impression material is syringedaround the coping (in this case Impregum) and the trayis filled and seated into place so that the impressionpins are visible.

f Following set of the material the pins are unscrewedand the copings will come out in the impression onwithdrawal from the mouth. Replicas of the implantsare attached to the copings ready for making themaster cast.

g Records of the shade are taken; in this case aPolaroid photograph was taken with shade tabs inposition.

d A stock impression tray is modified so that the pinsholding copings project through the tray.

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i Completed metal–ceramic crowns.

j The abutments are placed in a holder and seated intothe implants. The abutment screws are tightened.

k On initial seating the gingiva are put under pressureas demonstrated by blanching. The occlusion is checkedand the crowns are temporarily cemented.

h On the master model the gingival portion is formedin a removable silicone material to allow access tosubgingival areas and make it possible for the restora-tions to emerge through the gingiva in a natural way.

made in the laboratory or prepared in the mouth,depending on the clinical situation. Theseabutments are commonly known as customized(Figure 12.5). Ideally, with an implant in thecorrect position it is easier to use a prefabricated

abutment where the exact fit of the restorationcan be ensured rather than laboratory-madeabutments which require more complex technicalwork and are more akin to conventional crownand bridge procedures.

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m and n The completed crowns following final cemen-tation with periapical radiograph to verify fit and theposition of the bone in relation to the implant forfuture monitoring.

l Two weeks later the crowns are removed and ahealthy gingival contour is evident. The abutmentscrews are re-tightened.

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Figure 12.5

a A customized abutment made from a wax-up andgold casting technique allows a change in angulation forthe restoration compared with the implant. Theabutment screw projecting at the base will screw intothe implant so shows the implant’s orientation. Thecrown will have a path of insertion of about 30°compared to this.

b and c The abutment is screwed into place and thechange in orientation between implant and crown isclearly seen, bringing the crown back into alignmentwith the adjacent teeth. A crown placed in the orien-tation of the implant would have resulted in a severebuccal position.

Screw- versus cement-retained

Screw-retained restorations are highly complexwhen used for conventional crowns and bridgesand so are rarely seen. As there is significantlymore space with implants in which to accommo-date the metal framework needed to accommo-date the screw, and the implants are in a staticposition, the use of screw retention has becomecommon for implant restorations. The advantagesof screw retention are the ease of removal andthe stability of the restoration while in use. Screwretention demands a very accurate fit for therestoration onto the abutment so that the screwsare not placed under tension during tighteningand this places significant demands on the skill ofthe dental technician (Figure 12.6).

Prefabricated components are used with a goldcylinder incorporated into the restorationforming all of the connection onto the abutmentso that a precise fit is achieved (Figure 12.6e).Sometimes the hole on the occlusal surface maycompromise the appearance or is important inocclusal contact. Abutments with pre-determinedalterations in angulation (angled abutments) canbe used to overcome this (Figure 12.6g).

Laboratory-made abutments are normallyconstructed by the dental technician in much thesame way as a conventional post and core ismade. The abutment is made as though a crownpreparation had been performed onto a metalcore with margins and near parallel sides (Figure12.5a). A ‘conventional’ crown or bridge is thenconstructed which is cemented onto the

abutment. If the restorations may need to beretrieved at some time in the future soft or provi-sional cement can be used. This technique doesnot require quite such an exact fit for therestoration and small amounts of misfit can beaccommodated by the cement layer. It also doesnot have screw holes showing and so appearanceis not compromised (Figure 12.5d).

The soft tissues lying over the implant may beof different thicknesses around and betweenmouths. Abutments are therefore made withdiffering heights so the margin for the restorationcan be placed subgingivally to enhance the appear-ance. Screw-retained restorations are routinelyplaced 2–3 mm subgingivally as there are noconcerns about marginal gaps or excess cement.Cemented restorations are normally placed 1 mmsubgingivally where appearance is paramount butthis should not be confused with margin place-ment for crowns on teeth, where taking impres-sions to obtain an accurate fit is a concern,together with the risk of leakage and caries if thesubgingival margin is not so accessible for clean-ing.

Abutments can be made from titanium, castgold or high strength ceramics such as aluminaoxide or zirconia.

Restorations

Single tooth restorations are normally cementedmetal–ceramic or all-ceramic crowns. Where

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d The completed crown has a good contour, positionand appearance.

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Figure 12.6

a A patient being provided with a full arch upperimplant-supported bridge and lower anterior bridge.During treatment the canine teeth have been retainedto stabilize the partial denture.

b Six implants have been placed in the upper jaw inthe position of the incisor teeth and the second pre-molars. The healing abutments are visible.

c, d and e The completed bridge made with a cast goldsubframe and pink heat-cured acrylic and acrylicdenture teeth. This could have been made fromtitanium and have a porcelain or composite toothportion. The bridge is screw-retained and access holesfor the bridge screws are visible on the palatal andocclusal surfaces. The fit surface incorporates ready-made gold copings which will fit exactly onto theabutments.

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f A jig is used to transfer the abutments to the mouthin the same position as they were on the model so thebridge will be an exact fit. The canine teeth have beenextracted.

g The abutments in position.

h Completed bridge immediately on insertion. Thepink acrylic maintains the appearance of normal gingivawithout excessively long teeth but still allows accessfor cleaning.

i The bridge retaining screws allow easy removal of thebridge for maintenance in the future.

appearance is not paramount, for example amolar tooth, screw retention is more acceptable.

Multiple implants can be linked either withbridges or, if there are no pontics, splintedcrowns. Longer implants can help to supportshorter ones or allow for cantilever pontics.Conventional style metal–ceramic restorationsare normally provided but with extensive bridgeswhere this may be complex to produce andmaintain, a gold or titanium frame can be madewith the tooth portion made from ready-madeacrylic (denture) teeth or laboratory composite(Figure 12.6c). Such restorations will be easier tomaintain if wear or fractures occur in the futurecompared with porcelain restorations.

Implants can also be used to support completeor even partial dentures. This may be a cost-effec-tive treatment as fewer implants are required orfor those who need the lip support of a remov-able denture. The implants can be linked with agold bar and the denture has clips on the fitsurface which engage the bar to provide reten-tion. Alternatively, individual ball abutments canbe placed onto the implants and the dentureretained with internal ring clips. Lower overden-tures tend to have fewer complications thanupper dentures and require fewer implants.

Implant survival

Implants have been extensively investigated sincetheir initial development and many long-termstudies extending 10 years and more have beenpublished. Most studies concentrate on implantsurvival as well as restoration survival, the latterhaving a higher survival rate where extensivebridges are supported by multiple implants, asloss of an implant may not result in loss of therestoration. Implant loss can occur either at thetime of or soon after placement, normally due tosurgical complications. They may also fail at thetime of loading due to lack of osseo-integrationwhich failed to develop during the healing,perhaps due to movement of the implant duringhealing or failure of the bone to grow.

Finally implants can fail years later followingapparently successful osseo-integration. This maybe due to mechanical overloading of the implant(which may show as fracture of the implant) orprogressive bone loss around an implant termed‘peri-implantitis’, a process not dissimilar to

periodontitis. This may result from overload, poororal hygiene around the implant, the presence ofother periodontal lesions on adjacent natural teethor a combination of all these factors. Heavysmoking may contribute to bone loss around animplant. A rate of bone loss of 0.2 mm per yearhas been postulated as acceptable from long-termstudies when averaging out loss from manyimplants. This disguises the fact that most implantsachieve a stable state where no bone loss can bedemonstrated over many years. Any degree of lossis therefore unusual and should be investigated,monitored and treated, as it is not desirable.

Failures of the restoration above normal wearand tear are mechanical in nature such as screwfracture, screw loosening, or de-cementation orfracture of the superstructure, particularly porce-lain fracture. This particular form of failure cancommonly be seen with implant-supportedrestorations, as they can be subject to excessiveloads as patients have no normal proprioceptionfrom implants and there is no protective effectfrom a periodontal ligament. Retrievability of therestoration so that it can be repaired is thereforeessential.

Single tooth implants have reported successrates varying from 100% survival at 5 years tofailure of up to 15% within 2 years; for examplewith single missing lower molar teeth. All of thiswill depend upon the site and size of the missingtooth, the size and length of the implant that canbe placed into the site and the occlusal load.Ideally the implant should be as long as possibleand at least 10 mm in length. Increasing the diame-ter of the implant does not necessarily improvesuccess rates, as there has to be sufficient thick-ness of the bone to support it. Lower molars areat greater risk as the implants may well have tobe short to avoid the inferior dental canal. Thelength of the span may well be large compared tothe width of the implant and the greater loadscompared with anterior teeth. Where possibletwo implants linked together are a better solutionfor a missing molar tooth (Figure 12.7).

Bridges supported by implants can range fromfull arch bridges supported by three to eightimplants to shorter spans with perhaps only twoimplants. Linking multiple implants allows for useof shorter implants splinted together and if morethan two implants are used and one fails it maybe possible for the bridge to survive, perhaps ina shorter form using the remaining implants. If

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only two implants are used at the start, the lossof one implant invariably results in loss of thewhole restoration.

Several long-term studies have monitored fullarch fixed bridges for periods up to and beyond15 years and show a success rate in the regionof 85% for restoration survival.

Implant-supported overdentures have shown agenerally higher rate of implant complicationsthan implants used for fixed bridges and theoverdentures will require ongoing maintenance ofthe retentive elements which is more than a fixedrestoration.

Patients commonly ask how long an implant willlast and great care must be taken to avoid theimpression that implants will last forever – thereis no evidence to support this claim. It is accept-able to inform patients that failure rates forproperly planned and placed implants are low,comparable to or better than conventionalbridges and that if failure does occur this shouldnot compromise their remaining teeth (which isan advantage over conventional bridges). Patientsin high risk groups or with inadequate bonevolumes should be encouraged to try conven-tional treatment first. Patients should be warned

276 Dental implants

Figure 12.7

a and b A single lower molar tooth replaced on a widediameter single implant. The potential for overloadingthe implant is evident when comparing the size of thetooth to the implant.

that the restorations will wear out and requiremaintenance and replacement with time, buthopefully the implants themselves can be reusedfor a new restoration.

Planning

It is often said that implant treatment is not initself complex but the stage at which things cango wrong is in the planning and the decision toproceed with treatment. It is therefore essential

that thorough planning is carried out before animplant is placed. The restoration should beplanned from the final result backwards so thatthe implant is placed to allow the final result tobe achieved exactly as envisaged. All too often theimplant is placed first and only then is consider-ation given as to how to restore it (Figure 12.8).

Implant numbers and position

As a general rule the following can be applied:

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c and d A similar situation where two narrowerimplants have been used to support a lower molartooth. This is more like a natural situation but may bedifficult to clean and more expensive. At least 12 mmof space is required to place two implants.

• 1 implant can support 1 tooth• 2 implants can support 2–4 teeth• 3 implants can support 3–5 teeth.• Full arch upper jaws require 6–8 implants• Full arch lower jaws require 4–6 implants.

The diameter and length of the implant is clearlyimportant and short implants below 9 mm should

only be used with care and protected by splint-ing to other implants. A space of 6–7 mm isrequired between adjacent teeth, measured at theminimum convergence along the entire length ofthe teeth including the roots. This gives only a1 mm space between the implant and the adjacenttooth when a standard 4 mm implant is used andtherefore there is no margin for error in placing

278 Dental implants

Figure 12.8

a and b Following loss of the upper anterior teeth thispatient wears a partial denture with an extensive labialflange to replace the missing hard and soft tissues. Thisprovides her with lip support and cannot be replicatedin a fixed bridge as it would not be cleansable.

c A new partial denture is made without a labial flangeand the pontics placed on the crest of the ridge toreplicate the likely result expected from an implant-supported bridge. The patient can then assess theappearance and lip support provided before havingimplants placed.

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Figure 12.9

a A radiographic stent has been made by copying theexisting bridge in clear acrylic and radio-opaquemarkers placed on the labial surface of those teethwhere it is hoped to place implants.

b An image from a CT scan in the same patient. Themarker from the stent can be seen in the lower leftside and the section through the alveolar ridge allowsmeasurement of height and thickness. This is a one-to-one image and the markers are 1 mm.

c A Scanora panoramic radiograph for a hypodonticpatient with multiple missing permanent teeth. Theradiographic markers are in the sites chosen to placeimplants. There appears to be adequate bone height inthese sites.

d Sections through the mandible from the Scanora inc. These are magnified by the ratio 1:1.7. There is insuf-ficient bone thickness to place implants and eitheralternative sites must be found or the patient will needbone grafting.

the implant. When implants are placed adjacentto each other the implant centres should be7 mm apart to allow for bone and soft tissue toform between the implants. Careful measurementof the gap is therefore required.

The bone contour in the site where theimplant needs to be placed is assessed visually andby palpation to check for bony undercuts and thethickness of the ridge. As at least 1 mm of boneis required all round the implant the ridge willneed to be 6 mm thick. Special sectional radio-graphs are often required (Figure 12.9) to deter-mine the thickness of the ridge. The height of theridge is also assessed, particularly in relation tothe adjacent teeth and gingival margins.

Lack of bone and grafting

If inadequate bone or soft tissue is present at thesite where an implant is to be placed this can beaugmented by a graft. This increases the risk ofcomplications and adds to the surgery requiredand may lead to alternative restorations beingchosen rather than proceeding with implants.Bone can be harvested from other intra-oral sitessuch as the retromolar region, other edentuloussites or the mandible below the roots of theincisor teeth. In extreme cases the hip or craniumare used. The patient’s natural bone works betterthan artificial materials.

Appearance

However well an implant works, if it is in thewrong place resulting in a poor appearance thepatient will not be happy with the result. Theimplant cannot be moved once it has integrated.It is essential that the patient is given a realisticunderstanding of the result that can be achieved,particularly if a degree of compromise from theideal is going to be inevitable. The patient’s provi-sional restoration can be used. If this is a remov-able denture it will need to be produced withouta labial flange as this will not form part of the finalimplant restoration (Figure 12.8c). The toothmust be centred over the alveolar ridge wherethe implant is going to emerge. Often a denturewill look better than a fixed implant restoration

as the denture is able to replace the missing softtissue. If the diagnostic set-up is processed inacrylic this will allow the patient to assess theappearance and also the lip support providedwithout a flange before treatment is started.

A diagnostic wax-up of the desired result maybe adequate for simple cases but these are diffi-cult for patients to interpret and can make resultslook better than can actually be achieved.

Radiographs

For simple cases long cone radiographs may beadequate. These will not show the thickness ofthe bone or its contour. Where anatomic struc-tures such as the inferior dental canal or themaxillary sinus need to be accurately imaged,detailed radiographs such as sectional tomography(e.g. Scan-ora) or computer aided tomograms(CT scans) will be necessary (Figure 12.9). Aradiographic marker can be incorporated into theimage using the diagnostic set-up and an acrylicstent and this allows the exact relationship of thebone to the desired restoration to be evaluatedfor accurate implant placement.

Surgical stent

From the diagnostic set-up an acrylic surgicalstent is made to accurately show the idealposition of the new tooth and allows the implantto be positioned so that this can be achieved. Thestent fits onto adjacent teeth so it can be usedduring surgery with soft tissue flaps raised (Figure12.10). Full arch or overdenture stents can bemade by copying the complete denture in clearacrylic and cutting out the area where access isrequired for surgery.

Provisional restorations

As the implant cannot normally be placed andrestored immediately, most patients requiretemporary restorations while the treatment isbeing undertaken. Simple acrylic dentures are theeasiest as these are easy to modify to fit over the

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implanted area following surgery and the restora-tive phases. Alternatively minimum-preparationbridges can be provided, ideally of a perforatedRochette design so they can be removed easily.The pontic should be made from acrylic orcomposite so that it can be modified to accom-modate the healing abutment when placed.

Surgical procedures

Implants are normally placed using conventionallocal anaesthesia following exact planning. Patientsshould be advised that they may experience someswelling or bruising postoperatively and may havesome difficulty wearing their temporary dentureinitially. If teeth are to be extracted this isnormally carried out 6–12 weeks before theimplant placement. With a small extraction socketor one where there is plenty of bone beyond thesocket into which the implant will be placed, ashort healing time following extraction will beacceptable to allow for soft tissue healing. Moreextensive sockets will require longer for bonehealing to take place before the implant can besafely placed and be stable. Ideally the minimumtime should be left so that the patient is with thetemporary restoration for the least time possibleand there is less chance for excessive shrinkageof the site to occur.

Implant placement is performed under strictaseptic techniques. Flaps are raised to identify theridge, adjacent teeth and important anatomicstructures (Figure 12.11). Using a series of drillswith increasing diameter a hole is prepared to the

ideal depth and diameter under copious irrigation.The position, angulation and depth of the hole arecontinuously checked with relation to the surgi-cal guide so that changes can be made before thefinal drill. The implant is then slowly screwed intothe bone utilizing the self-cutting threads until itis in the correct position and stable in the bone.

One stage

At implant placement a healing abutment is placedand the flaps are sutured so that the healingabutment is just projecting through the softtissue. A healing abutment is a plain titanium cylin-drical abutment which screws into the implantand holds back the soft tissue to create a softtissue tunnel from the mouth to the implant head.Any temporary restoration should be adjusted sothat the implant is not loaded. Once the softtissues and bone have healed the implant is readyto be restored without the need for furthersurgery. This approach has become the mostcommon as it reduces the surgical stages, and aslong as the implant is protected from loading thesuccess rates are comparable to the traditionaltwo-stage approach.

Two stage

After insertion of the implant, the hole for theabutment screw is sealed with a cover screw andthe flaps are sutured over the area. Following a

Dental implants 281

Figure 12.10

A surgical stent is used to guide the implant placement.Guide posts have been placed in the initial holes cutfor the implants to verify their orientation. As the holesare not full size at this stage it is possible to changetheir angulation.

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Figure 12.11

a–d Some stages in the surgical placement of a singleimplant for a left central incisor tooth. A flap exposesthe ridge and a surgical guide verifies the implantposition. The implant mount seen in c allows theimplant to be screwed into position with a high degreeof torque and is then removed. A two-stage techniqueis employed and the implant has been buried to beexposed 3 months later.

healing period of 6–12 weeks a second stage ofsurgery is performed to locate the top of theimplant, remove the cover screw and place ahealing abutment. This technique requires extrasurgery but encourages an uneventful healing withno loading to the implants during healing. It isrecommended where bone grafts have been usedor where protection of the implant from loadingcannot be guaranteed.

Immediate placement/restoration

It is occasionally possible to extract a root andimmediately place an implant into the site. Thiscan only be considered if dealing with anuninfected tooth and if the residual socket issmaller than the implant being placed, so theimplant will be stabilized in sound bone, notsitting in the socket. Immediate placement carriesan increased risk of failure and should only beconsidered where maintenance of the gingivalcontour is essential to preserve appearance.There may still be some gingival recession follow-ing immediate placement.

Where complete stability of a newly placedimplant can be guaranteed, it may be possible toplace a provisional restoration at the same timeas the implant is placed. Again, this carries anincreased risk of complication and care must betaken to ensure that the restoration is completelyout of occlusal contact in all positions of themandible. After the normal 3 month healingperiod the provisional restoration can be changedfor the permanent restoration. In situationswhere providing a temporary restoration may bedifficult or unacceptable to the patient thisimmediate approach may be considered but theincreased risk must be fully explained to thepatient and recorded in the notes.

Restorative procedures

As discussed in the planning section, it is essen-tial that the restoration has already been plannedbefore the implant is placed to ensure its correctposition. Even with such planning, variations in theimplant position following surgery can beexpected, as at the time of surgery the final

position of the implant is dictated by the shapeof the residual bony ridge and the operator’sability to place the implant exactly as required.

The restorative phase consists of choosing theabutment, taking accurate impressions, fitting theabutment and restoration and finally monitoringthe result. The patient will present with a healingabutment attached to the implant which can beremoved and the position of the implant, itsangulation and depth in the soft tissues can beassessed. It is normally possible for the abutmentto be unscrewed without requiring a local anaes-thetic.

Abutment choice

If the implant position is satisfactory and a clearchoice of manufactured abutment has been madethen the abutment can be ordered and attachedto the implant. Manufacturers will have a proto-col for securing the abutment and tightening theabutment screw which must be followed. Thismay include taking a radiograph to confirm fullseating of the abutment before tightening thescrew.

Making the wrong choice of abutment is acostly mistake and if a clear decision cannot bemade in the mouth it is better to take an impres-sion of the implant itself and make the choice outof the mouth following a diagnostic set-up (Figure12.12). A ready-made impression coping isattached to the implant and a master modelproduced using a replica of the implant. As theimplant will be significantly subgingival, the use ofa solid plaster model will not allow access to thedeep margins. A removable silicone replica of thegingiva is therefore incorporated into the modelto allow access to the top of the implant and alsoto allow the restoration to be made with anemergence that can shape the soft tissue for amore natural appearance.

To ensure that the correct abutment is chosento make the desired restoration a putty labialmask taken from the ideal diagnostic wax-up canbe placed on the master model. If a laboratory-made abutment is to be used this needs to bedesigned to leave adequate space for the restora-tion with optimal retention and margins placed1 mm subgingivally on the labial aspect so that nometal shows.

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Impressions

Implant impressions are more straightforwardthan for conventional crowns and bridges but thesame principles are followed. A special tray isoften required but a stock tray will be adequate,if it is a good fit for simple cases. Impressioncopings are attached to the implant or abutmentand are normally held in place with an extendedpin. The tray is modified to allow the pin toextend through an opening. A conventionalimpression material (normally a polyether orsilicone) is syringed around the coping and thetray is filled and seated so that the pin is visiblethrough the tray (Figure 12.4e). Once set the pin

can be unscrewed and the impression removedalong with the coping. The pin is used to securethe replica to the coping before the model is cast(Figure 12.4f).

Alternative techniques are available, particularlywhere access may be difficult to use a screw-driver to undo the pin with an impression in thepatient’s mouth. Impression copings can beattached to the implant or abutment and theseare designed to remain in the mouth followingremoval of the impression. The coping can thenbe removed and re-seated into the impression.

The healing abutment is replaced on theimplant while the restoration is made and thepatient can wear their temporary restoration.

284 Dental implants

Figure 12.12

a A silicone putty matrix has been made from adiagnostic wax-up and is placed onto the master modelof the implant positions. With a guide pin screwed intoone of the implant replicas the correct abutment canbe chosen.

b An abutment selection kit with replicas of thevarious abutment sizes and angulations.

Placing the restoration

If the abutment was left in the mouth, the tempor-ary cover is removed and the restoration is triedin. Alternatively, a laboratory-made abutment isattached to the implant in exactly the sameposition as it was on the model. This may requirea location jig to verify its position (Figure 12.6f).It is normal for the soft tissues to be placed underslight pressure as the restoration emergesthrough the gingival margin. The restoration isassessed for appearance, contour, contact pointsand occlusion as for any conventional restoration.The fit of the restoration should be ensured ifready-made components have been used.

Screw-retained restorations can be tightenedand temporary covers placed over the screws toallow patients to live with the restoration beforeit is completed. Cemented restorations can betemporarily cemented to also allow for retrievaland alteration before being definitively cemented.It is frequently important to allow for this trialperiod while the patient adjusts to the newrestoration and the soft tissue adapts to the newcontour (Figure 12.4l). This trial period allows therestoration to be altered before finally screwingor cementing into place.

Implant restorations, other than full arch recon-structions, have the occlusal contacts adjusted sothat on light contact the implant restoration isonly just in contact and is not able to hold occlusalcontact foils such as shim stock. On applyingpressure the implant restoration will come intofull contact and hold shim stock and this allowsthe implant to be protected from overloading, asit is not in a periodontal ligament and cannotprotect itself from occlusal overload as teeth can.Extensive restorations cannot be protected fromocclusal contacts and care is taken to ensure thatthe occlusal loads are in the long axis and centredover the implants where possible.

Following approval by the patient and verifica-tion by the dentist that the restoration is satis-factory it can be completed. Cementedrestorations can be placed using conventionalcrown and bridge cements, although often tem-porary cements are adequate due to the tightnessof fit of components. Care must be taken toensure removal of all excess cement. Screwedrestoration must have the screws re-tightened

and the access holes restored with composite.The patient should be given specific oral hygieneinstruction.

Long cone radiographs should be taken of thecompleted implant restoration and monitored forthe first few years with annual radiographs tocheck the maintenance of the bone support forthe implant. The patient’s oral hygiene also needsto be maintained.

Maintenance

The continued success of an implant should bemonitored by clinical and radiographic examina-tion every year for at least 2–3 years and there-after at longer time periods. Periodontal probingof the soft tissues is not very useful other thandemonstrating poor plaque removal if the gingivaare bleeding on probing. Metal scalers should notbe used around implants as they may damagecomparatively ‘soft’ titanium surfaces. Plasticscalers are available.

Complications

The signs of implant failure are:

• Implant mobility• Pain or discomfort• Bone loss more than 1 mm in the first year or

more than 0.2 mm per year thereafter.

If bone loss around the top of the implant isobserved, this may not in itself lead to completefailure of the implant but may indicate occlusaloverloading of the implant or poor oral hygiene,both of which should be addressed.

Complications can commonly arise withimplant restorations and there is evidence thatthis may be more common than with tooth-supported restorations. Fractures of porcelain oracrylic portions of a restoration can be repaired,particularly if screw-retained restorations havebeen provided. Abutment and bridge screws cansometimes fracture but fracture of the implantsis rare.

Dental implants 285

Many of the techniques used in constructing fixedsplints are similar to those used to make crownsand bridges. Large splints often contain one ormore pontics, and are therefore combinationbridge/splints. There is no attempt here todescribe removable splints in detail, since thetechniques for constructing them are more akinto partial denture construction. There is,however, a section of this chapter comparingfixed and removable splints.

Different types of splint are used depending onthe length of time for which they will be needed.Short-term splints are made as an emergencymeasure, intermediate splints are made to last fora few months, usually while other forms of treat-ment are being carried out, and permanent splintsare intended to last for the lifetime of the denti-tion or the patient.

Indications for fixed splints

Trauma

A blow may result in an incisor tooth beingpartially or completely subluxated. If the tooth isrepositioned correctly in its socket very shortlyafter the accident, particularly with youngpatients, it has a good chance of surviving for auseful period, provided that it is kept clean andother conditions are favourable. It will normallyneed to be stabilized by being attached toadjacent teeth while the periodontal ligamentheals and the alveolar bone remodels. It is notusually necessary to provide intermediate orpermanent splinting for traumatized teeth.

Periodontal disease

At one time splints were prescribed as a way oftreating periodontal disease and preventing theloss of teeth through progressive loosening. Thisis no longer accepted as a reasonable form of

treatment; and the proper treatment of periodon-tal disease itself is beyond the scope of this book.However, once the disease has been successfullytreated, there may be two conditions when afixed splint is indicated:

• When the residual mobility of the teeth is suchthat the patient finds them uncomfortable andnormal masticatory function is impractical.

• When teeth are missing and must be replacedfor one of the reasons listed in Chapter 7. Inmany cases the remaining teeth are not satis-factory as denture abutments in view of theirmobility or because it is considered that apartial denture will make oral hygiene proce-dures more difficult and will be likely toshorten the life expectancy of the remainingteeth. Individual teeth may also be unsuitableas bridge abutments, but a number of teethsplinted together may form a satisfactoryabutment for a bridge or perhaps a precision-attachment retained partial denture.

Orthodontic retention

In the great majority of courses of orthodontictreatment the teeth are moved into new positionswhere, following a period of settling in, they arestable. There is sometimes a persistent tendencyto relapse, and for fuller explanations for this thereader is referred to textbooks of orthodontics.Orthodontic relapse is more likely, and may indeedbe anticipated, if the tooth movement is to realignteeth that have drifted following periodontaldisease. Figure 4.5c and d illustrates a case where,if orthodontic treatment is to be provided, fixedsplint retention is very likely to be necessary.

Cleft palate

One method of treating cleft palate cases is toexpand the palate rapidly by orthodontic means

13 Fixed splints

287

and to insert a bone graft. In some cases theresult is not completely stable, and if there aremissing anterior teeth (which is common), abridge replacement may be made and a numberof teeth on each side of the cleft splinted to formthe abutments. These splinted abutments will alsostabilize the two halves of the upper arch.

Occasionally the premaxilla is separated fromthe remainder of the upper arch and, togetherwith any teeth carried in it, it will be mobile. This

can be splinted by means of a fixed splint/bridge(Figure 13.1).

The surgical and orthodontic management ofcleft lip and palate has improved in recent yearsand the need for stabilization by means of bridgeshas reduced. However, the surgical andorthodontic treatment is not always successfuland so the need for a restorative option stillremains. Also a large number of cleft lip andpalate patients were treated in the past by fixed

288 Fixed splints

Figure 13.1

A bridge to stabilize a mobile premaxilla resulting froma bilateral cleft palate.

a The preoperative condition following surgicaltreatment.

b The prepared teeth. Two abutments in each buccalsegment are used together with the two teeth in thepremaxilla.

c The completed bridge stabilizing the premaxilla.

splint/bridges and these need to be maintained.There have been unfortunate incidents whenenthusiastic dentists faced with a failing bridge ofthis type think it would be better to make individ-ual crowns and place implants to replace themissing teeth. In some of these cases the arch hascollapsed without the stabilization of the bridge,causing great difficulty.

Additional retention

With precision-attachment retained partialdentures, the abutment teeth carrying the attach-ment may need to be splinted together. Thisprovides extra retention to resist the additionalforce during removal of the appliance (Figures7.3a and 13.2).

Fixed splints 289

d Another similar case with considerable bone loss andinsufficient blood supply for further bone grafts orimplants. Hence the very long pontics.

e However, the tight lip hides the pontics.

Figure 13.2

Splinted abutment teeth for a precision-attachmentretained free end saddle denture. The six anteriorteeth have reduced alveolar support and have beensuccessfully treated periodontally. They were alsoheavily restored. Splinted crowns provide support forextracoronal precision attachments that retain andstabilize a denture and avoid visible clasps.

Short-term, intermediate andpermanent splints

Short-term splints

When a tooth is loosened by a blow or iscompletely lost and replanted, an immediatetemporary splint is necessary. The usual methodof splinting is to attach the tooth involved toadjacent teeth with wire attached with compos-ite (Figures 13.3, 13.4 and 8.9). Various othertechniques were previously used for temporarysplints, such as wiring the teeth together orcementing a cap splint made of acrylic, someother vacuum-formed material or cast metal.However, if sufficient enamel is present for acidetching, these other techniques are less satisfac-tory than wire/composite splints because they areless hygienic and interfere with the occlusion.

Intermediate-term splints

These are used when teeth need to be immobilizedfor periods of between a few weeks and a few

months, for example while periodontal treatment iscarried out, before permanent restorations aremade. They are usually one of the less permanentminimum-preparation types, such as a Rochettesplint (Figure 13.5c), but may be intracoronal.

Permanent splints

Conventional permanent splints for restoredteeth are usually complete crowns connected(Figure 13.2). With unrestored teeth a minimum-preparation splint is the treatment of choice(Figure 13.5d).

Fixed splints compared withremovable splints

A variety of removable splints were used in thepast but only two are still common: long-termorthodontic retainers and short-term vacuum-formed soft splints for traumatized teeth. Apart

290 Fixed splints

Figure 13.3

The right central incisor was partly displaced by a blow.It was mobile and uncomfortable. This wire-and-composite splint is rigid, allows the tooth to bepositioned correctly in the occlusion while the splint isbeing attached, allows the tooth to be tested for vital-ity, does not interfere with the occlusion, and is access-ible for cleaning. It was removed after 3 weeks, bywhich time the injured tooth was firm.

Figure 13.4

A wire-and-acrylic splint. This splint served a usefulpurpose during initial periodontal therapy, after whichthe roots of the four incisors were resected. The splinthas remained effective until complete alveolar healinghas occurred, and now the patient is ready to have abridge – a temporary partial denture having beenavoided.

Fixed splints 291

d Preparations for a six-unit minimum-preparationsplint. The notches are a little deeper than usualbecause of the difficulty of seating a multi-unit splint onmobile teeth.

Figure 13.5

a Quartz-fibre-reinforced composite splint.

b A flexi-wire orthodontic retainer retained bycomposite.

c A Rochette minimum-preparation splint made for apatient who found the mobility of their teeth to beuncomfortable following successful periodontal stabil-ization.

from these fixed splints should be used ratherthan removable. The advantages of fixed splintsare:

• They are the most reliable splints for mobileteeth or those with a tendency to drift

• Can be kept entirely clear of the gingival tissues• Occupy minimum or no additional space• Cannot be left out by the patient.

Types of short-term, intermediateand permanent fixed splint

Although short-term splints are different from theothers, there is less distinction between interme-diate- and long-term splints.

Examples of short-term splints

Acid-etch retained composite splints(Figure 13.3)

A simple short-term splinting technique is to acid-etch the approximal surfaces of adjacent teethand connect them with composite. The techniqueis not sufficiently rigid to function as a permanentsplint. To strengthen the splint, it is usually neces-sary to add stainless-steel wire or ribbon fibres.

Wire and composite splint (Figure 13.4)

A satisfactory form of intermediate splint, partic-ularly for mobile lower incisors, is the wire-and-acrylic splint. The technique can be used whensome of the teeth are crowned and so cannot beetched for the retention of composite.

Examples of intermediate- and long-term splints

Fibre-reinforced splint (Figure 13.5a)

These have only been available for a fairly shorttime and so it is sensibly cautious to regard themas intermediate for the time being.

Flexi-wire splints (Figure 13. 5b)

These are commonly used by orthodontists follow-ing a course of orthodontic treatment producing anunstable result. Orthodontists can predict thatmedium- or long-term splinting will be necessarybefore the orthodontic treatment starts. The patientand parents can therefore give informed consent.

The ‘flexi-wire’ is attached to the lingual orpalatal surfaces of teeth with composite. Theflexibility of the wire allows some movement ofthe teeth to which it is attached but the wire isrelatively weak and breakages are common.

Cast metal intermediate and permanentminimum-preparation splints

These are the most common types of intermedi-ate and permanent splints. They have the advan-tage that they do not require much or anypreparation of the tooth and yet are thin andunobtrusive and do not significantly affect thepatient’s appearance. However, because they areapplied to the surface of the teeth, they inevitablyadd to their bulk and make oral hygiene moredifficult. There is also the problem of them inter-fering with the occlusion; and in some cases theideal design for retention and splinting cannot beused because of the occlusion.

Figure 13.5c shows a Rochette splint. Thesesplints have the advantage that it is possible toremove them fairly atraumatically by cutting thecomposite out of the retentive holes.

Figure 13.5d shows the preparations for a six-unit minimum-preparation splint. The only prepa-ration has been the palatal notches to locate thesplint firmly to the teeth, without slipping, whilethe cement sets.

For permanent retention of orthodonticallytreated teeth with sound periodontal health, aminimum-preparation splint is preferred since itis more retentive and smoother lingually. Thesesplints are made and cemented in the same wayas the minimum-preparation bridges described inChapter 11.

Complete-crown splints

Despite the advantage of minimum-preparationsplints, complete-crown splints are still common.

292 Fixed splints

This is because the natural crowns of the teethbeing splinted often already have large restorationsor crowns, and some teeth may have beenextracted, so that the appliance becomes asplint/bridge. Figure 13.6 shows a typical 12-unitsplint/bridge where six teeth are missing andwhere the remaining teeth were uncomfortablymobile. A partial denture to replace the missingteeth would probably have increased the mobilityof the remaining teeth; and the patient was mostunhappy about wearing a removable appliance. Theradiographs of this patient are shown in Figure 7.5.

The disadvantages of this technique are that itis very time-consuming, both at the chair side andin the laboratory, and therefore very expensive.If failure occurs, it may be necessary to removethe entire splint and maybe extract several otherteeth. This type of appliance should thereforeonly be provided for very highly motivatedpatients with extremely good oral hygiene.

Intracoronal splints

A variety of techniques have been suggested forsplinting adjacent teeth with intracoronal restora-tions using either amalgam or composite, with theteeth linked by wire or a proprietary device.

The major problems with this type of splint arefirst that forces applied to the unprotected partof the tooth surface tend to break down the seal

between the restoration and tooth, with marginalleakage occurring followed by secondary caries.Second, mechanical failure at the connectors isfairly common. Third, because they are difficult tofinish and polish, it is often harder to cleanaround this type of splint than around partial orcomplete crown splints with polished connectors.

For these reasons and because there are nowbetter alternatives intracoronal splints are nolonger used, although some lucky patients whohave avoided these problems still have them.

The appearance of anterior splints

A patient who has had extensive periodontaldisease and treatment (particularly surgical treat-ment) often has upper anterior teeth that appearvery long. When the lipline is high this is anaesthetic problem (Figure 7.8a). If the incisorteeth are extracted and a partial denture is made,artificial teeth, fitted to the ridge, will also appearto be too long; otherwise a flange may be used,and the edges will have an extremely artificialappearance. In any case, if the patient has beencooperative during periodontal treatment and thishas been successful, he or she will obviously notwant the teeth extracted.

If a complete-crown splint is made, it will benecessary to prepare crown margins at the

Fixed splints 293

Figure 13.6

A 12-unit fixed splint/bridge. There are six abutmentteeth and six pontics. Note the supragingival marginswhich have helped to maintain a good level of gingivalhealth. Note also the opaque appearance of theretainer margins. This is because in order to make thesix preparations parallel to each other it was not poss-ible to prepare sufficiently wide shoulders withoutover-preparing the whole tooth and risking exposure.This was anticipated from the trial preparations and thepatient was fully informed about this before the prep-arations were undertaken so that her consent to theprocedure included understanding that the appearancewould be compromised in this way.

cement–enamel junction (CEJ) and try to disguisethe crown margin as the CEJ, or at the gingivalmargin, producing very long thin preparations thatendanger the pulp. The first alternative is oftenunsatisfactory, since the opacity of themetal–ceramic retainer is greater than that of theroot surface. Neither makes any improvement inthe appearance of the length of the teeth (Figure13.6).

With all these aesthetic problems, somepatients and dentists would elect to extract theupper incisor teeth, re-contour the ridge by boneaugmentation and replace the teeth with implants.However, this would be even more expensivethan the splint/bridge and so many patients haveto accept the cheaper option of dentures.

Selecting an anterior splint

It is important to make sure that the patientunderstands how the splint will look, and whatcompromises are necessary. In a typical case thepatient has:

• Mobile, uncomfortable upper anterior teeththat have been successfully treated periodon-tally or have been retracted orthodontically

• A high lipline with unattractive appearance ofthe upper incisor teeth

• An extreme reluctance to wear a removableappliance.

There is no ideal solution to these problems; theoptions are as follows, in increasing order of cost:

• Offer no treatment; the result will be a patientwho continues to complain about mobility, thelack of comfort, the appearance and possiblefurther drifting of the upper incisors.

• Provide a minimum-preparation splint with orwithout a removable gingival prosthesis; thecompromise here is the ‘metal shine-through’,but on the plus side are the conservativenature of the preparations and the relativelylow cost compared with the alternatives.Sometimes this option is not possible becausesome of the teeth are heavily restored orcrowned or the occlusion is unfavourable.

• Extract the upper incisor teeth and provide apartial denture; some patients will refuse, and

in any case it will provide only limited improve-ment in appearance.

• Extract the upper incisors and provide a bridge.There is still the problem of the length of thepontics, but this may be the preferred treat-ment in some cases – there is little point inkeeping teeth with a very poor prognosis if thesame number of additional abutment teethwould be necessary to support them as wouldbe prepared for a bridge; a removable gingivalprosthesis may also be provided.

• Provide a complete-crown splint with orwithout a removable gingival prosthesis; thishas the disadvantages of time, cost and appear-ance described above, but may still be thepreferred treatment in some cases.

• Extract the teeth and provide ridge augmenta-tion and implants.

Clinical techniques for permanentsplints

The reader is referred to the literature on thetreatment of traumatized teeth and the periodon-tal literature for fuller descriptions of temporaryand intermediate splinting techniques. The clinicaltechniques for minimum-preparation splints arethe same as for bridges (see Chapter 11), there-fore clinical techniques will be described only forcomplete-crown splints.

Tooth preparation for complete-crown splints

One of the techniques described in Chapter 11should be used to ensure that the preparationsare parallel. In the case of multiple-unit complete-crown splints, it may be necessary to take severalintermediate impressions to check the parallelismof the preparations with a surveyor before thefinal impression is taken. Figure 13.6 shows a 12-unit splint/bridge with six abutment teeth. Sixintermediate impressions were taken. This soundsvery time-consuming, but with fast-setting plasterand a surveyor at the chair side, only twoappointments were needed.

It is highly advisable to carry out trial prepara-tions on a study cast to ensure that the ideal path

294 Fixed splints

of insertion is selected. This may not be in thelong axis of all the teeth.

Temporary splints

With complete-crown splints it is possible tomake a temporary splint at the chair side or inthe laboratory using one of the techniquesdescribed in Chapter 11.

Impressions

When teeth are mobile, they should be splintedso as to be in an unstrained position within theirremaining periodontal support, and preservingoptimum occlusal relationships. There is a dangerof them being moved away from this position bythe force of the impression being inserted, so thatthe finished splint, although it may fit, will distortthe alignment of the teeth. This risk is greater ifa viscous material is used, particularly in a close-fitting special tray. This means that the putty-washtechnique is not ideal for these impressions.

There are two ways around this problem. Oneis to use an impression technique in which theteeth can ‘float’ into their natural positions beforethe material sets, the ideal material beingreversible hydrocolloid. The second way is totake an impression in any material and have

separate transfer copings made for each tooth.These are located in the mouth using a gentletechnique that does not disturb the alignment ofthe teeth, for example painting on a self-curingacrylic material (see Chapter 11).

Cementation

Some teeth being splinted are likely to be moremobile than others, and this produces a cemen-tation problem. Although the splint may fit wellat the margins when it is tried in, if some teethcan be moved apically in their sockets and otherscannot, when the splint is being cemented themobile teeth may be depressed by the hydrostaticpressure in the unset cement. The marginal fit ofthe retainers of these teeth will therefore beunsatisfactory. Precautions should be taken toavoid this happening. The splint should be pressedfirmly home onto the stable abutments and thenan instrument such as a Mitchell’s trimmerhooked onto the mobile abutment teeth, prefer-ably at the cement–enamel junction, and the teethdrawn down into its retainer. Because this takestime, zinc phosphate cement, mixed to producean extended working time, is preferred.

Alternatively, floss is tied round each of themobile teeth between the preparation margin andthe gingival margins. After fully seating on thestable abutments, the mobile teeth can be pulleddown into their retainers with the floss.

Fixed splints 295

General considerations of successand failure and the value of surveydata

How long will it last?

This is a question which many patients ask whena crown, bridge or implant is being discussed anda decision is being made between them or leavingthe tooth without a crown or the spaceunrestored.

The honest answer is ‘I do not know’ but thatis not going to satisfy the patient.

There have been many surveys of crown,bridge and implant success and failure; however,there are a number of major problems with thesesurveys.

Prospective surveys, following up restorationsfrom when they are placed, are usually of selectedand therefore biased samples – restorations madein dental schools, or specific practices. Also, if itis known that the treatment is part of a survey,extra care may be taken in the treatment, thusproducing further bias.

Retrospective surveys, looking at a cross-section of restorations placed in differentlocations – general or specialist practice or dentalhospitals – by a variety of dentists and by a varietyof techniques involve so many variables that it isdifficult to analyse the results realistically.

Retrospective surveys of failures are helpfulwhen they look at the causes of failure and the timefrom the restoration being placed to its failure.

A reasonable way to record failures is as apercentage per year. For example, large surveysof bridges made in practice and elsewhere indifferent countries show that about 90% ofbridges last at least 10 years.

There are difficulties in definingfailure

Looking at any crown, bridge or implant, it isalways possible to find some minor fault with thefit or the appearance of some other aspect. Inmany cases it is a matter of degree. There isnothing seriously wrong with the restoration,only that one dentist, looking at another’s work,would have applied his or her skills in differentways – would have introduced a little more incisaltranslucence or placed the margin a little moresubgingivally or supragingivally, or finished itbetter. These variations in judgement are to beexpected and need to be encouraged. If everycrown or bridge were standardized, there wouldbe no room for development and improvement.

At the other extreme there are undisputedfailures, for example, the fractured ceramic crownor the loose bridge where extensive caries hasdeveloped. Between these extremes lies a largegrey area of partial failures and partial successes.With these it is better to consider levels ofacceptability to patient and dentist (which may bedifferent) and to consider what needs to be doneto improve matters.

Because the prognosis for a crown, bridge orimplant cannot be guaranteed, potential failureshould be regarded as a disadvantage andbalanced against the advantages. It is not realisticto ignore the possibility of failure, and its finan-cial implications for patient and dentist must berecognised.

For restorations which have only been availablefor a short time or where significant changes indesign, materials or techniques have been madeit is not possible to give a reliable prognosis, onlya best guess.

297

14 Crown and bridgefailures and repairs

Many patients die with their restorations intactand so these restorations, however long theyhave been in place, have served their purpose andare successful.

Later in this chapter the causes of failure andsome solutions are described, together with sometechniques for adjustment or repair.

How long do crowns andconventional bridges last?

Most crowns and bridges do not wear out,neither do the supporting teeth. Failure is theresult of an isolated incident, a progressivedisease process, or bad planning or execution inthe first place. Isolated incidents such as a blowcannot be predicted and may occur on the daythe restoration is fitted, in 40 years time ornever.

The prevention of caries and periodontaldisease is largely under the control of the patient,assisted and monitored by the dentist and hygien-ist. Some changes affecting caries and periodontaldisease cannot be predicted. These includedietary changes, drugs producing a dry mouth, theonset of a general disease such as diabetes andgeriatric changes that may make cleaning difficult.

A number of long-term surveys of success andfailure have produced results varying from verylow to high rates of failure. It is possible to calcu-late from the published figures an average lifeexpectancy of a bridge but this is the wrongstatistic to use, and it should not be quoted topatients. Some restorations are failures from theday they are inserted, for example because theydo not fit properly, and some last for over 40years with a range between. To quote an‘average’ of 20 years is meaningless.

In the more recent surveys more sophisticatedstatistical methods have been used to describesurvival rates of restorations. In addition, anumber of factors affecting the survival rate havealso been analysed, for example for bridges, thedesign, the number of teeth being replaced, theperiodontal support for the abutment teeth andtheir vitality and factors to do with the patientsuch as age and gender. Some of the surveysshow a survival rate that remains high for the first10 years or so with more than 90% of the bridgesstill in place at that time. After this, the survival

rate declines, with 60–70% of bridges still in placeat 15 years. There are not sufficient studies toestablish the number of years at which thesurvival rate is 50%, in other words when thereis an even chance that the bridge will still be inplace. However, looking at the published survivalcurves and extrapolating them, the figure is likelyto be between 30 and 40 years survival for small,well-made conventional bridges.

One of the difficulties in interpreting thesesurveys is the fact that many of the bridges weremade a long time ago using techniques, materialsand concepts that are now regarded as out ofdate.

There is therefore no reliable, consistent figurewhich can be given to a patient when they ask:‘How many years will the bridge last?’. It is oftennecessary to give the patient a fairly detailedexplanation of why such a figure cannot be givenor to say something more vague such as (depend-ing on the patient’s age, condition of the abutmentteeth, etc.): ‘A bridge made for you may have tobe replaced with another bridge or an alternativeprosthesis once (or twice) during a normallifetime, at times that cannot be predicted’.

How long do minimum-preparationbridges last?

These were introduced long after conventionalbridges and so there has been less time for long-term surveys. However, when they began tobecome popular two factors influenced thereviews of these bridges. First there was scepti-cism among many dentists about the likelihood oftheir success and this was reinforced by manyearly failures while they were being developed.This view still prevails among some traditionalistsbut has almost disappeared with more moderndentists who have been trained and have devel-oped for themselves policies of minimum inter-vention, maximum conservation and considerableconfidence in a range of bonding techniques andmaterials for many applications in dentistry.

The second factor is that the time thatminimum-preparation bridges started to becomepopular and more reliable coincided with a fastgrowing emphasis on clinical audit and review, andthey were therefore prime candidates for well-designed prospective surveys of success and failure.

298 Crown and bridge failures and repairs

Many good but relatively short-term surveysand audits have been carried out and the resultsare very encouraging. It is now reasonable to saythat a minimum-preparation bridge should be thedefault option, rather than a conventional bridge,when the circumstances are appropriate.

How long do porcelain veneers andinlays last?

Porcelain veneers rarely fail by becoming de-cemented unless this occurs soon after placement,indicating poor technique. More commonly porce-lain fractures occur with or without loss of part ofthe veneer. These are normally vertical or obliquefractures extending down from the incisal tip.Fractures without loss can be monitored as theymay not progress and the cement lute under theveneer, if intact, will protect the tooth from decay.Eventually the crack will stain and the veneer willusually need to be replaced. Small fractures toveneers can be repaired with composite.

Removing failed veneers is difficult and is bestdone with a diamond bur, regularly stopping toensure that the enamel is not damaged. It is notalways possible to replace a failed veneer withanother veneer if there is insufficient enamelremaining.

Porcelain or composite inlays and onlays canfail by fracture, commonly across the narrowestpart of the occlusal lock, in which case repair isnot possible. Fracture may cause loss of part ofthe cuspal coverage. Repair with composite maybe possible.

The reason for failure of porcelain inlays orveneers should always be investigated to ensurethat a design problem is not repeated.

How long do implants last?

The early Swedish studies of highly controlledgroups of patients and treatment protocolsproduced impressive results but are unlikely to bereproduced in the less controlled environment of‘high street’ general practice or specialist practiceelsewhere in the world. Dento-legal experienceshows that many failures do not get into surveydata, which are distorted towards suggesting ahigher than realistic success rate.

Dealing with failures of implant fixtures and/orthe prosthetic elements is a specialist subjectbeyond the scope of this book. If a dentist findsevidence of failure in an implant, the patientshould be referred unless the dentist has hadspecialist training. Preferably the referral shouldbe back to the dentist who placed the implant.

How long have modernrestorations been available?

The following approximate figures relate to thetime when the various restorations first made amajor impact on dentistry in the UK. They haveall been developed since then, producing contin-ual improvement to a greater or lesser extent.

For example, conventional crowns and bridgeshave been made for over 100 years but haveimproved significantly with new techniques andmaterials in the last 30–40 years. However some,made with older materials and techniques, havelasted for more than 50 years.

These figures relate to the publication date ofthe fourth edition of this book: 2007.

• Metal–ceramic restorations about 40 years• Modern ceramic crowns about 30 years• Ceramic veneers about 20 years• Minimum-preparation bridges:

– Fixed–fixed and cantilever (Rochette 1975) about 25 years

– Fixed–movable and hybrid about 15 years • Multiple implant retained restorations.

The Brannemark system was imported into the UK and USA in 1982 and it took about 5 years before a significantnumber of dentists were properly trained in its use. Therefore the substantial experience is about 20 years

• Single tooth implants about 15 years

A rough guide for advice topatients, to be modified byknowledge of the patient’s specificcircumstances

This guide is based on survey data where this isreasonably robust, anecdotal evidence from

Crown and bridge failures and repairs 299

colleagues and the author’s personal experience.It is therefore not scientific and should not bequoted in, for example, legal reports.Nevertheless, we feel it right to make an attempt,however flawed, to help dentists to answer thequestion ‘How long will it last?’.

Provided that the patient fully understands thelikely effects of their own circumstances and thatthe following figures are very approximate with awide range, it may be reasonable to give patientsthese figures. However, it is often better toestimate how many replacements will be necessaryin the patient’s anticipated lifespan if everything elseremains stable. In other words add lots of caveatsand estimate low rather than high. A patient whoserestoration lasts a few years longer than youthought it would will be content but if it fails a fewmonths beforehand they will be disgruntled.

Years• Ceramic veneers 8–10 • Individual anterior crowns 15–40• Individual posterior crowns 20–50• Small anterior conventional bridges 20–40• Small posterior conventional bridges 20–50• Large conventional bridges 10–30• Minimum-preparation bridges

– Cantilever 15–30– Fixed–fixed 10–15 – Fixed–movable 10–20– Hybrid 10–20

• Multiple implants– Connected to each other 25+– Connected to natural teeth 20+

• Single tooth implants 20+

Causes of failure and somesolutions

Loss of retention

With the exception of post crowns, where failureis usually due to inadequate post design orconstruction (Figure 14.1), loss of retention is nota common cause of failure of individual crowns.However, because of the leverage forces onfixed–fixed bridges, one of the more commonways in which they fail is by one of the retainersbecoming loose but the other remaining attachedto the abutment tooth.

Conventional fixed–fixed bridges andsplinted retainers

When only one retainer of a conventional bridgebecomes loose, this can be disastrous. Without acement seal, plaque forms in the space betweenthe retainer and the abutment tooth and cariesdevelops rapidly across the whole of the dentinesurface of the preparation (Figure 14.2).

Sometimes the patient is aware of movementdeveloping in the bridge or experiences a badtaste from debris being pumped in and out of thespace with intermittent pressure on the bridge. Agood diagnostic test for a loose retainer is toexamine the bridge carefully without drying theteeth, pressing the bridge up and down andlooking for small bubbles in the saliva at themargins of the retainers.

Minimum-preparation bridges

Loss of retention of one part of a fixed–fixedminimum-preparation bridge also occurs but,although caries does sometimes develop rapidly,because the surface of the tooth is enamel ratherthan dentine, the development of caries is usuallyslower than with a convential bridge. If oneretainer does become loose, it is a matter ofurgency to remove at least that retainer, andusually the whole bridge. If a fixed–fixedminimum-preparation bridge becomes loose atone end but seems firmly attached at the other,one option is to cut off the loose retainer, leavingthe bridge as a cantilever.

Partial or complete loss of retention is thecommonest cause of failure of these bridges. It isargued by some that if the bridge can be removedwithout distorting it, cleaned, re-grit-blasted andre-cemented without further treatment, it is nota true failure but only a partial failure. This is areasonable point of view, and when minimum-preparation bridges are made, the patient shouldbe warned that re-cementation may be necessaryas part of normal maintenance and should notalways be regarded as a disaster.

There is some evidence that minimum-prep-aration bridges are retained for longer periodswhen they have been re-cemented. It is difficultto imagine why this should be, other than perhapsthe operator taking greater care the second timearound. If a minimum-preparation bridge debondssoon after placement, this is often due to poor


cementation technique and if re-cementation isdone with more care, the bridge is likely to lastfor longer. Bridges that survive for many yearsand then debond may well not last for as long asecond time.

There is now good evidence that fixed–fixedminimum-preparation bridges fail through loss ofretention more readily than cantilever (with oneabutment tooth) and fixed–movable designs. Thisis why these designs have been advocated earlierin this book. It is very unusual for a minorretainer for a fixed–movable minimum-prepar-ation bridge to lose its retention because thereare no significant forces to dislodge it.

Other bridges

In the case of simple cantilever bridges with oneabutment tooth, or the major retainer of a three-unit fixed–movable bridge, the loss of retentionwill result in the bridge falling out. The same istrue if both ends of a fixed–fixed bridge becomeloose. There is usually less permanent damage inthese cases, since plaque is not retained against

the surface of the preparation, and the patient isobviously aware of the problem and seeks treat-ment quickly.

Solutions for loss of retention

If there is no extensive damage to the prepara-tion, it may be possible to re-cement the crownor bridge, provided that the cause can be identi-fied and eliminated. It may be that a bridge wasdislodged by a blow or that some problem duringcementation was the cause. However, if theunderlying reason is that the preparations are notadequately retentive, they can sometimes be mademore retentive and the crown or bridge (or atleast the unsatisfactory retainer) remade. A moreadhesive cement could be tried such as a chemi-cally active cement as used for minimum-prepara-tion bridges. It is always wise to re-check theocclusion in case this contributed to the failure.

Alternatively it may be necessary to includeadditional abutment teeth in a bridge to increasethe overall retention or to change the design insome other way.


Figure 14.1

The upper central incisor had a post-retained crownbut no diaphragm covering the root face. The toothhas split longitudinally and the crown has fallen off. Itmust now be extracted.

Figure 14.2

Carious abutment teeth (the upper right canine andupper left first premolar) revealed by removing a bridgethat was still firmly attached to the sound centralincisors.


Figure 14.3

Repairing porcelain facings.

a The porcelain of the lateral incisor facing haschipped. The bridge is more than 10 years old.

b After polishing with pumice and water, a silanecoupling agent is painted over the surface, followed bya resin bonding agent and light-cured composite.

c Polishing the composite.

d The finished result. This would have been better ifan opaquer had been used over the metal to stop the‘shine through’.

Mechanical failure of crowns orbridge components

Typical mechanical failures are:

• Porcelain fracture• Failure of connectors: solder and laser welded

joints• Distortion• Occlusal wear and perforation• Lost acrylic facings.

Porcelain fracture

At one time pieces of porcelain fracturing offmetal–ceramic restorations, or the loss of theentire facing due to failure of the metal–ceramicbond, were relatively common. With modernmaterials and techniques this is much lesscommon; but when it does occur it is particularlyfrustrating since, even though the damage may beslight, and can often be repaired with composite,the repair is less satisfactory than the originalporcelain (Figure 14.3). Even if the repair only


e Fractured porcelain at the tip of a bridge retainerthat was 5 years old and otherwise satisfactory.

f An intra-oral grit-blasting machine.

g The fracture repaired with composite following grit-blasting with alumina oxide particles and silane priming.

lasts a few years before discolouring or wearing,it is a cost-effective way to extend the life of arestoration.

To prevent this type of damage tometal–ceramic bridges, the framework must beproperly designed with an adequate thickness ofmetal to avoid distortion, particularly with long-span bridges. If there is any risk of the pontic areaflexing, the porcelain should be carried on to thelingual side of pontics to stiffen them further.

A ceramic crown or bridge that is fracturedmust be replaced. Sometimes the cause is a blow,and then the choice of material can be regardedas fortunate: had a metal–ceramic material beenused it is more likely that the root of the toothwould have fractured. If the fracture is due totrauma, and particularly if the crown or bridgehad served successfully for some time, it shouldbe replaced by means of another all-ceramicrestoration. However, if the failure occurs duringnormal function, shortly after the crown orbridge is fitted, the implication is that the condi-tions are not suitable for a ceramic restoration,and the replacement should be metal–ceramic.

Failure of connectors: soldered and laserwelded

Solder joints. Occasionally a solder joint thatappears to be sound fails under occlusal loading.This may be due to:

• A flaw or inclusion in the solder itself• Failure to bond to the surface of the metal• The solder joint not being sufficiently large for

the conditions in which it is placed.

A problem, particularly with metal–ceramicbridges, is that soldered connectors should berestricted from encroaching on the buccal sidetoo much to avoid metal showing, restrictedgingivally to provide access for cleaning, andrestricted incisally to create the impression ofseparate teeth. Too much restriction can lead toan inadequate area of solder and to failure.

It is better whenever possible to join multiple-unit bridges by solder joints in the middle ofpontics before the porcelain is added. This givesa much larger surface area for the solder joint,and it is also strengthened by the porcelain cover-ing. Ideally one-piece castings are preferred andthese are now more successful with moderninvestment materials and casting techniques.

A failed solder joint is a disaster in a largemetal–ceramic bridge, and often means that thewhole bridge has to be removed and remade.Figure 9.4e shows a failed solder joint which wastoo small. There are no satisfactory intra-oralrepair methods, and it is not usually possible toremove the bridge to re-solder the joint withoutdoing further damage.

Laser welded joints have been available forless time than soldered joints, but it is likely thatthey will be stronger. Figure 14.4 shows a laserwelding machine.

Avoiding soldered or welded joints in thefirst place

The current, ideal laboratory technique is toproduce both a working model with removable,trimmed dies to wax-up the separate componentsand a solid model to ensure that the components


Figure 14.4

A laser welding machine.

are located together in an accurate relationshipwith good contact points (Figure 14.5).

Distortion

Distortion of all-metal bridges may occur, forexample, when wash-through pontics are made toothin or if a bridge is removed using too much force.When this happens the bridge has to be remade.

In metal–ceramic bridges distortion of the frame-work can occur during function or as a result oftrauma. This is likely if the framework is too smallin cross-section for the length of span and thematerial used. Distortion of a metal–ceramic frame-work invariably results in the loss of porcelain.

Occlusal wear and perforation

Even with normal attrition, the occlusal surfaces ofposterior teeth wear down substantially over a

lifetime. Gold crowns made with 0.5 mm or so ofgold occlusally may wear through over a period oftwo or three decades. If perforation has been theresult of normal wear and it is spotted before carieshas developed, it may be repaired with an appro-priate restoration. Occasionally, particularly if theperforation is over an amalgam core, it is satisfac-tory simply to leave the perforation untreated andcheck it periodically (see Figures 8.1b and 14.12d).

Occlusal perforations may also be made delib-erately for endodontic treatment or vitalitytesting (see Figure 10.2c).

Lost acrylic facings

Old laboratory-made or acrylic facings may beentirely lost, and wear and discoloration are alsocommon (Figure 14.6). If the metal part of therestoration is satisfactory then removing all orpart of the facing, grit-blasting the metal andrepairing with composite is worth trying.


Figure 14.5

a Sectioned and b unsectioned models of the samepreparation need to be used to verify the contactpoints and confirm the fit of a crown.

Changes in the abutment tooth

Periodontal disease

Periodontal disease may be generalized, or in apoorly designed, made or maintained restorationits progress may be accelerated locally. If the lossof periodontal attachment is diagnosed earlyenough and the cause removed, no further treat-ment is usually necessary. However, if the diseasehas progressed to the point where the prognosisof the tooth is significantly reduced then thecrown or bridge, or the tooth itself, may have tobe removed.

With a bridge the original indication will still bepresent, and so something will have to be doneto replace the missing teeth. It may be possibleto make a larger bridge, or the abutment teethmay be reduced and used as abutments for anover-denture. Teeth that have lost so muchsupport that they are not suitable as bridgeabutments are also not suitable either asabutments for conventional partial dentures.

Figure 14.7 shows how successful periodontaltreatment and adjustment of bridge margins canbe achieved.

Problems with the pulp

Unfortunately, despite taking the usual precau-tions during tooth preparation, abutment teethmay become non-vital after a crown or bridge hasbeen cemented. It is usually reasonable toattempt endodontic treatment by making an

access cavity through the crown. There are ofcourse problems in the application of a rubberdam with bridges, although these can usually beovercome by punching a large hole and applyingthe rubber dam only to one tooth, stretching therubber over the connectors.

It is difficult to gain access to the pulp chamberand remove the coronal pulp completely withoutenlarging the access cavity to a point where theremaining tooth preparation becomes too thinand weak to support the crown satisfactorily, orwhere the (pin) retention of a core is damaged.The crown may have been made with ratherdifferent anatomy from the natural crown of thetooth for aesthetic or occlusal reasons, so thatthe angulation of the root is not immediatelyapparent. Provided that these problems can beovercome and a satisfactory root filling placed,the prognosis of the crown or bridge is onlymarginally reduced.

Teeth that were already satisfactorily root-filled when the crown or bridge was made maylater give trouble. It may be possible to re-root-fill the tooth through the crown, but apicectomyis an alternative solution. Care must be taken notto shorten the root of an abutment tooth morethan is absolutely necessary so that the maximumsupport for the bridge can be maintained.

Caries

Secondary caries occurring at the margins ofcrowns or bridge retainers usually means that thepatient has changed his or her diet, the standard


Figure 14.6

Badly worn acrylic facings.

The bridge has been present for many years and fitswell. It is worth replacing all or part of the canine facing with composite.

of oral hygiene has lapsed or there is some inade-quacy in the restoration that is encouraging theformation of plaque. The cause of the problemshould be identified and dealt with before repairor replacement is started.

Fracture of the prepared natural crownor root

Fractures of the tooth occasionally occur as aresult of trauma, and sometimes even duringnormal function, although the crown or bridgehas been present for some time. With a bridgeabutment it is usually necessary to remove thebridge, but occasionally the abutment tooth canbe dispensed with and the root removed surgi-cally, the tissue surface of the retainer beingrepaired and converted into a pontic.

Movement of the tooth

Occlusal trauma, periodontal disease or relapsingorthodontic treatment may result in the crownedtooth or bridge abutment becoming loose, drift-ing, or both. When the cause is periodontal

disease or relapsing orthodontic treatment, thismust be remedied before the crown or bridge isremade.

Design failures

Abutment preparation design

The pitfalls of inadequate crown preparationdesign were described in Chapter 3, and are theunderlying cause of many of the problems listedso far in this chapter.

Inadequate bridge design

Designing bridges is difficult. It is neither a precisescience nor a creative art. It needs knowledge,experience and judgement, which take years toaccumulate. So it is not surprising that somedesigns of bridge, even though well intentionedand conscientiously executed, fail. A simple classi-fication of these failures is as ‘under-prescribed’and ‘over-prescribed’ bridges.


Figure 14.7

a An old bridge with defective margins and extensivegingival inflammation.

b The same bridge after a periodontal flap has beenraised, the retainer margins adjusted by grinding andpolishing, and the flap then apically repositioned. Thegingival condition is now healthy.

Under-prescribed bridges

These include designs that are unstable or havetoo few abutment teeth – for example acantilever bridge carrying pontics that cover toolong a span or a fixed–movable bridge whereagain the span is too long, or where abutmentteeth with too little support have been selected.

Another ‘under-design’ fault is to be tooconservative in selecting retainers, for exampleintracoronal inlays for fixed–fixed bridges. Withthese design faults little can be done other thanto remove the bridge and replace it with anotherdesign (Figure 14.8).

Over-prescribed bridges

Cautious dentists will sometimes include moreabutment teeth than are necessary, and fateusually dictates that it is the unnecessary retainerthat fails. The first lower premolar might beincluded as well as the second premolar andsecond molar in a bridge to replace the lowerfirst molar, no doubt so that there will be equalnumbers of roots each end of the bridge so as tocomply with the redundant ‘Ante’s Law’. This isnot necessary.

Another example would be to use the uppercanines and both first premolars on each side inreplacing the four incisor teeth. As well as beingdestructive, this gives rise to unnecessary practi-cal difficulties in making the bridge and cleaning it.This, in turn, reduces the chances of the bridgebeing successful.

When an unnecessarily large number ofabutment teeth have been included in a bridge

and one of the retainers fails, it is sometimespossible to section the bridge in the mouth andremove the failed unit, leaving the remainder ofthe bridge to continue in function. The failed unitis remade as an individual restoration (Figure14.9).

The retainers themselves may be over-prescribed, with complete crowns being usedwhere partial crowns or intracoronal retainerswould have been quite adequate; or metal–ceramic crowns might be used where all-metalcrowns would have been sufficient. When thepulp dies in such a case, it is interesting to specu-late whether this might not have occurred with aless drastic reduction of the crown of the naturaltooth.

Inadequate clinical or laboratorytechnique

It is helpful to allocate problems in the construc-tion of crowns and bridges to one of threegroups:

• Minor problems to be noted and monitoredbut where no other action is needed

• The type of inadequacies that can be correctedin situ, and

• Those that cannot.

This is often a matter of degree, and many ofthe following faults can fall into any of thesegroups.


Figure 14.8

A bad design.

The bridge is fixed–fixed and is firmly held by thepremolar retainer. The inlay in the canine is, however,loose and caries has developed beneath it. Either thedesign should have been fixed–movable with a mesialmovable connector or, if fixed–fixed, the retainer onthe canine should have covered all occluding surfacesof the tooth and have been more retentive.

Marginal deficiencies

Positive ledge (overhang)

A positive ledge is an excess of crown materialprotruding beyond the margin of the preparation.Considering that this is a fairly easy fault to recog-nise and correct before the crown or bridge isfitted, it is surprising how frequently overhangsare encountered (see Figure 6.26a). However, itis often possible to correct them without other-wise disturbing the restoration.

Negative ledge

This is a deficiency of crown material that leavesthe margin of the preparation exposed but withno major gaps between the crown and the tooth.Again it is a fairly common fault, particularly withmetal margins, but one that is difficult or imposs-

ible to correct at the try-in stage (Figure 6.26b).It often arises because the impression did not givea clear enough indication of the margin of thepreparation and the die was over-trimmed, result-ing in under-extension of the retainer (Figure6.27).

Provided that the crown margin is supragingi-val or just at the gingival margin, it is sometimespossible to adjust and polish the tooth surface.When the ledge is subgingival, and particularlywhen there is localized gingival inflammationassociated with it, it may still be possible to adjustthe ledge with a pointed stone or bur, althoughthis will cause gingival damage. However, it isusually necessary to remove the crown or bridge.

Defect

A defect is a gap between the crown and prepa-ration margins. There are four possible causes:


Figure 14.9

Overprescribed design.

a This four-unit bridge replaces only one central incisor.The partial-crown retainer on the canine has becomeloose. When the bridge was removed, the central andlateral incisors were found to be sound and adequateabutments, without the inclusion of the canine. Carieshas spread across the canine, and the pulp has died.

b and c Fortunately it was possible to remove thebridge intact, and, after removing the canine retainer,the remaining three units could be re-cemented. Aseparate post crown was made for the canine toothfollowing endodontic treatment.

• The crown or retainer did not fit and the gapwas present at try-in

• The crown or retainer fitted at try-in, but atthe time of cementation the hydrostaticpressure of the cement (particularly if thecement was beginning to set) produced incom-plete seating

• With a mobile bridge or splint abutment, thecement depressed the mobile tooth in itssocket more than the other abutment teeth,thus leaving the gap

• No gap was present at the time of cementa-tion, but one developed following the loss ofcement at the margin, and a crevice has beencreated by a combination of erosion/abrasionand possibly caries.

In any of these cases, the choice is to remove thebridge, restore the gap with a suitable restora-tion, or leave it alone and observe it periodically.

Purists may say that all defective retainersshould be removed and replaced. But this is notalways in the patient’s best interest, and the skilfulapplication of marginal repairs may extend the lifeof the restoration for many years (Figure14.10).

Poor shape or colour

More can be done to adjust the shape of a crownor bridge in situ than to modify its colour,although occasionally surface stain on porcelaincan be removed and the porcelain polished. Theshape of metal–ceramic crowns or bridges can beadjusted if they are too bulky (and this is usuallythe problem), provided that it is done slowly. Atthe first sign of the opaque layer of porcelain, theadjustment is stopped.

Successful modifications can often be made toopen cramped embrasure spaces, reduce exces-sive cervical bulbosity, shorten retainers andpontics, and of course adjust the occludingsurface. In all cases the adjusted surface, whetherit is metal or porcelain, should be polished.

Occlusal problems

As well as producing abutment tooth mobility,faults in the occlusion involve damage to theretainers and pontics by wear and fracture.


b The defect repaired with glass ionomer cement. Thepatient has been shown gentler oral hygienetechniques.

Figure 14.10

Repair for a retainer margin.

a A small gap at the mesial margin of the upper canineretainer on an otherwise very satisfactory bridge thathas been in place for several years. The gap was notnoticed at previous recall appointments, and althoughit may now have become apparent through gingivalrecession, it is more likely that the gap has beenenlarged by over-vigorous use of dental floss. Thepatient demonstrated a faulty and damaging sawingaction, with floss running into the gap.

The occlusion can change as a result of theextraction of other teeth, or their restoration, orthrough wear on the occlusal surface.

Techniques for adjustments,adaptations and repairs to crownsand bridges

Assessing the seriousness of theproblem

In existing restorations there is not infrequentlyone or other of the faults listed above. A decisionhas to be made between:

• Leaving it alone, if it is not causing any seriousharm

• Adjusting or repairing the fault• Replacing the crown or bridge.

When action is necessary, it is clearly better toextend the life of an otherwise successful crownor bridge with the second option than replacerestorations too frequently. If there is any doubt,or when adjustment or repair must be carriedout, the restoration must be kept under frequentand careful review.

Adjustments by grinding and polishingin situ

In some situations the margins of crowns withpositive ledges can be satisfactorily adjusted. If themargin is porcelain, specially designed porcelain


Figure 14.11

a A set of instruments for polishing porcelain. Theseare also available with contra-angle shanks for intra-oraluse.

b Scanning electron micrographs, at the same magnifi-cation, of three areas of the same porcelain surface. Left:the glazed surface showing some undulation andoccasional defects. Centre: the surface ground with a fineporcelain grindstone. Right: the same surface re-polished, after grinding, with the instruments shown ina. The surface is smooth, without undulations, but withsome fine scratch marks and occasional residual defects.

finishing instruments should be used. Alterna-tively, a heatless stone or diamond point can beused, followed by polishing with successive gradesof composite finishing burs and discs. These arecapable of giving a very good finish to non-porousporcelain, which the patient can keep as clean asglazed porcelain (Figure 14.11). The contour ofporcelain restorations can be modified in situusing the same instruments.

In the case of metal margins, a diamond stonefollowed by green stones, tungsten carbide stonesor metal and linen strips may be used.Interdentally, a triangular-shaped diamond and anabrasive rubber instrument in a special recipro-cating handpiece designed specifically for remov-ing overhangs may be used. The margin should bepolished with prophylactic paste and a brush orrubber cup, and interdentally with finishing strips.

Repairs in situ

Occlusal repairs

Occlusal defects in metal retainers can berepaired with amalgam, which usually gives quitea satisfactory result. In ceramic or metal–ceramicrestorations composite can be used, but therepair may need to be redone periodically.

Repairs at the margins

Although repairs are justified to extend the life ofan established crown or bridge, they should neverbe used to adapt the margins of a poorly fittingbridge on insertion. Secondary caries that isidentified at an early stage or early abrasion/erosion lesions at crown margins can be repairedusing composite or glass ionomer cement. Thecause should be investigated and preventivemeasures applied.

The cavity preparation at the margin must notbe so deep that it endangers the strength of thepreparation, although of course all caries must beremoved. If there is poor access it may be betterto remove part of the crown margin rather thanan excessive amount of tooth tissue.

In some cases raising a full gingival flap may bejustified. Retainer margins can be adjusted andrestored under conditions of optimum access and

visibility, and any necessary periodontal work orendodontic surgery carried out at the same time(Figure 14.7).

Repairs to porcelain

Materials are available to repair or modify theshape of ceramic restorations in the mouth.These are basically composites with a separatesilane coupling agent to improve bonding. It is notan acid-etch bond like the bond to enamel and isnot strong, so the use of the material is limitedto sites not exposed to large occlusal forces(Figure 14.3).

An alternative and better solution is to use anintra-oral grit-blasting device if one is available.This will produce a retentive surface on porcelainto which bonding resin and composite can beapplied. In many cases the repair is strong enoughto be used on chipped or fractured incisal edgeswhere there is sufficient porcelain to bond to(Figure 14.3e, f and g).

Repairs by removing or replacingparts of a bridge

Replacing lost facings

It is sometimes possible to replace a failed facingon a bridge, usefully extending its life. But this isnot worth attempting on individual crowns – it isbetter to replace the whole crown.

When the porcelain is lost from ametal–ceramic unit and a composite repair is notpossible, there is often little choice but to removethe whole crown or bridge.

However, it is sometimes possible with retain-ers or pontics to remove all the porcelain and re-prepare the metal part, producing enoughclearance without damaging the strength of themetal. A new complete crown covering the skele-ton of the old retainer or pontic can then beaccommodated. These are sometimes made inheat-cured acrylic, laboratory light-cured compos-ite or metal–ceramic material. They are known as‘sleeve crowns’. A metal–ceramic sleeve crown isshown in Figure 14.12a, b and c.

Before the routine use of metal–ceramicmaterials, bridges were sometimes made with a



Figure 14.12

Techniques for repairing bridges.

a The porcelain on this bridge retainer has fractured.It has all been removed and the tooth prepared for a‘sleeve-crown’.

b The sleeve-crown with a metal lingual surface replac-ing the original lingual porcelain. This could be bondedwith the resin bonding material used with minimum-preparation bridges. The metal of the sleeve-crownshould be grit-blasted in the laboratory immediatelybefore bonding and the metal of the bridge could beblasted with an intra-oral grit-blaster if one is available.This would give a very retentive result.

c The sleeve-crown in place.

d A fractured traditional porcelain jacket crown, whichhas been made over a gold coping as the canineretainer for a bridge. Apart from this, and the holeworn in the occlusal surface of the premolar partialcrown retainer, the bridge is still serving satisfactorilyafter more than 20 years.

e The replacement ceramic crown cemented. Picturesof the bridge illustrated in d and e were published inthe first edition of this book in 1986. By the 3rd editionin 1996 the bridge with its replacement ceramic crownwas still in place. The bridge eventually had to beremoved for other reasons in 2002. It had been inplace for 37 years – showing that repairs of this sortare well worthwhile.

metal framework and separate ceramic crownscemented to it. This design was known as ‘unit-construction’. The individual ceramic crownsoften broke, since they were considerablyreduced approximally to accommodate theconnector. However, a new ceramic or prefer-ably a metal–ceramic crown can easily be made(Figure 14.12d and e).

Removing and/or replacing entiresections of a bridge

Bridges are sometimes so designed that if adoubtful abutment tooth becomes unsavable, itcan be removed with its associated section of thebridge, leaving the remainder undisturbed. This isone of the purposes of removable, telescopiccrown-retained bridges and of dividing multiple-unit bridges into smaller sections. When part ofa bridge is removed, the remainder cansometimes be modified, perhaps by cutting a slotfor a movable joint and then replacing the lostsection.

Removing abutment teeth

Sometimes with large bridges, particularly thosemade as bridge/splints for patients with advancedbone loss and following periodontal treatment,

such as that shown in Figure 13.6, it is possibleto remove a failing abutment and fill the retainerwith composite. The bridge shown in Figure 13.6was made with six abutment teeth but over theyears two abutment teeth developed untreatableperiodontal disease and were extracted. Despitethis the bridge survived for more than 20 yearsuntil the other abutment teeth deteriorated. Thepatient had learnt and practised meticulous oralhygiene methods.

Extending bridges

Provision is sometimes made to extend a bridgeif further teeth are lost. Figure 14.13 shows alarge bridge with a slot in the distal surface of thepremolar retainer on the left of the picture sothat a further fixed–movable section can be addedif the second premolar (which has a questionableperiodontal prognosis) is lost. The slot is filled inthe meantime by a small gold inlay.

Removing crowns and bridges

In removing any crown or bridge, and in particu-lar posts and cores, it is often helpful to break upthe cement by vibrating the restoration with anultrasonic scaler. This works best with zincphosphate cement.


Figure 14.13

Provision for the extension of a bridge (see text fordetails).

Crowns

Removing metal crowns

Complete and partial metal crowns cansometimes be removed intact by levering at themargins with a heavy-duty scaler such as aMitchell’s trimmer. Alternatively, a slide hammertype of crown- or bridge-remover may be used,or one of the other devices specially designed toremove crowns; Figure 14.14 shows a selection.If these techniques do not work, the crown willhave to be cut off (see under ‘Removingmetal–ceramic crowns’).

Removing posts and cores

Unretentive posts can sometimes be removed bygripping the core in extraction forceps and givingit a series of sharp twists to fracture the brittle

cement, rather than tear the periodontalmembrane and extract the tooth. This should notbe attempted by the inexperienced!

There are several devices designed to removeposts and cores intact and to remove brokenposts (Figure 14.14).

Removing ceramic crowns

These cannot usually be removed intact, and shouldbe cut off. A vertical groove is made with a diamondbur in the labial or buccal surface, just through tothe cement, and then the crown is split with asuitable heavy-duty instrument (Figure 14.15).

Removing metal–ceramic crowns

It is sometimes possible to remove metal–ceramiccrowns intact by using one of the devices shown


Figure 14.14

A selection of instruments for removing crowns,bridges and posts.

From the left:

• a slide hammer remover with two alternative screw-in tips: the tip is hooked into a crown margin orunder a bridge connector, and the weight slid downthe handle and tapped against the stop at the end;

• a spring-loaded slide hammer, also with replaceabletips;

• a special heavy-duty instrument that is hooked undercrown margins and twisted to remove them;

• below: a turquoise-coloured adhesive polymer that issoftened in hot water and bitten upon by the patient.The material is cooled with water and the patient isasked to jerk the jaw open;

• above: this instrument is clamped beneath the crownand the two screws (the heads visible here) arescrewed down on to the occlusal surfaces of adjacentteeth, lifting the crown;

• two clamps that fit on to posts and cores, with ascrew that presses on to the shoulder of a post-crown preparation and draws the post and core outof the tooth.


Figure 14.15

Removing crowns and bridges.

a Removing a ceramic crown. A cut is made with adiamond bur down the labial surface and across theincisal edge. The crown can then be split with a suitableheavy-duty instrument.

b Removing a metal–ceramic bridge by cutting throughthe labial porcelain with a diamond bur.

c Then changing to a special metal-cutting (beaver) burto cut through the metal until the cement just shows.

d Springing open the retainer with a heavy-duty instru-ment. It is sometimes necessary to continue the cutround to the occlusal or lingual surface.

in Figure 14.14, but they are more rigid than goldcrowns and the porcelain is liable to break, sothey usually have to be cut off.

A groove is cut vertically from the gingivalmargin to the occlusal surface, preferably on thebuccal side just through to the cement, and thenthe crown is sprung open with a heavy instrumentsuch as a Mitchell’s trimmer or a heavy chisel,breaking the cement lute. Sometimes the cut willneed to extend across the incisal or occlusalsurface (Figure 14.15b, c and d).

Cast metal is best cut with a special solidtungsten carbide bur with very fine cross-cuts(beaver bur). This is capable of cutting metalwithout juddering or jamming, and there is lessrisk of the bur itself breaking than with a conven-tional tungsten carbide bur. Eye protection shouldalways be worn by the patient, the dental nurseand dentist, particularly when cutting metal.

Diamond burs cut cast metal slowly, but areideal for rapidly cutting porcelain, and sometal–ceramic units are best sectioned usingdifferent burs for the two materials. Since it is

possible to cut porcelain much more quickly thanmetal, the metal on the buccal surface is usuallythinner than that on the palatal or lingual surface,and visibility and access are far better buccally,the groove is easier to make on the buccal side.

Removing bridges

There are three sets of circumstances:

• When the abutment teeth are to be extractedand so it does not matter if the preparationsare damaged, the bridge will be removed in themost convenient way, often with a crown- andbridge-remover. In some cases it may not benecessary to remove the bridge at all, forexample with simple cantilever bridges withone abutment tooth. In others it is quicker todivide the bridge through a pontic or connec-tor and extract the abutment teeth individuallywith their retainers in place.


Figure 14.16

a Specialized equipment for removing crowns. Thepistol-shaped instrument is driven by compressed airand vibrates one or other of the attachments againstthe crown or bridge. The equipment is expensive andis usually only available in specialist centres.

b The equipment being used clinically.

• When it is the intention to retain the abutmentteeth – either to make a new bridge or to usethem to support a partial denture or an over-denture – it does not matter whether thebridge is damaged during its removal, but thepreparations should be protected. The retain-ers should be cut and the bridge carefullyremoved with the bridge-remover.

• There are occasions when it would be helpfulto remove the bridge intact, modify or repairit and then replace it, if only as a temporarymeasure. In this case neither the bridge nor thepreparations should be damaged.

Removing bridges intact

The slightly more flexible structure of all-metalbridges and of minimum-preparation bridgesallows them to be removed intact rather morereadily than metal–ceramic conventional bridges.However, all types can sometimes be removed bysharp tapping, which fractures the cement lutewithout too much risk to the periodontal

membrane of the abutment teeth. The nature ofthe force is quite different to the slow tearingapplied in extracting teeth.

Slide hammers are specially designed for thepurpose with replaceable tips to fit under retainermargins, under pontics or into embrasure spaces(Figure 14.14). Sometimes it is necessary to drilla hole in the palatal surface of the retainer orpontic and fit an attachment from the slidehammer into it.

Various other techniques can be used. Figure14.16 shows a specialist air-driven appliance.Ultrasonic vibration with a scaler can loosencrowns and bridges.

A good technique is to make a loop of soft wirebeneath the connector of the bridge and use a slidehammer in the wire loop (Figure 14.17).Alternatively, if a slide hammer is not available, aheavy metal object is passed through the loops welloutside the mouth, and sharp blows applied to itwith a mallet or other heavy instrument. This is arather dramatic approach, and the patient needs tohave a phlegmatic personality and to be properlyinformed of what is proposed beforehand.


Figure 14.17

Removing a bridge with a soft brass wire loop.

The locking forceps are clipping the twisted ends of thewire together to prevent the sharp ends damaging thechin. The slide hammer (see Figure 14.14) is being usedin the wire loop rather than under the bridge pontic. Thisis more controllable and effective and less dangerous.

abrasion 5abutment teeth 246–8

assessment for bridges 229–31caries, secondary 306–7definition of 197directions of forces 231fixed–fixed bridges 199fracture of natural tissue 307minimum-preparation bridges 199, 242,

244occlusal loading 226, 228, 231

adaptation to extra load 226–7paralleling

by extra-oral survey 247–8by eye 244, 246–7paralleling devices 248

periodontal disease 306preparation design failures 307prognosis 189pulp problems 306radiographs 184and retainer choice 212, 213selection 235support

in bridge design 225–8and length of span 227

abutments, implant 180, 271choice of 283custom-made 271definition 261design 265, 270, 272healing 267screw- vs cement-retained 272, 273–4seating 269, 270see also transmucosal abutments (TMAs)

acid-etch retained composite splints 292acid etching 17acrylic

biteplane 89, 90bridges

provisional 216, 250temporary 248

on cast metal crowns 39, 107changing occlusal vertical dimension 116,

117crowns, provisional 87lost/worn facings 305partial dentures 267, 280–1polyacrylic veneers 15special trays 127–30temporary bridges 241, 242for temporary crowns 154, 155wire/acrylic splint 290

adhesive bridges 209, 210adhesive cements 61, 171–2adrenaline-impregnated cord 164, 166age of patient

crown planning 106, 107fixed vs removable prostheses 187, 188

bridge indicated 192and implant restorations 264

alginate impressions 101, 123for temporary bridges 247, 249

alloys, dental 37, 45aluminium crown forms 154–5alveolar bone loss 189, 192

implant restorations 262, 280amalgam cores 47, 51

pin-retained 46, 47–8, 131post-retained 70, 131

amalgam restorations 21bonded 20, 21–2, 27dislike of, by patients 22failing 49pin-retained 20, 21–2, 27

amelogenesis imperfecta 4, 31anterior crowns

alternatives to 13–18, 21complete 29–39

preparations 137–40finishing 138, 140incisal/proximal reduction 137, 138,

139labial reduction 139palatal reduction 138, 140

indications 11, 13life-expectancy 300pin-retained 66post-retained

preparations 140–5anti-rotation notch 145finishing 145post hole preparation 140–2, 145post hole shaping 145, 146–7post insertion/cementation 142–4shoulder/margin preparation 140

preparations 71–2inadequate 72vital teeth 73, 75

for root-filled teeth 39–40anterior splints

appearance 293–4selecting 294

Ante’s law 225, 227, 308appearance

bridges 228–9and crown design 55–6and crown planning 109, 110, 112–16, 124–6fixed vs removable prostheses 187,

188–9, 190, 191bridge indicated 192

and gender 106and implant restorations 280importance 9indicating crowns 7, 8, 9pontics 214porcelain jacket crowns (PJCs) 37and retainer choice 212, 213splints, fixed 293–4veneers 13, 14–16

appointments 118–19arcon articulators 94, 99

articulating paper/foil 90–1, 154articulating wax 91articulators 94–6

arcon 99fully-adjustable 100hand-held 98inadequate 99non-arcon 99–100semi-adjustable 94–6, 98simple hinge 98, 99

Astra implant 261attrition 5autogenous bone grafting 189, 279automix guns 153, 159, 161, 166axial contours/surfaces of crowns

checking/adjusting 170reduction 135, 137

banding 10, 11beaver burs 316, 317Bennett angle 80Bennett movement 80bevelling functional cusp 54biteplanes

acrylic 89, 90fixed anterior (Dahl appliances) 132–3

bleaching 12as alternative to crowns 13crown shade matching 117external 9, 12, 13internal 8, 9, 12, 13shade planning 125in tetracycline staining 11vital 10, 13

bonded amalgam 20, 21–2, 27bone grafting 189, 279bone loss 189, 192, 279, 280

implant restorations 262, 280, 289Brannemark system 299bridge abutment preparation 70bridge-retaining crown preparations 72bridges

cleaning aids 256, 257clinical/laboratory techniques, inadequate

308components 211–24

see also specific componentsconstruction 241–57conventional 177, 178, 197

anterior, life-expectancy of 300cementation 255large, life-expectancy of 300life-expectancy 298loss of retention 300paralleling 244, 246–8posterior, life-expectancy of 300

designabutment teeth selection 235anterior missing teeth 236

alternative designs 239multiple 237–8

319

Index

bridges (cont.)basic 197–203

advantages/disadvantages 204combinations 203–4comparison 206–7connector selection 238examples of process 239failures 307–11hybrid 203, 204inadequate 307–8overprescribed 308, 309pontic selection 238posterior missing teeth 236retainer choice 235, 238selection

criteria 225–9practical steps 234–9

support rule 225–8underprescribed 308variations 204

distortion 305failure 185, 217fixed vs removable prostheses 186–95implant-supported 273indications 192–3

compared with partial dentures/implants177–95

intra-oral trials 233–4life-expectancy 186localization 251, 252loss of retention 301maintenance 256materials 204–5, 208oral hygiene instructions 255–6patient confidence 188planning 232

abutment teeth assessment 229–31length of span 227, 231occlusion 225–9, 238–9predicting final result 232–3ridge shape 232study casts 233whole patient considerations 229

removal 317–18intact 318reasons 317–18

repairing fractured porcelain 303–4repairs 313

abutment teeth, removing 314extending bridge 314facings, replacing lost 312, 314

screw-retained 181terminology 177–82try-in stage 250

finished bridges 253types 177–9, 197–210use in UK 177for young patients 107see also specific types of bridge

brittlenessporcelain 53porcelain jacket crowns (PJCs) 37

broken-down teethindicating crowns 5, 21

preparation 20indicating veneers 14restoration 21

bruxismcontraindicating implants 264and crown choice 47and crown design 55

buccal flangeson dentures 188

partial 188, 233, 234removable 189, 204

burs 133, 134for anterior crowns

complete 137, 138, 139, 140post-retained 141, 145

for axial reduction 135, 137for occlusal reduction 135for restoration removal 316, 317see also drills

CAD/CAM see computer-aideddesign/manufacture (CAD/CAM)

canine guidance 78, 83and crown planning 111

canines, pointed 112cantilever bridges 71, 178, 181, 198, 201, 229

all-ceramic 201approximal surfaces 221in combination designs 203conventional, advantages/disadvantages of

206indications 193large splint bridge 202life-expectancy 300minimum-preparation 201, 213

designs comparison 207occlusal assessment 232occlusal forces 230temporary 241see also spring cantilever bridges

carbon-fibre posts 41caries

and bridges 185and crown design 59development 3indicating crowns 3, 11secondary 185, 199, 293

abutment teeth 306–7repair 312

castsarticulating 94, 102occlusal surface shaping 102opposing 123study 71

bridges 232, 233as contract 116diagnostic wax-ups on 114, 115for occlusion 93pre-preparation 123trial preparations on 112, 117, 118,

119, 124trimming 101, 102working

articulation 94–6articulators 98–100

see also wax-ups cast ceramic crowns 32, 33cast connectors 222–3cast (laboratory-made) posts 42, 43, 44cast metal abutments 271cast metal bridges 204cast metal cores 51

construction stages 120, 121preparations 147

cast metal crowns 69with acrylic/composite 39, 107construction stages 120, 121design 53–75

fit surfaces 67fracture and retention 67groove-retained 47posterior 45, 46preparation 69removal 315, 317retention problems 67–8

cast metal posts 51construction stages 120, 121design 74preparations 146, 147

cavosurface angle 53, 54, 57cementation splints, fixed 295cements/cementation 59–61

adhesive 61, 171–2cement removal 314choice of cements 171–2non-adhesive 59penetration into dentine 17, 61technique 172temporary/provisional 157

‘centric occlusion’ 79‘centric relation’ 79ceramic inlays/onlays 26

computer-aided design/manufacture 26, 27life-expectancy 299

ceramic (porcelain) bridges 205cantilever bridges 201failures 304

ceramic (porcelain) crowns 32, 33advantages/disadvantages 37anterior crowns

complete 29–37prepared teeth 40

choosing 36construction stages 120, 121date of development 299design 53failures 304implant-supported 272occlusal shaping 102posterior 47preparations 71, 72

inadequate 72occlusal reduction required 135vital teeth 75

reinforced 33removal 315, 316resin-bonded 19, 21retention problems 66–7see also porcelain jacket crowns (PJCs)

ceramic veneers see porcelain veneers Cerec CAD/CAM machine 26chair-side bleaching 13chair-side milled restorations 33chair-side temporary bridges 241, 248–9chair-side temporary crowns 130

injection technique 150–2, 155with laboratory-produced mould 153

moulding technique 154, 155children

bridges for 106, 107crowns for 106, 107oral hygiene 108

cleansabilitybridges 228intracoronal splints 293pontics 214, 218, 219

cleft palatesbridges supporting 184, 185indications for fixed splints 287–9

320 Index

collapsed bite 86complete-crown splints 292

preparation 294–5preparations 293

composite cores 41, 51, 72, 130pin-retained 20, 48, 49, 136

composite crowns 41composite inlays/onlays 22, 24, 27composite luting cement 61composite materials 9

bridge facings 204composite restorations

as alternative to crowns 13, 23appearance 8cores 20Dahl effect 87, 89, 133failing 8posterior teeth 22, 23veneers 13–14, 18, 21

composite splints 292compressed air for gingival retraction 162,

163, 166computer-aided design/manufacture

(CAD/CAM)ceramic inlays/onlays 26, 27cores 33crowns 33

connectors (joints)cantilever bridges 203combination bridges 205definition of 197failures 304fixed 222–3fixed–fixed bridges 197fixed–movable bridges 199, 200, 203hybrid bridges 204precision-attachment 224selection 238

connectors, movable 224construction

bridges 241–57abutment teeth preparation

conventional bridges 244–8minimum-preparation bridges

242–4cementation

conventional bridges 255minimum-preparation bridges 252–3permanent 252temporary/provisional bridges 250trial 252

occlusal records 251preoperative procedures 241–2provisional 249–50temporary 248–9, 250try-in stage 253

finished bridges 252metal framework 251separate units 252

working impressions 250crowns

clinical stages 120–1laboratory stages 120–1

impression trays 127–30porcelain veneers 29

contacts see occlusal contacts contraindications

of any prostheses 195implant restorations 264–5

copper bands 48, 51cord for gingival retraction 164–5, 166

cores 27–8assessing need for 117bonded 20building up 130and crown planning 110pin-retained 20, 45, 46, 47–8, 49, 131

in children 107pin placement 130in vital tooth 51

post-retained 49, 70, 131design 74

for posterior crowns 47–51with posts 41, 43removal/removal instruments 315in root-filled teeth 21, 41, 51see also specific types; specific types of core

costbridges 186ceramic crowns 37crowns 107–8implants 186metal–ceramic crowns 37, 39partial dentures 186retainers 214

crossarch splinting 183, 228, 238crossbite, posterior 83crown lengthening 57–8, 63, 124

minimum-preparation bridges 244for retention 118for worn teeth 133

crownsalternatives to 13–18, 21anterior see anterior crowns assessment on recall 173bevelling functional cusp 54complete, as retainers 211

repairing fractured porcelain 312designing preparations 53–75

specific crowns 73–5dislodgement 60

directions 63–4failures, mechanical 302–5vs fillings 3, 111fitting

checking/adjusting stagecontact points/axial contours 170die overtrimming 169, 170fit 168–9, 170occlusion 170–1retention 169–70

try-in stage 167–8indications 3–13

anterior 11, 13appearance 8, 9badly broken-down teeth 5caries 3, 11combined 7–8function 7, 9hypoplastic conditions 5, 7, 11mechanical problems 9, 11multiple crowns 8support of other restorations 7, 13tooth characteristics 7tooth wear 5–6, 11trauma 4, 5, 8, 11

life-expectancy 298maintenance by patient 172–3making, clinical/laboratory stages of 120–1partial 20, 27path of insertion 56, 62, 73

difficult 134

planning 105–18clinical/laboratory stages 118–21core, need for 117detailed planning 111–17individual tooth considerations 110–11,

117material, choice of 117type, choice of 117whole mouth considerations 108–10whole patient considerations 105–8

posterior see posterior crowns preparation of teeth 20, 63

design 53–75on extracted teeth 59, 60finishing 134instrument choice 134parallel opposing walls 65planning 111, 123–4stages 134–5taper/length and retention 63, 64, 65,

69removal 315–16, 317

instruments 315, 317, 318repair 173retention 61–7, 72

against displacing forces 62–6vertical loss 62–3

failure, other causes of 66–7, 72retentive features 63

section at dentine/cement interface 62,63

shade planning 116–17, 124–6selection technique 124–6

spacing 56–7support from other teeth 109–10types 29–51

see also specific typescusps, functional, in crown design 54

Dahl appliances 87, 88–9, 103, 248for axial tooth movement 214for worn teeth 132–3

Dahl effect 230, 232with composite 87, 89

Denar articulator 94dentine destruction in caries 3dentinogenesis imperfecta 5, 106, 107dentures

implant-supported 275occlusal design 78–9premature contact 84see also partial dentures

designbridges

anterior missing teeth 236alternative designs 239multiple 237–8

basic 197–203, 204combinations 203–4comparison 206–7connector selection 238examples of process 239hybrid 203, 204pontic selection 238posterior missing teeth 236retainer choice 235, 238selection

criteria 225–9practical steps 234–9

support rule 225–8variations 204

Index 321

design (cont.)crown application to tooth 117crown preparations 53–75

adjacent teeth, related to 56–7appearance-related 55–6function-related 54–5material-related 53periodontal tissue-related 57–9pulp-related 59specific crowns 73–5

failures 307–11implant restorations 265–75minimum-preparation designs comparison

207see also planning

diastemma 6, 7, 112and bridge planning 234and implants 195, 198and spring cantilever bridges 198

Dicor crowns 32, 33direct bridges 208dome-shaped pontics 215, 216, 219

buccal surface 222cleansability 256

drills 142, 148see also burs

eating disorders and tooth erosion 132eating with missing teeth 182, 183elastomeric impression materials 157, 160,

252, 268relining 161

elderly patients 106, 188electrosurgery for gingival haemorrhage 166embrasure spaces, open 220, 221Empress crowns 31enamel hypoplasia 5

and crown design 57erosion 5, 38

incisal 132palatal 88, 112, 132reflux 89, 116

management 132temporary crowns for 151

see also broken-down teeth; tooth/teeth,wear

facebow record 94failures

amalgam restorations 49bridges 185, 217causes/solutions 300–11ceramic bridges 304ceramic crowns 304composite restorations 8connectors (joints) 304–5crown retention 66–7, 72defining, difficulties with 297–8fillings 9implant restorations 275, 285recording 297retrospective surveys 297soldered connectors 304–5veneers 15, 18see also design, failures

fibre-reinforced composite crowns 36, 39fibre-reinforced splint 292fillings

vs crowns 3, 111failing 9

fixed–fixed bridges 197–8, 199abutment teeth 199

approximal surfaces 221in combination designs 203conventional 206failures 301hybrid designs 204life-expectancy 300loss of retention 300–11minimum-preparation designs comparison

207non-parallel abutments 223paralleling 244, 246–8temporary 241unsatisfactory design 199

fixed–movable bridges 198, 199–200, 202,213

in combination designs 203conventional 206hybrid designs 204life-expectancy 300minimum-preparation 200, 202

designs comparison 207temporary 241trial preparations for 119

fixed partial dentures see bridges fixed splints see splints, fixed fixed vs removable prostheses 78–9, 186–95

dental issues 189, 191patient issues

age/sex/occupation 188, 192appearance 187, 188–9, 190, 191, 192attitude 186–8health/confidence 188

fixtures see under implant restorations flexi-wire splints 289, 292full arch mirrors 246fully-adjustable articulators 100function, indicating crowns 7, 9functional cusps 135‘fuse-box’ principle 205

genderand crowns 106fixed vs removable prostheses 188

gingival blanching 269gingival hyperplasia 57gingival margins

in crown planning 113minimum-preparation bridges 242oral hygiene 255retraction 162–6

compressed air 162, 163, 166cord/impregnated cord 164–5, 166electrosurgery 166kaolin pastes 163–4, 166

gingival prostheses 189gingival tissue

and bridges 186and crowns 58

glass ionomer core 40, 41glass ionomer crown 41glass ionomer luting cements 61, 172gold

abutments 271, 272casts 69copings 273, 313crowns 46, 67, 69

checking/adjusting 168metal– ceramic 37preparation for 135as retainers 200wear 305

diaphragm for retention 111framework 273, 275inlays/onlays

bridge repair 314as retainers 200, 205, 213

post crowns 41, 45restorations 21

gold inlays/onlaysinlays/onlays

in metal–ceramic bridges 205posterior teeth 20, 22–5, 27preparation 25

groovesbuccal depth indicator 139reference 134, 138, 139for retention 54, 63, 64, 65, 135

partial crowns 66, 75posterior crowns 68, 73, 75

group functionand crown planning 111occlusion 78, 83, 111

‘half’ crowns 47healing abutments 180Hi-Ceram crowns 29, 31home bleaching 13hot transfer-moulded ceramic crowns 33hybrid bridges 203, 204

life-expectancy 300hydrophilic impression materials 161hypodontia 7, 107, 194, 262hypoplastic conditions indicating crowns 5,

7, 11

implant restorations 182, 261–85abutment transfer jig 274alveolar bone loss 262, 280appearance 280vs bridges 264, 265bridges 275caution with 280completed 270, 272

radiograph of 267, 270components

abutments 271angled 272choice of 283customized 270, 271definition 261design 265, 270, 272healing 267materials 272screw- vs cement-retained 272,

273–4seating 269, 270selection kit 284titanium 266

implants 261, 266definition 261description 265, 278numbers/positions 277, 278, 280placement 281, 282survival 275–7

restorations 272, 275assessment 285cemented 285definition 261placement 285provisional 280–1screw-retained 266

contraindications 264–5

322 Index

cross-section 180date of development 299design 265–75failure 275, 285gingival blanching 269gingival portion 269impressions 284

copings 267, 283, 284impression-taking 268

indications 194–5, 262–4life-expectancy 299maintenance 285multiple, life-expectancy of 300multiple, linked 275osseointegration 261planning 265, 277–81principles 261–2procedures

restorative 283surgical

immediate placement/restoration283

one-stage implants 281stents 279, 280, 281two-stage implants 281–3

radiographs 267, 270, 280shade planning 268single tooth 180, 275, 276

life-expectancy of 300implant-retained prostheses 182

advantages 185use in UK 177

implant-supported bridge 181, 182impression materials 157, 158, 160–1

and gloves 162impression trays

choosing 126making 127–30

impressions 126, 157–62of bridges 250

minimum-preparation 252gingival retraction 162–6implant copings 267, 283, 284localizing 252for splints 295techniques 162

In-Ceram crowns 29, 31, 33incisors

crowns for 4lower incisor preparation 61peg-shaped lateral incisors 4, 19rotated laterals 118

length 116reshaping, temporary 116, 117veneers, preparation for 17

inclination of tooth indicating crowns 7indications

bridges 192–3compared with partial dentures/implants

177–95crowns 3–13

anterior 11, 13appearance 7, 9broken-down teeth 5, 21caries 3, 11combined 7–8function 7, 9hypoplastic conditions 5, 7, 11mechanical problems 9, 11multiple crowns 8posterior 21

root-filled teeth 21support of other restorations 7, 13, 21tooth characteristics 7tooth wear 5–6, 11trauma 4, 5, 8, 11

implant restorations 194–5, 262–4partial dentures 194splints, fixed 287–90

cleft palate 287–9periodontal disease 287retention

additional 289orthodontic 287

trauma 287informed consent 105, 188inhaled crowns 167–8inlays

ceramic 26composite 22, 24gold

posterior teeth 20, 22–5, 27preparation of 25

insertion/cementation of crown 172insertion paths

bridges 223crowns 62, 73, 223

difficult 134intercuspal position (ICP) 79, 80

adjustments 103contacts 82dentures 84occlusal adjustment in 102–3recording 97, 166–7

intermediate splints 290, 292interocclusal space, creating 85, 87intracoronal retainers 211, 230intracoronal splints 293

joint, definition of 197

kaolin pastes for gingival retraction 163–4,166

keratinization under pontics 218, 220

laboratory-made (cast) posts 42, 43, 44laboratory-made provisional bridges 249–50laboratory-made provisional restorations

157laboratory-made temporary bridges 241, 242laboratory-milled cores/crowns 33, 35laboratory temporary crowns 130laser welded connectors 223lateral excursion 77, 78, 82, 83

difficulty with 84lateral guidance 80layered restorations 22life-expectancy 297

anterior crowns 300bridges 186

conventional 298anterior 300large 300posterior 300

minimum-preparation 298–9cantilever bridges 300crowns 298fixed–fixed bridges 300fixed–movable bridges 300hybrid bridges 300implant restorations 299implants

multiple 300single tooth 300

minimum-preparation bridges 300porcelain inlays/onlays 299porcelain veneers 18, 299, 300posterior crowns 300

light-cured acrylic special trays 127–9, 130light-curing box 129loss of retention

conventional bridges 300fixed–fixed bridges 300–11minimum-preparation bridges 300–1solutions 300–1

maintenanceof bridges 185of crowns 172–3

making restorations see construction malocclusions see occlusal problems mandible 83

dysfunction 87, 89–90, 93movements 79–83

border 80lateral 77, 78, 82, 83protrusive 80retrusive 80, 82

manufacture see construction marginal deficiencies 309–10

repairs in situ 312margins of crown

assessment on recall 173and crown design 55–6marginal configurations 53, 54marginal fit 37subgingival vs supragingival 56, 57

Maryland bridges 209, 210mechanical problems indicating crowns 9, 11mercury allergy/toxicity 22metal bridges see cast metal bridges metal–ceramic bridges 204, 251

with all-metal posterior retainers 208combined with gold inlay 205metal framework 250processing temperatures 205provisional 250removal 316

metal–ceramic crowns 32, 37–9advantages/disadvantages 38–9, 45–6construction stages 120, 121design 53, 60, 73, 75groove-retained 46implant-supported 272pin-retained 45, 46and pin-retained cores 51post-retained, one-piece 38, 45posterior 45–6preparation 38preparations 70, 72, 136

anterior vs posterior 37–8bridge-retaining crowns 72occlusal reduction required 135

removal 315, 317metal–ceramic pontics 214, 215metal–ceramic restorations 299metal–composite bridges 250

provisional 250temporary 249

metal crowns see cast metal crowns metal posts 41Metalor Stylepress crowns 31metals, dental 37, 45

Index 323

Michigan splint 89microdontia 107

and crown design 59milled crowns 33, 35minimum-preparation bridges 177, 178,

192–3, 197, 229, 238abutment teeth 199, 242, 244advantages 185advantages/disadvantages 210appointments

first 253second 254

on augmented ridge 191cantilever 201, 213cementation 252–3contraindications 195date of development 299designs comparison 207direct bridges 208fixed–fixed 199fixed–movable 199, 200, 202laboratory stage 253life-expectancy 298–9, 300loss of retention 300–1macro-mechanically retentive (Rochette)

bridges 210occlusal assessment 232overpreparation 242preparations 242, 243as provisional implant restorations 281spring cantilever 197success with 255wax-up for 73for young patients 188

minimum-preparation retainers 197, 204,211, 213

design 245tooth tissue conservation 211, 212

minimum-preparation splints 292missing teeth see replacement of missing

teeth Mitchell’s trimmer 315, 317modified ridge-lap pontics 215, 219movement of teeth 307multiple crowns 8multiple implant retained restorations 299mylar matrix strips 93

Nayyar cores 50, 51, 131nickel chromium 37, 209non-arcon articulators 99–100

occlusal contacts 82checking/adjusting crowns 170

occlusal disharmony 84occlusal forces 200occlusal harmony 84occlusal interference 80, 83–4

detection 89–90in intercuspal position 84non-working-side 82and overeruption 83of restorations 103and temporomandibular dysfunction 87treatment 90

occlusal loading of abutment teeth 226, 230occlusal marking materials 90–1, 102occlusal problems

indicating bridges 189, 191in restorations 310–11

occlusal records 82, 166silicone 92–3, 166

trimming 166, 167wax 166–7

occlusal relationships, maintaining 97–8occlusal stability 85, 86

and missing teeth 183, 184of restorations 103

occlusal stops 85occlusal surface reduction 68occlusal vertical dimension (OVD) 85

bridges restoring 184crown planning 116, 117increased 86loss with erosion 89occlusal records 100for worn teeth 133

occlusal wear/perforation 305occlusion 7, 170–1

adjustments/alteration 7, 93, 97bridges 252in intercuspal position 102–3restorations 103, 238

before cementing 102–3, 170–1techniques 103

‘balancing side’ 83bridges 231, 232, 238–9canine-guided 78, 83, 111clinical/laboratory management 97–8crowns

checking/adjusting 102–3, 170–1planning 110–11preparation design 54–5surface reduction 55

design 78–9examination/analysis 90–7

clinical aids 90–3clinical examination 90

fixed vs removable prostheses 78–9functional approach 77–9group function 78, 83lateral excursion 77, 78, 82, 83measurement 85objectives in crown-/bridge-making 97orthodontic approach 77–8premature contact 84recording 97, 98–100and retainer choice 214of tooth, indicating crowns 7

occupation and choice of restoration 107,184–5, 188

bridge indicated 192oligodontia 107onlays

composite 22gold 22–5, 27

oral hygienebridges 189

instructions 256crowns 108, 109, 119

design 56–7instructions for 172–3on recall 173

orthodontic retention 183osseointegration 261overbite, complete 191overeruption

anticipated 86bridge planning 235, 238and occlusal interference 83, 238and occlusion 97–8and palatal erosion 88

overtrimmed die 169, 170

palatal reduction 38parallel-sided posts 49

smooth/serrated 42, 43, 44, 51threaded 42, 44–5

paralleling abutment teeth 244–8by extra-oral survey 247–8by eye 244, 246–7paralleling devices 248

partial crowns 47, 49fit surfaces 66fracture and retention 67posterior 75as retainers 211rigidity 66

partial dentures 109, 179, 182and bridge planning 233, 234as first alternative 186–7and implant restoration 267indications 194patient anxiety 188for young patients 188

patientadvice to 299attitude

crowns 105fixed vs removable prostheses 187

informed consent 105–6peg-shaped lateral incisors 4, 19, 58periodontal disease

abutment teeth 231, 306indications for fixed splints 287

periodontal splinting 183periodontal tissue

and bridges 186, 189, 235and crowns 186

design 57–9planning 109, 110

permanent splints 290, 292petroleum jelly-zinc oxide cement 157, 252photographs

clinical, in planning 123image manipulation 115–16

pier, definition of 197pin-retained amalgam 20, 21–2, 27, 47–8pin-retained cores 20, 46, 47–8, 49, 130, 131pin-retained crowns 66planning

bridges 229–34abutment teeth assessment 229–31length of span 227, 231occlusion 231, 238–9ridge shape 232whole patient considerations 229

crowns 105–18appearance 124–6clinical modifications 117clinical stages 118–21core, need for 117detailed planning 111–17individual tooth considerations 110–11,

117laboratory stages 118–21material, choice of 117pre-preparation 123preparation 123–4type, choice of 117wax-ups 112–13whole mouth considerations 108–10whole patient considerations 105–8

implant restorations 265, 277–81see also design

324 Index

plastic strips 93polyacrylic veneers 15polycarbonate temporary crown preparations

145, 149polycarboxylate cements 157polyether impressions

material 158, 161mixing machine 159–60, 161technique 162

polymerization contraction 22pontics 214–22

for cantilever bridge 181definition of 197design principles 214effects on ridge 218, 220inflammation around 218, 220materials 222strength of 214, 215surfaces

approximal surfaces 221buccal 221lingual 222occlusal surface 221ridge surface 215–20

cleaning technique 255porcelain

advantages/disadvantages 37chips/fractures, repairs to 302–4polishing instruments 311properties 53repairs in situ 312strength of 29surface defects 311

porcelain bridges see ceramic (porcelain)bridges

porcelain connectors 223–4porcelain crowns see ceramic (porcelain)

crowns porcelain facings, repairing 302porcelain inlays/onlays see ceramic

inlays/onlays porcelain jacket crowns (PJCs) 29–31

advantages/disadvantages 37feldspathic 29fractured, repair of 313strength of 29, 33strengthened 29, 31

porcelain veneers 13, 14–17, 18alternatives to 21cements 172date of development 299life-expectancy 298–9, 300manufacture 29

posts/post systemscast (laboratory-made) 41, 42, 43, 44choosing 44–5and cores 41, 49and crown planning 110for crown retention 35, 39, 41, 51direct/indirect 41impressions 158materials 41preformed 41, 42, 44, 49removal/removal instruments 315in root-filled teeth 51shapes 41–4temporary 156

post hole cutters 148post-retained cores 49, 131post-retained crowns 41

as abutment teeth 231

design 75metal– ceramic 38one-piece 38, 45

posterior crowns 20, 21alternatives to 21–7complete 45–7, 73indications 21life-expectancy 300partial 47, 49, 75preparations 68–9, 70, 135–7

axial reduction 137finishing 137margins 137occlusal reduction 135

posterior restorationschoosing 27–8root-filled teeth 21tooth-coloured 22for weak cusps 22–3, 27

precision-attachment connectors 224precision-attachment partial dentures 179,

182preformed posts/post systems 41, 42, 44, 49preformed temporary crowns 130pressed ceramic crowns 31, 34Procera 33

cores 35crowns 35, 152

prospective surveys 297prostheses contraindicated 195provisional bridges 107, 155–6, 234

fibre-reinforced 39laboratory construction 249–50long-term 156–7metal–composite 250see also temporary bridges

provisional crowns 48, 87, 155–6cementation 157fibre-reinforced 39long-term 156–7vs temporary crowns 155–7for worn teeth 133see also temporary crowns

provisional restorations 39, 97, 103laboratory-made 157see also temporary restorations

pulp tissueabutment teeth, problems in 306bridges damaging 185and crown design 59and crown planning 110

putty and wash impressions 162PVC slips 150, 155

quartz-fibre posts 41, 43quartz-fibre-reinforced composite splint 289

radiographyhypodontia 279implant restorations 267, 270, 280, 285on recall 173

recall of patients 173reinforced porcelain crowns 33removable bridges 177, 179, 204removable prostheses see fixed vs

removable prostheses repairs

crowns 173porcelain chips/fractures 302–4

replacement of missing teethadvantages 183–5

decisions 182–3disadvantages 185

resin-based/resin-modified cements 61,171–2, 254

resin-bonded ceramic crowns 19, 21, 33, 36resin-retained bridges 210resorption and pontic modification 220restorations 182

dates of development 299failing

adjustments in situ 311–12assessing seriousness 311repairs in situ 312

marginal deficiencies 309defects 309–10negative ledge 309positive ledge (overhang) 309repairs in situ 312

occlusal considerations 77polishing/grinding 311–12shape/colour problems 310

retainers 211–14choosing 212–14definition of 197major/minor 211marginal deficiency repairs 309–10materials 212‘metal shine-through’ 212, 213, 230

retentioncrowns 27, 59–67

‘adhesive’ 59, 61and casting difficulties 67–8checking/adjusting 169–70crown fracture/distortion 66–7crown lengthening for 118crowns 61–7, 72displacing forces 60, 62–6mechanical 59retentive features 64tooth tissue fracture/distortion 66trial wax-ups 112

and fixed splints 289orthodontic

indications for fixed splints 287and missing teeth 184

and retainer choice 212, 214retrospective surveys 297retruded arc of closure/movement 79retruded contact position (RCP) 79, 80

contacts 82dentures 84difficulty with 84occlusal interference 83recording 97, 166–7

reversible hydrocolloid impression materials157, 158, 160, 161, 295

ridge augmentation 189, 190–1ridge-lap pontics 215, 217, 219ridge shape and bridges 232ridge surface of pontic 215–18Rochette bridge 208, 234, 281Rochette splint 292root-filled teeth 40

as abutment teeth 231anterior crowns 39–40composite restoration 40and crown planning 110and pin-retained cores 51posts in 51restoration 21

root length and crown planning 111

Index 325

saddle, definition of 197saddle pontics 218safety precautions at try-in stage 167–8self-curing acrylic trays 127–8, 130semi-adjustable articulators 94–6, 98‘seven-eighths’ crowns 47sex see gender shade

checking/adjusting 170planning 116–17, 124–6

implant restorations 268shade guide 124–6shade maps 126shade-taking device 125shape see under tooth/teeth shimstock 91, 93, 103‘short root syndrome’ 262short-term splints 290, 292shortened dental arch 182silicone impressions

materials 158occlusal record 92–3techniques 162

silicone occlusal records 166silicone putty matrix 139, 283, 284silicone ‘seating’ cement 252sleeve crowns 312, 313slide hammers 315, 318smoking 264social development and dental appearance

11social history and crowns 107soldered connectors 223

failures 304span, definition of 197speech and missing teeth 183splinting 183, 189

in combination with bridges 202, 204crossarch 183, 228, 238periodontal 183

splints, fixed 287–95advantages 292anterior

appearance 293–4selecting 294

cementation 295indications 287–90

cleft palate 287–9periodontal disease 287retention

additional 289orthodontic 287

trauma 287intermediate 290, 292permanent 290, 292

clinical techniques 294–5vs removable 290short-term 290, 292temporary 295

sport players, crowns for 106spring cantilever bridges 197, 198, 203

minimum-preparation 197staining of teeth 7

banding 10and bleaching 11

correction 9tetracycline 10, 11, 12vital 12

stainless-steel temporary crowns 145, 154,155

stents, surgical 279, 280, 281Straumann implant 266strengthened ceramic crowns 29, 31superfloss 255, 256, 257surgical ridge augmentation 189, 190–1surveys

prospective 297retrospective 297

swallowed crowns 168

Tanner appliance 89tapered posts

smooth/serrated 43–4, 51threaded 42, 44, 45

temporary bridges 234chair-side construction 248–9materials 248see also provisional bridges

temporary crowns 155cementation 157planning 130post-retained 155, 156preformed 145, 154–5vs provisional crowns/bridges 155–7removing 167see also provisional crowns

temporary posts 156temporary restorations 97–8

see also provisional restorations temporomandibular dysfunction (TMD) 87,

89–90terminal hinge axis (THA) 79, 80tetracycline staining 10, 11, 12‘three-quarter’ crowns 47tilted teeth

in crown design 54–5crown preparation 71

titanium crowns 33titanium implants 261titanium posts 41tooth-grinding see bruxism tooth/teeth

inclination 7occlusion see occlusion replacement see replacement of missing

teeth shape/size

indicating crowns 6, 7peg-shaped lateral incisors 4, 19

supporting reconstructions 109–10tilted 54–5value of, in crown planning 110wear

and crown design 59future, and crown design 55indicating crowns 4, 5–6, 11

management 6, 130–3tooth tissue

conservationand bridge design 228and retainers 211, 212

destruction/removalin bridge design 227in bridge preparation 185in crown preparation 37, 39, 55, 56,

134central incisors 71

and retention 66transmucosal abutments (TMAs) 180–1, 182

see also abutments, implant trauma 4, 21

indicating crowns 5, 11indicating fixed splints 287

ulceration around pontics 218, 220unit, definition of 197

veneers 15appearance 13, 14–16cement penetration 17composite 13–14, 18, 21cost 18failing 15, 18life-expectancy 18, 299physical properties 18, 21polyacrylic 15porcelain 13, 14–17, 18, 115

alternatives to 21preparation 15, 16, 17reversibility 13, 18

vital teeth, anterior crowns on 73, 75

wash-through pontics 215, 216, 219buccal surface 222cleansability 256

wax-added technique 102, 103wax carving 102, 103wax occlusal records 91, 133, 166–7

relining 167wax-ups

‘cheating’ 113diagnostic (trial) 112–13, 124

bridges 233constructing temporary restorations-

114, 115implant restorations 280for temporary crowns 151

wear see tooth/teeth, wear wind instrument players 107, 184–5, 188wing, definition of 197wire/acrylic splint 290wire/composite splints 290, 292wire loop for bridge removal 318working impressions see impressions

zinc oxide-petroleum jelly cement 157, 252zinc phosphate cements 157, 171, 255, 295

removal 314zirconia 33, 47, 272

326 Index

Planning and Making CrPlanning and Making Crowns and Bridges - Smith. 4th Ed.2007owns and Bridges -...

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Transcript of Planning and Making CrPlanning and Making Crowns and Bridges - Smith. 4th Ed.2007owns and Bridges -...