ALL-CERAMIC FIXED PARTIAL DENTURES Studies on aluminum oxide- and zirconium dioxide-based ceramic systems Per Vult von SteyernISBN 91-628-6444-0

Swedish Dental Journal Supplement 173, 2005

ALL-CERAMIC FIXED PARTIAL DENTURES

Studies on aluminum oxide- and zirconium dioxide-based ceramic systems

Per Vult von Steyern, Odont lic, DDS

Department of Prosthetic Dentistry Faculty of Odontology

Malmö University Sweden, 2005

Per Vult von Steyern, Odont lic, DDS Department of Prosthetic DentistryMalmö University SE-205 06 Malmö SwedenTel. +46 40 6658583 Fax: +46 40 6658503 E-mail: per.vult@od.mah.se

Cover: Machu Picchu, Peru.Photo: Per Vult von Steyern

ISBN 91-628-6444-0

Swedish Dental Journal Supplement 173, 2005 ISSN 0348-6672

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To the endless ocean path

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CONTENTS

Preface 7

Abstract 9

Swedish summary (Populärvetenskaplig sammanfattning) 11

Introduction 13

Aims 25

Materials and Methods 27

Results 41

Discussion 51

Conclusions 61

Acknowledgements 63

References 65

Appendix:

Paper I

Paper II

Paper III

Paper IV

Paper V

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PREFACE

This thesis is based on the following papers, which will be referred to by their Roman numerals:

I. Vult von Steyern P, Al-Ansari A, White K, Nilner K, Dérand T. Fracture Strength of In-Ceram All-Ceramic Bridges in Relation to Cervical Shape and Try-in Procedure. An In-Vitro Study. Eur J Prosthodont Rest Dent 2000; 4: 153–158.

II. Vult von Steyern P, Jönsson O, Nilner K. Five-year Evaluation of Posterior All-Ceramic Three-Unit (In-Ceram ) Fixed Partial Dentures. Int J Prosthodont 2001;14: 379–384.

III. Vult von Steyern P, Ebbesson S, Holmgren J, Haag P, Nilner K. Fracture Strength of Two Oxide Ceramic Crown Systems After Cyclic Preload and Thermocycling. An In-Vitro Study. Submitted.

IV. Vult von Steyern P, Carlson P, Nilner K. All-ceramic Fixed Partial Dentures Designed According to the DC-Zirkon Technique. A 2-Year Clinical Study. J Oral Rehabil 2005; 32: 180-187.

V. Vult von Steyern P, Kokubo Y, Nilner K. Use of abutment-teeth vs. dental implants to support all-ceramic fixed partial dentures: An in-vitro study on fracture strength. Submitted.

The papers have been reproduced with the kind permission of the publishers.

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You blow, fresh wind of the ocean from the southwest and sweetly caress the sailor's cheek best of all the winds

Toward sea and storms you boldly course toward sea and storms be then on guard!

On an endless ocean path, life is free and cares are forgotten When the green-white sea sings its high freedom song

Swell, great lovely sail, swell on the push of the wind with resounding delight towards mountainous waves in the moment's utmost joy

Towards sea and storms...

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ABSTRACT

Background: The development of refined, tougher, and stronger ceramic core

materials in recent years has led to the wider use of new, strong all-ceramic systems

based on oxide ceramics. Results from in-vitro studies investigating the use of oxide

ceramics in shorter all-ceramic fixed partial dentures (FPDs) have been positive, but

clinical studies and additional in-vitro studies are needed to confirm the advisability of

such procedures. Aims: One aim of this thesis was to investigate whether alumina-

based and zirconia-based material systems are adequate for use in shorter ( five-

unit) FPDs and to evaluate the clinical results. Additional aims were to investigate how

to achieve optimal fracture strength in an all-ceramic FPD by varying the try-in

procedure, the cervical shape of the abutments, and the support of the FPD (abutment

teeth or dental implants). The final aim was to compare the strength of a zirconia

material system with that of an alumina equivalent with known long-term clinical

performance. Materials and Methods: Two clinical studies investigating one alumina-

based and one zirconia-based material system were performed. Twenty posterior,

three-unit FPDs (glass-infiltrated alumina) were followed for 5 years and 20 three–five-

unit FPDs (HIP zirconia) for 2 years. Long-term follow-ups were made after 11±1

(glass-infiltrated alumina) and 3 years (HIP zirconia). In three in-vitro studies, the

following variables were investigated: 1a) the flexural strength of porcelain specimens

depending on whether they were exposed to saliva before the glaze firing (n=20) or

first after the glaze firing (n=20), 1b) the fracture strength of three-unit all-ceramic

FPDs (glass-infiltrated alumina) supported by abutments prepared with cervical

shoulder preparations (n=9) and abutments with cervical chamfer preparations (n=9),

2) the fracture strength of crowns (n=30) made of a zirconia material system (densely

sintered zirconia) and of crowns (n=30) of an alumina material system (densely

sintered alumina) that had undergone three different pre-treatment modalities (water

storage only; water storage and cyclic pre-loading; water storage, cyclic pre-loading,

and thermocycling), 3) the fracture strength of all-ceramic FPDs (densely sintered

alumina) supported by simulated teeth (n=12) or by dental implants (n=12). Results:

The success rate of the clinical alumina study was 90% after 5 years. Six (±1) years

later (after a total of 11 ± 1 years), the success/survival rate was 65%. In the second

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clinical study, the success rates of the 2- and 3-year follow-ups were 100%. In the

three in-vitro studies, the following results were found: 1a) the mean flexural strength

of the specimens in the group that was exposed to saliva first after glazing was

significantly higher (P < 0.001) than that of the specimens in the group that was

exposed to saliva before glazing, 1b) the FPDs luted on shoulder preparations resisted

higher loads than the FPDs luted on chamfer preparations (P = 0.051), 2) total

fractures were more frequent in the alumina than in the zirconia group (P < 0.001), 3)

FPDs loaded on implants resisted higher loads (mean = 604 N, SD=184 N ) than

FPDs loaded on abutment teeth (mean= 378 N, SD=152 N, P = 0.003).

Conclusions: This thesis justifies the use of shorter alumina- ( three-unit) and

zirconia-based ( five-unit) FPDs as the clinical results are acceptable. The clinical

performance of alumina is, however, not as good as that of comparable high-gold alloy

based porcelain-fused-to-metal FPDs concerning fracture resistance. Within the

limitations of the in-vitro studies: Saliva exposure of porcelain before glaze firing

should be avoided to optimize the strength of the porcelain. Shoulder preparations can

be beneficial for the strength of all-ceramic FPDs compared to chamfer preparations,

as can support by dental implants compared to abutment teeth. The fracture mode of

alumina crowns (total fractures) differs from that of zirconia crowns (veneer fractures),

suggesting that the zirconia core is stronger than the alumina core.

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HELKERAMISKA TANDBROAR

Keramiska material har länge använts inom tandvården för framställning av

tandersättningar. Keramer har många goda egenskaper som gör dem särskilt lämpliga

att använda i munnen. Viktigast är kanske att de är biokompatibla, det vill säga att de

inte skadar omgivande vävnader, att de inte ger upphov till allergier eller utgör någon

risk för förgiftning och att de inte bryts ner i den miljö i vilken de är tänkta att fungera.

Särskilt intressanta bland keramerna är porslin som förutom nämnda fördelar har

optiska egenskaper som liknar tandemaljens. Detta har bidragit till att dentalt porslin

sedan många år används för att ge olika typer av tandersättningar ett ytskikt med

tandliknade utseende.

Broar är fastsittande tandersättningar som används när man behöver ersätta förlorade

tänder. Det idag mest använda materialet för framställning av tandbroar är så kallad

"metallkeramik", en kombination av en metallegering, ofta högädel, och ett porslin.

Den viktigaste rollen som metallegeringen spelar är att förstärka porslinet så att det

motstår de belastningar som förekommer i munnen.

Metallegeringar har emellertid flera nackdelar. Dels finns det en risk att patienten är

allergisk mot någon av legeringsmetallerna om de läcker ut. Metallers optiska

egenskaper begränsar dessutom möjligheterna att få tandersättningarna så tandlika

som man många gånger önskar, vilket försvårar förutsättningarna att framställa

tandersättningar med gott estetiskt resultat.

Sedan mer än 40 år har forskning pågått för att utveckla helkeramiska material som

har egenskaper som tillåter framställning av broar utan metallunderstöd. Olika metoder

och material har testats, men resultaten har många gånger varit nedslående; broar har

spruckit efter en allt för kort tid i funktion. Inte förrän 1985 kom ett material som

verkade kunna fungera och som hade teoretiska hållfasthetsvärden långt över de

traditionella keramernas. Nittonhundranittiotvå presenterades flera laboratoriestudier

med samma slutsats - nu fanns ett material som verkade vara tillräckligt starkt för att

kunna användas till broframställning; glasinfiltrerad aluminiumoxid. Man betonade

dock att kliniska långtidsstudier behövdes innan materialet kunde rekommenderas för

allmänt bruk.

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Föreliggande arbete består av 5 delarbeten, samtliga rörande helkeramiska broar eller

material som används i broarna. Delarbete 1, 3 och 5 är laboratoriestudier medan

delarbete 2 och 4 är kliniska studier. I delarbete 1 undersöktes dels vilken

inprovningsmetod som ger högst hållfasthet hos porslin (göra bron helt färdig på

laboratoriet alternativt att prova den i munnen som halvfabrikat innan bron färdigställs),

dels vilken typ av tillslipning man bör göra av tänderna som skall bära bron för att få

högst brohållfasthet ( s.k. skulderpreparation alternativt chamferpreparation

[hålkälsprofil] ). Delarbete 2 är en klinisk 5-årsuppföljning av broar framställda i

glasinfiltrerad aluminiumoxid och delarbete 3 jämför hållfastheten hos 2 olika

bromaterial (aluminiumoxid och zirkoniumdioxid). Delarbete 4 är en klinisk 2-

årsuppföljning av broar framställda i zirkoniumdioxid och slutligen delarbete 5 jämför

helkeramiska broars hållfasthet beroende på om de är förankrade med tänder eller

tandimplantat. Utöver dessa delarbeten finns en sammanställning av kliniska resultat

från delarbete 2 och 4 efter 11±1 år (glasinfiltrerad aluminiumoxid) respektive 3 år

(zirkoniumdioxid).

Slutsatserna från avhandlingen är att små broar baserade på aluminiumoxid kan

framställas med acceptabelt kliniskt resultat, men att hållfastheten över tid inte är lika

bra som för motsvarande broar i metallkeramik. Zirkoniumdioxidbaserade broar med

storlek upp till 5 tänder uppvisar lyckandefrekvenser motsvarande metallkeramik inom

ramen för den tid föreliggande studie pågått. Laboratorieavsnitten i avhandlingen ger

följande slutsatser: Inprovning av porslin i mun på patient bör inte göras innan

glansbränning av porslinet gjorts. Vidare bör skulderpreparation väljas framför

chamferpreparation för att motstå höga belastningar i bettet. Zirkoniumdioxid är

starkare än aluminiumoxid, särskilt efter förbelastning, men ytporslinet på en

zirkoniumoxidkrona spricker vid ungefär samma belastning som en aluminiuoxidkrona

spricker genom båda skikten (både kärna och ytporslin). Slutligen antyder resultaten i

delarbete 5 att helkeramiska broar bör kunna göras på implantat med minst lika bra

hållfasthet som på naturliga tänder. Resultaten från laboratoriestudierna behöver

emellertid bekräftas i kliniska studier innan metoden kan rekommenderas för allmänt

bruk.

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INTRODUCTION

Ceramics in dentistry

The word “ceramics” is derived from keramikos, which is the ancient Greek word for

"earthen", and most commonly used for inorganic materials consisting of one or more

metals combined with a non-metallic element, usually oxygen20. The present high

interest in the use of dental ceramics is illustrated by the rising demand for ceramic

crowns: an increase of 50% every 4 years in recent years61. The optical properties of

dental ceramics in general are considered to be similar to those of the natural tooth;

this makes them suitable for reconstructions designed to fulfill high esthetic demands.

There are, however, dental ceramics that do not possess such optical properties but

are used for other reasons—strength being the main one. Hence, in discussions on

which material produces the most pleasing result esthetically and gives the best

impression of vitality, the qualities of dental porcelains are not matched by those of

any other material. The expression dental ceramics is more a general description of a

large group of materials within which the dental porcelains are but one subgroup.

Dental porcelain

The word porcelain is derived from porcellana, which is the Italian name for a small

seashell. Traditionally, it is used for strong, vitreous ceramic materials consisting of a

continuous glass matrix in which different fractions of crystals and particles are

interspersed. The glass phase is predominantly a noncrystalline, amorphous, fairly

transparent material produced by fusion, which forms silica networks as structural

units and a matrix consisting of potassium and/or soda feldspar (potassium feldspar;

K2O-Al2O3-6SiO2)65.

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Porcelain has been used in dentistry for more than 200 years and was first introduced

in the eighteenth century in an all-porcelain denture. In 1903 a procedure for making

porcelain crowns (“porcelain-jacket crowns”) was described, an achievement that

unfortunately encountered problems because the crowns easily fractured30,46. The

development of stronger porcelains and improved firing techniques, particularly during

the 1960s, made it possible to use porcelain restorations in the anterior regions with

acceptable success. Today, however, the inherently low tensile strength of dental

porcelains still does not allow their use in high stress-bearing applications without the

use of a high strength support – or by using bonding technique as a strengthening

mechanism for the porcelain25.

The primary reason for using porcelains in dentistry is their superior esthetic

appearance, which is a result of the light absorbing and light scattering behavior of the

material and its potential to reproduce the depth of translucence, the color, and the

texture of natural teeth. Porcelain is chemically stable, has good wear resistance and

color stability in the oral environment, and is relatively affordable compared to precious

alloys. Thermal expansion and conductivity are similar to those of enamel and dentine,

resulting in a low risk of temperature sensitivity and marginal percolation27,37,59.

Furthermore, there is no known risk of developing adverse reactions to the porcelains,

as has been described for metal alloys6. Glazed porcelain is, moreover, the only

restorative material from which bacterial plaque can be easily removed37.

There are, however, drawbacks to the use of dental porcelains. Despite high bonding

forces between the atoms, the material cannot withstand deformations of more than

0.1% without fracturing. This brittleness is due to the nature of the strong covalent

bonds that do not allow plastic deformation when subjected to tensile or shear forces.

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The atoms in ceramics cannot, in contrast to metal, which has relatively low atomic

bond forces, slide along the atomic planes when the applied load exceeds the elastic

capacity of the material. Such loads result in a brittle fracture originating from the point

of the highest concentration of stress, which often is at the location of a microstructural

flaw37.

Porcelain components and specimens have a large variation in types and sizes of pre-

existing flaws that act as starting points in the formation of cracks. Such flaws could be

areas of porosity, agglomerates, inclusions, and large-grained zones, which can all be

processing related. Machining and grinding determine the size and number of surface

flaws. Finally, during firing, formation of weak, secondary grain boundary phases can

occur as well as microcracking associated with phase transformations or differences in

granular contraction during cooling14,35.

Under continuous loading, cracks propagate and insidiously weaken the porcelain

restoration, a phenomenon described as slow crack growth. If a loading cycle exceeds

the mechanical capacity of the remaining sound portion of the material, catastrophic

failure will occur56. Thus, the major problem in designing porcelain restorations resides

in the unpredictable strength of the material itself. Differences in the shape, size, and

distribution of flaws and cracks in dental porcelain make it difficult to predict the

longevity of one porcelain restoration based on experience with other, equivalent

restorations used under similar conditions and often lead to unexpected

failure33,47,48,49.

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Glaze and polishing

Different kinds of surface treatments have been investigated to find the optimal

procedure for reducing surface flaws. A glaze layer can be used to fill in the flaws; or

polishing can be used to reduce their depth. Studies have reported that highly polished

porcelain can be even stronger than glazed equivalents. The improved strength might

be attributable to the elimination of surface flaws and to the development of residual

compressive stresses in the porcelain surface21.

To prolong the longevity of a porcelain restoration, it is important to address the

question of how to achieve microstructural refined dental porcelain to reduce—or

preferably avoid—the size and number of cracks and flaws33. Hence, it is essential to

optimize the production techniques and thereby improve control of the quality of the

ceramic restorations.

Strengthening dental porcelain

The problem of improving the strength of porcelain restorations so that they will be

able to withstand the loads they will be subjected to during service can be solved in

two ways. One is to make the porcelain itself stronger and tougher; the other is to

provide the porcelain with a stronger substructure that supports the porcelain. At the

present time, the mechanical strength of porcelain ( 120 MPa) and glass ceramics

(~180 MPa) is too low for use in high stress applications without some kind of

supportive substructure25. Such supporting structures can entail:

1 – Etching and bonding with methacrylate-based cement.

2 – Use of high strength (oxide) ceramic substructures.

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In the first case above, etching the cementation surface and coating it with a polymer

can substantially improve the strength of porcelain and glass ceramics. The

strengthening effect may be caused by the elimination—or blunting—of cracks or by

reduced stress corrosion through a reduction in the transport of water to the crack tips

by the polymer coatings37. Another possible explanation is that the bond between

porcelain and enamel/dentine improves strength by reducing tensile forces on the

cementation surface of the restoration61.

In the second case above, the strengthening mechanism of high strength ceramic

substructures supporting dental porcelain is similar to that of porcelain-fused-to-metal

(PFM) where strength is added by a metal substructure with a thermal coefficient

compatible with that of porcelain. In all-ceramic, high strength (oxide) core materials,

the shape of the substructures serves to support the weaker veneering porcelain.

During loading of a laminate crown where the porcelain is supported by a strong core,

the resulting forces in the veneer will be compressive rather than shearing or tension

stresses. As ceramics in general can withstand higher compressive loads than

shearing and tension ones, this will be beneficial for the load-bearing capacity of the

crowns.

The dense core blunts flaws and cracks in the critical inner surface of the crown,

hence preventing time-dependent slow crack growth emanating from this area.

Another mechanism could be that ceramic substructures prevent water from getting

access to the crack tip and thus indirectly decrease the risk of stress corrosion in this

surface area. The stiff, strong inner construction resists radial expansion of the dentine

core and wedging of the crown during loading and prevents tension on the inner

surface of the crown.

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Oxide ceramics

The quest for all-ceramic materials with properties that would enable their use in fixed

partial dentures (FPDs) led to the development of many new materials and processing

techniques in the last decade. Whereas traditional dental ceramics primarily comprised

a glass matrix with a crystalline phase as filler, newly developed ceramic materials are

primarily crystalline in nature36. These new materials—often referred to as oxide

ceramics—are based on crystalline alumina, magnesia, or zirconia. The use of new

processing techniques in combination with oxide ceramics has made it possible to

fabricate FPD frameworks with a flexural strength and fracture toughness that are

considerably higher than those of the ceramics that have been previously used, thus

increasing the material’s resistance to crack propagation22,43,52,60. The strongest and

toughest oxide ceramics used today are based on aluminum oxide (alumina), and on

the latest material contribution—zirconium dioxide (zirconia)1.

Alumina

Alumina has been used to increase the strength of dental porcelains for more than 4

decades37. Alumina-based core ceramics consisting of a partially sintered porous

alumina structure infiltrated by molten glass are available in two forms:

1 - As slip powder, used dispersed in water to build up crown copings and FPD

cores.

2 - As dry pressed material processed for milling FPD frameworks and crown

copings. This form can be used with several milling systems.

In both instances, lanthanum glass is used after the final shaping of the frameworks

and copings to infiltrate the porous alumina structure. Finally, the substructures are

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veneered with dental porcelain to create the appearance of a natural tooth. Studies

have shown that glass-infiltrated alumina has a flexural strength up to four times

greater than that of conventional ceramics. The authors concluded that it seemed

possible to make restorations with all-ceramic FPDs in cases not only of anterior but

also posterior tooth loss. They emphasized, however, that long-term follow-up studies

were necessary to establish the advisability of such a procedure22,40,43,52,60,64.

Another all-ceramic system based on alumina employs a technique where high purity

alumina crown copings or FPD cores are fabricated using computer-aided

design/computer-aided manufacturing (CAD/CAM) techniques2. Subsequent to CAM,

the alumina substructures are densely sintered and veneered with dental porcelain.

Clinical studies have indicated that such alumina-based crowns may be used for

crowns in all locations of the oral cavity41,42. The system includes a technique for

producing all-ceramic FPDs. This technique combines alumina copings with an

alumina pontic that is joined to the copings using a specially formulated connecting

and fusing material32.

Zirconia

The dental ceramic with the best mechanical properties is yttrium-stabilized zirconium

dioxide17,59. Zirconia is well known as an orthopedic implant material and has been

used in hip surgery for many years9. By adding a small amount of Y2O3 to ZrO2, it is

possible to stabilize the ceramic in a tetragonal phase that normally is unstable at

room temperature. The energy that arises around crack tips and sharp corners when

loading a ceramic specimen above a certain level transforms metastable tetragonal

grains into monoclinic ones that are larger, thus sealing the cracks and stopping

further propagation11. This mechanism is based on martensitic phase conversion, as

20

found in steel, giving the material a beneficial toughening property that could not be

found in any other dental ceramic19. Several studies have indicated that flexural

strength values of 1200 MPa and fracture toughness values of 9 MPa m½, which are

possible with zirconia and substantially higher than for other ceramics makes this

material useful for highly loaded, all-ceramic restorations. Hence, suggestions have

been made that zirconia could also be a viable alternative to metal in reconstructive

dentistry, especially for crowns in the molar region and FPDs 17,19.

Degradation of dental ceramics

Stress corrosion

Even though dental ceramics are chemically stable, they are still susceptible to

chemical corrosion. It has been described that porcelain undergoes an abrupt

transition of damage mode and strength degradation after multi-cyclic loads compared

to static loading tests26. When water is present, stress corrosion enhances further

crack propagation62. When tension periodically occurs at the crack tip as a result of

load cycles, the damage is increased in the presence of water. Oxygen atoms that are

debonded when the interatomic distance increases during tension in the crack tip area

are blocked as a result of hydration and are then unable to re-establish the previous

bond when expansion ceases during the unloaded phase. A 27% decrease in fracture

strength has been reported for aluminous and feldspathic porcelains tested in water

compared to specimens tested in air53. In the clinical situation, fatigue is important

because dental restorations are subjected to small alternating forces during

mastication. Water in the saliva plays an important role as a catalyst for this fatiguing

mechanism31,49,56.

21

Moisture plays a vital part in the time-dependent reduction of the strength of dental

porcelain38. The presence of water and organic molecules in the oral cavity are, of

course inevitable, as they are always present in the saliva. A frequently used

technique for occlusal adjustments of tooth replacements is to make the adjustments

in raw porcelain, before the glaze firing. This try-in is made in the mouth. Whether

saliva molecules, if present in the subsequent firing process, could react with the

porcelain has, however, not been investigated. If so, and if these reactions affect the

porcelain, it would be advisable to postpone the try-in stage in the mouth until after the

final glaze firing.

Support gained from the FPD abutments

Cervical shaping

The brittle nature of ceramics makes the fracture resistance of all-ceramic fixed partial

dentures highly dependent on a solid support and on reduced strain in the beam of the

prosthesis. Several authors have discussed the influence of cervical shaping on the

fracture resistance of all-ceramic crowns. Today’s knowledge thus indicates that all-

ceramic crowns luted with non-adhesive luting techniques should be designed with a

cervical shoulder preparation to resist high loads18,55. Whether this is applicable to all-

ceramic FPDs, however, has not yet been investigated.

Abutments

As teeth are lost due to caries or periodontal disease, implants can be used to replace

the natural abutments. The biomechanical support gained from implants differs,

however, from the support provided by natural teeth because the implants are directly

connected to the bone without any other intermediate tissue, a biomechanical situation

22

similar to the one of tooth ankylosis where no periodontal ligaments exists. The

periodontal membrane of a tooth acts as a shock absorber, has sensory functions, and

allows minor tooth movement13.

If supporting bone has been lost due to periodontal disease, the capacity of the

involved teeth to serve as abutments is lower since their amplitude of movement

increases when loaded54. The direction and magnitude of these movements varies

considerably depending on among other things the anatomy of the root, remaining

bone height, bone density and other periodontal conditions, and it has been concluded

that the tensile stress in an FPD can reach critical values when abutment teeth with

excessive loss of bone support are loaded4,34. Because ceramics are brittle as

mentioned above, they cannot withstand deformations. Hence, when planning an all-

ceramic FPD, it is essential to evaluate abutment support since the resistance of all-

ceramic constructions to fracture depends on the stability of the support to reduce

strain in the beam of the prosthesis. One question that remains to be answered is

whether all-ceramic FPDs benefit from implant support when the prosthesis is loaded

on implants compared to natural teeth.

Hypotheses

The following hypotheses are based on the above:

Dental porcelain that is glazed prior to saliva exposure will resist higher loads

than equivalents that have been subjected to saliva prior to the final firing.

All-ceramic FPDs supported by abutments cut with circumferential shoulders

will resist higher loads than equivalents that are supported by abutments cut

with circumferential chamfers.

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Oxide ceramics veneered with dental porcelain can be used for FPDs with

extensions up to five units if based on zirconia and three units if based on

alumina.

Zirconia-based reconstructions veneered with dental porcelain and subjected to

fatiguing and stress-corrosion can resist higher loads than alumina-based

equivalents.

All-ceramic FPDs supported by dental implants will resist higher loads than

equivalents that are supported by natural teeth.

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25

AIMS

The aims of the study were:

To compare the flexural strength of a feldspathic porcelain exposed to saliva

before the final firing with an equivalent exposed first after the final firing.

To determine how the cervical shape of the preparations influences the fracture

strength of shorter all-ceramic FPDs made of a glass-infiltrated slip-cast

alumina-based material system.

To investigate in a long-term perspective whether the strength of a glass-

infiltrated slip-cast alumina-based material system is sufficient for use in

posterior three-unit FPDs when a standardized protocol regarding preparation

technique, FPD design, and choice of cement is adopted.

To evaluate and compare the strength of a zirconia material system for crowns

and FPDs with an alumina material system with known long-term clinical

performance.

To investigate whether the properties of a zirconia-based material system is

adequate for use in three–five-unit FPDs and to evaluate the clinical results.

To compare in an in-vitro study the fracture strength of all-ceramic fixed partial

dentures supported by simulated teeth with the same supported by dental

implants.

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MATERIALS AND METHODS

Table 1. Materials and methods; summary of studies I–V

Paper

Study design

I a I b II III IV V

Type of study In-vitro In-vitro In-vivo In-vitro In-vivo In-vitro

Core material - Alumina Alumina Alumina/zirconia Zirconia AluminaVeneer material

Porcelain Porcelain Porcelain Porcelain Porcelain Porcelain

Glaze firing / saliva

exposure

Before / after

- After - After -

FPD units - 3 3 1 3-5 3Abutments - Duralay® Teeth Duralay® Teeth Duralay®/implantsAbutmentposition

- End End End End End

Connector Ø - 3 mm 3 mm - 3 / 4 mm 2 x 3 mm** FPD position - Posterior Posterior - All mouth Posterior

Follow-up - - 5 years - 2 years -Extendedfollow-up

- - 11 years ±1

- 3 years -

Type of specimen

Porcelainrectangular

bars

Three-unit FPDs - Norm crowns - Three-unit FPDs

Cervical shape

- Shoulder/chamfer Shoulder Chamfer Shoulder Chamfer

Preloading 300 N

10.000 cycles

- Yes - Yes / no - Yes*

Water exposure

Human saliva

Yes Human saliva

Yes Human saliva

Yes

Thermocycling No No - Yes / no - No

*100 N ** According to the manufacturer’s instruction

The different materials and methods used in this thesis are summarized in this section. For details (including manufacturers´ details), please see the Materials and Methods sections in the individual papers.

Fracture strength of a veneering porcelain in relation to try-inprocedure. (I a)

In the first part of the first in-vitro study (I a) in this thesis, 40 rectangular bar porcelain

specimens with standardized dimensions and a small projection on one side were

fabricated (Fig. 1). After firing, the luster of the specimens was removed using a white

stone. The specimens were subsequently randomly divided into two groups of 20 and

subjected to two different treatment regimens, simulating different protocols of occlusal

adjustment in the mouth, as described in Table 2.

A three-point flexural test was executed subsequent to pre-treatment to evaluate the

fracture resistance of the specimen. For this purpose, a test rig was used with the

porcelain specimen resting on two metal stainless steel rods with a diameter of 1.5

mm and a span length of 7 mm. The unground surface of the specimen was loaded

with another 1.5-mm stainless steel rod, centered between the other two rods, until

fracture occurred. The crosshead speed of the load was 0.255 mm min-1 (Fig. 2).

Finally, flexural strength in the two groups was compared. Statistical differences were

calculated using Student's t-test.

The load was registered and the flexural strength was defined as:

28

22

3

hb

lFfs

where fs is the flexural strength; F the load in Newtons; and l the length, b the breadth, and h the height in mm.

29

Figure 1. The porcelain specimens.

A = 1.7 mmB = 15.3 mmC = 3.2 mmD = The projectionE = After removal of the projection

Try-in stage

Group 1 n=20

The specimenswere stored in human salivafor 15 minutes

The projectionwas removed to

simulateocclusal

adjustments*

The specimenswere stored in human salivafor 30 minutes

The specimenswere cleaned

withElma®clean**

The specimenswere

autoglazed in the final firing

Group 2 n=20

The specimenswere

autoglazed in the final firing

The projectionwas removed to

simulateocclusal

adjustments*

The specimenswere stored in human salivafor 45 minutes

The specimenswere

polished***

Try-in stage

Figure 2.

Three-point flexural test.

Cross section of bulk material beneath the ground surface

The solid support of the test machine

Table 2. *The projection was removed with a fine diamond burr. **The specimens were

utrasonically cleaned in the Elma Transonic T310 ultrasonic bath using Elma clean 10'

cleaning detergent. ***The specimens were polished using a diamond-impregnated wheel.

30

Fracture strength of all-ceramic (alumina) FPDs in relation to cervical shape. (I b)

Eighteen posterior, three-unit In-Ceram Alumina (slip-cast) FPDs with end abutments

were fabricated. Nine of the FPDs were made on the preparations with 1.0-mm wide

90° shoulders with a rounded inner angle and the other nine on the preparations with

120 chamfers. The angles of convergence of the preparations were 15 (Figs. 3–4).

The FPDs were luted on dies made of inlay pattern resin with zinc phosphate luting

cement. The surface of the root section of the dies was covered with anti-slip varnish

to simulate a periodontal ligament. The dies were fixed in holes in acrylic blocks using

die stone plaster (Fig. 5).

All FPDs were subjected to preloading in a cyclic preloading procedure. This cyclic

preload was applied to the FPDs for 10,000 cycles at loads between 30 and 300 N

with a load profile in the form of a sine wave at 1 Hz. All FPDs were stored in distilled

water during preloading and mounted with a 10-degree inclination relative to the

vertical plane (Fig. 6).

After preloading, the FPDs were mounted in a testing jig, still inclined 10 degrees as

described above, and subjected to a load applied by a universal testing machine. The

crosshead speed was 0.255 mm min-1 and the load was applied with a 2.5-mm

stainless steel ball placed in the mesial fossa of the FPD pontic. The FPDs were

loaded until fracture occurred, and the required loads were registered. Differences

between the two groups were tested with Student’s t-test.

Figure 5. The test models. A = Inlay pattern resin, B = Anti-slip varnish, C = Die stone plaster, D = Acrylic block

Figure 4. Design and dimensions of the FPDs: A) The core rests on the entire extension of the shoulders/chamfers. B) 1.0-mm core material in the area adjacent to the connectors. C) U-shaped interproximal grooves. D) 8.0-mm

length of the pontic. E – Depth of the cervical preparation = 1.0 mm. F) 0.7-mm core and 1.0-mm veneer

porcelain. G) Diameter in the connector in the marked plane 3.0 mm. H) Total length 26 mm.

Figure 6. Jig and application of the load in the preloading and loading tests.

A = Brass foundationB = Acrylic blockC = The FPD pontic

Figure 3. The two different preparation modes.A = Shoulder preparationB = Chamfer preparation

A B

31

32

Five-year evaluation of posterior all-ceramic (alumina) three-unit FPDs. (II)

The first clinical study in this thesis involved 18 patients who were treated with a total

of 20 posterior, three-unit FPDs according to the In-Ceram Alumina slip cast

technique. The FPDs were constructed with bilateral support and one pontic and all

replaced one premolar (n=11) or molar (n=9). Four dentists performed the treatment.

The supporting teeth were cut for cervical shaping according to study I (I b) with a 90°

shoulder and a slightly rounded inner angle. The aim was to cut the cervical shoulder

to a depth of 1.2 mm (Fig. 7).

The try-in procedure was performed after the firing as described in study I (I a, group

2), without any saliva exposure prior to the final firing. Subsequent to try-in, the FPDs

were permanently cemented with zinc phosphate cement in one sitting. No temporary

cementation of the finished FPD was allowed, to avoid creating microcracks or flaws in

the material during removal (Figs. 8–12).

The FPDs were evaluated in clinical and radiographic examinations after 6 months

and then once yearly for 5 years. Fisher’s exact probability test was used to assess

statistical differences between the FPDs replacing the premolars and molars.

7.

8.

9.

Figure 7-12

7). Shoulderpreparation with rounded inner-angle

8). Try-in of the glassinfiltrated Al2O3 core on the master die.

9). Porcelain added to complete the restoration.

10). The finished restoration.

11). Permanently cemented with zinc phosphate cement

12). 12-year follow-up.

10.

11. 12.

33

34

Fracture strength comparison between two oxide ceramic systems (alumina and zirconia). (III)

Sixty specimens designed as "norm crowns" were made: 30 identical crowns of

alumina and 30 of zirconia. Compatible porcelain was used as veneer material

(Fig.13). Each group of 30 was randomly divided into three groups of ten crowns that

were to undergo different treatments according to a test protocol. Subsequent to

fabrication, the crowns were cemented to dies made from inlay pattern resin using zinc

phosphate cement. Excess cement was removed, and the crowns were stored in

distilled water with a temperature of 37˚C until they were subjected to different

treatments according to a test protocol.

The control group underwent no pre-treatment. Both of the experimental groups

underwent preloading with 10,000 cyclic loads between 30 and 300 N at 1 Hz. In

addition, one of the experimental groups underwent thermocycling in two water

baths—5˚C and 55˚C—20 seconds in each bath before loading (Table 3). Subsequent

to pre-treatment, all 60 crowns were subjected to load until fracture. Load was applied

with a 2.5-mm stainless steel ball placed on the occlusal surface of the crowns and a

crosshead speed of 0.255 mm min-1. The loads at fracture were registered, and

differences between the groups were calculated using Student’s t-test. Any differences

in fracture mode were calculated using Fisher’s exact probability test.

Figure 13. Shape and dimensions of the norm crowns.

Table 3. Test protocol with a description of the test groups and number of crowns in each group.

Pre-treatment

Core material Group 1 (control) Group 2 Group 3

Water storage only Preloading* Thermocycling**+

Preloading*Alumina 10 10 10Zirconia 10 10 10

Porcelain Core

* Force was applied with a 2.5-mm stainless steel ball placed on the occlusalsurface of the crowns. All crowns were stored in distilled water during preload andmounted at a 10-degree inclination relative to the long axis of the crowns. The crowns underwent 10,000 cycles at 30–300 N and 1 Hz.

**The crowns in group 3 (10 Alumina and 10 Zirconia) underwent 5,000thermocycles prior to the preloading procedure. Two water baths—5˚C and 55˚C—were used. Each cycle lasted 60 seconds: 20 seconds in each bath and 10 secondsto complete the transfer between baths.

35

All-ceramic (zirconia) CAD/CAM-produced FPDs. A 2-year clinical study. (IV)

Eighteen patients—nine women and nine men—were selected and accepted for

participation in the study. The FPDs were to replace one missing tooth or two missing

teeth with a total gap not exceeding a length equal to the width of one premolar and

one molar and constructed with end abutments. In total, 20 DC-Zirkon® FPDs were

made to replace 26 missing teeth.

The overall design is described in Figure 4. Differences between this study and study

l b, however, were that the aim for the interdental connectors in cases of molar

replacements was a minimum cross-sectional diameter of 4 mm and that the depth of

the cervical shoulder was 1.2 mm. The cores were subsequently veneered with

compatible veneering porcelain (Fig. 14). The patients were examined 1 and 2 years

after cementation and the FPDs were evaluated regarding secondary caries,

endodontic complications, clinical wear, marginal integrity, and presence of cracks or

fractures. Any structural flaws were described. The margins were rated as excellent,

acceptable, or not acceptable according to the modified Californian Dental Association

(CDA) quality assessment system (Table 4).

Table 4 Marginal integrity according to modified CDA criteria

Score Criteria

Alpha (excellent) No visible evidence of crevice along margin into which the explorer can penetrate.No discoloration on the margin between restoration and tooth structure.

Bravo (acceptable) Visible evidence of slight marginal discrepancy with no evidence of decay; repair can be made or is unnecessary.Discoloration between restoration and tooth structure.Faulty margins that cannot be repaired.Penetrating discoloration along the margin of restoration in the pulpal direction.Retained excess cement.Mobile reconstruction.

Charlie(unacceptable)

Fractured reconstruction.Caries continuous with margin of restoration.Fractured tooth structure.

36

A

B

C

D

E

Figure 14. Different steps during production and treatmentA: Finished core on the master dieB: Radiographic examination of the finished core C: Occlusal view before cementationD: Occlusal view after cementationE: 1-week follow-up

37

38

Fracture resistance of all-ceramic (alumina) FPDs supported by simulated teeth vs dental implants. (V)

Two titanium implant abutments—one for position 24 and one for position 26—were

cut to preparations representing one premolar and one molar. By scanning those

abutments and by using the CAD/CAM technique, 24 alumina all-ceramic FPDs were

made. Subsequent to fabrication they were randomly divided into two groups—12 to

be supported by dental implants (Fig. 15) and 12 by simulated teeth (Fig. 16).

Two abutments mounted on implants—one representing a premolar and one

representing a molar—were modified by making a root-shaped wax-up of the implant

parts. Finally, they were copied and 24 simulated abutment teeth were made from the

copies, 12 cut premolars and 12 cut molars (Fig. 17a).

The FPDs were subsequently luted on the implants and the simulated teeth using zinc-

phosphate cement. Finally all the FPDs, both those supported by implants and those

by simulated teeth, were fixated in holes in acrylic blocks using die stone as described

in study I (l b) (Figs. 17b-d).

All FPDs were subjected to preloading as described in study I (l b). The cyclic preload

was between 30 and 100 N with a load profile in the form of a sine wave at 1 Hz.

Finally, the FPDs were loaded until fracture occurred, and the required loads were

registered (Fig. 17e).

Differences between the two groups were tested with Student’s t-test.

39

Figure 16 A simulated abutment tooth, cut with a 120° chamfer and a 15° angle of convergence. The root section is covered with an anti-slip varnish to simulate the periodontal ligament.

Figure 15 Implant abutment (10 mm, wide platform with a connected PROCERA®titanium abutment, cut with a 120° chamfer and 15° angle of convergence)

A

B

C

D

E

Figure 17 A= The two type of abutmentsB=View of a completed FPDC=FPD supported by implants and simulated teethD=Test model (implant supported)E=Settings for preload and load until fracture

40

Long-term follow-ups. (II and IV)

The patients in studies ll and lV were re-examined after 11±1 years (II) and 3 years

(IV) respectively. During the re-examinations, the patients were interviewed regarding

their experience with the FPDs. One dentist made all clinical examinations, and the

FPDs were evaluated regarding secondary caries, clinical wear, and presence of

cracks or fractures. The FPD was considered “successful” if it was in service and

showed no signs of secondary caries, excessive clinical wear, cracks, or structural

flaws. If, on the other hand, the FPD was still in use but showed signs of secondary

caries, excessive clinical wear or fractures, cracks, or flaws, it was considered a

"survivor". An FPD that had been removed was considered a “failure”. If an FPD had

been removed, we checked whether the abutment teeth were still in place and we

interviewed the patients regarding their opinion of the underlying reasons for removal

of the FPD.

41

RESULTS

Table 5. Summary of results, studies I–V

Paper

I a I b II III IV VType of study In vitro In vitro In vivo In vitro In vivo In vitro

Core material - Alumina Alumina Alumina/zirconia Zirconia Alumina

Veneer material

Porcelain Porcelain Porcelain Porcelain Porcelain Porcelain

The group that resisted the

highest loads

Firedprior to saliva

exposure

Shoulder preparation

- Zirconia,preloaded only

- Implant supported

p p<0.0001 p=0.051 Se table

Mean (MPa) 81 / 116 510** / 606 Se table 378 / 604

1-yr follow-up; Investigated patients (%)

- - 100% - 100% -

1-yr follow-up; Success

100% 100%

3-yr follow-up up Investigated

patients (%)

- - 100% - 100% -

3-yr follow-up; Success

95% 100%

Remaining abutment teeth

(%)

100% 100%

5-yr follow-up Investigated patients (%)

- - 100% - - -

5-yr follow-up; Success

90%

11-yr follow-up Investigated patients (%)

- - 85% - - -

11-yr follow-up; Success

65%* -

Remaining abutment teeth (% after 11 yrs)

- - 100% - - -

* 59% success and 6% survival, 35% of the FPDs removed. **Total fracture

The results from the different studies in the thesis are briefly described in this section.

In the first part of study I (I a), the mean flexural strength of the specimens in the

group that was exposed to saliva after glazing (group 1) was significantly higher

(P < 0.001) than that of the specimens in the group that was exposed to saliva before

glazing (group 2). The mean flexural strength of the specimens in group 1 was 81 MPa

13 (SD) and the strength of the specimens in group 2 was 116MPa 15 (SD). The

results indicate that saliva exposure before glaze firing can decrease the flexural

strength of porcelain (Table 6).

I

Force (MPa)

Group 1 Group 2

Mean 81 P < 0.0001 116SD 13 15

Table 6. The force (MPa) required to fracture the specimens in group 1 (exposed to saliva before glazing) and group 2 (glazedbefore exposure to saliva).

In part I b, the FPDs luted on shoulder preparations resisted higher loads than the

FPDs luted on chamfer preparations. This result was significant at the P = 0.051 level.

In four of the FPDs luted on chamfer preparations, the porcelain veneer fractured

before the core. In contrast, all the FPDs on shoulder preparations fractured instantly

through all the layers (Table 7). The orientation of the fracture was from the inferior

surface of one of the connectors towards the loading-point in all cases.

42

43

Loads at fracture (N)

Chamfer Shoulder

FPD no. Veneer fracture Total fracture Total fracture

1 325 375 5372 300 412 5373 - 426 5374 - 438 5375 - 450 5556 387 537 5757 - 575 6008 - 588 7629 413 787 812

Mean 434 510 606SD 98 128 106

Table 7. The loads (N) at fracture and the type of fracture in the two groups

of fixed partial dentures (FPDs): chamfer preparations and shoulder

preparations.

A second FPD was found to be fractured after 35 months (Table 8). Of the

remaining 18 FPDs, none had any defects. No caries and no signs of gingivitis or

closer examination of the fractured FPDs, the following observations were made:

II

The FPD that was lost after 24 months was the last one in the series

of 20. It replaced the upper right first molar in a man who was 50

years old. The fracture was located at the connection between the

pontic and the distal abutment tooth. The operator responsible for the

treatment of this patient had 8 years of experience in dentistry and

had made seven of the FPDs.

On

periodontitis exceeding those found in the rest of the dentition were registered.

In the second study (II) all FPDs were found to be functioning at the

6- and 12-month follow-ups, but at 24 months, one FPD had fractured.

The second fractured FPD replaced the lower left first molar and, like

the first one, fractured at the connection between the pontic and the

distal abutment tooth. This FPD had the longest pontic of the 20

FPDs. The patient was a man who was 50 years old, and he was

treated by an operator with 3 years of experience in dentistry and

who was responsible for 11 of the FPDs.

It could thus be established that all the FPDs that replaced premolars were in function

at the end of the observation period. Two of the nine FPDs that replaced molars were

fractured. There was, however, no significant difference in the success rate between

FPDs replacing molars and the FPDs replacing premolars (Fisher’s exact probability

test P = 0.190). 95 09 25

96 10 15*

95 04 03

93 05 07

94 02 24

94 05 06

95 01 27

93 12 21

94 02 07

93 02 17

94 01 14

93 09 29

93 06 04

93 06 15

93 06 11

93 12 18

93 04 02

93 04 28

93 02 19

92 12 11

96 01 30*

7 6 5 4 3 3 4 5 6 7R i g h t L e f t

Date of cementation (year, month,day) and position in the mouth

*Date when failure was dicovered

denotes upper jaw

denotes lower jaw

Table 8.

44

In the third study (III) two types of fractures occurred: total fracture, through

both core and veneer, and partial fracture, through the veneer only. Total

fractures were more frequent in the alumina group compared to the zirconia group,

and this difference was statistically significant (P < 0.001). In all instances of partial

fracture, the fracture was cohesive within the veneer material.

III

During thermocycling, 7 of 20 crowns (4 alumina and 3 zirconia) underwent loss of

retention. The other 13 crowns in the group experienced no such loss (Table 9).

Group 1 Group 2 Group 3

Corematerial

Fracturestrength (N)

Fracturemode ratio

Fracturestrength (N)

Fracturemode ratio

Fracturestrength (N)

Fracturemode ratio

Alumina 905 8:2 904 9:1 917 9:1

Zirconia 975 2:8 1108 6:4 910 3:7

P = 0.38 P = 0.01 P < 0.007 P > 0.05 P > 0.05 P < 0.01

Table 9. Fracture strength and mode [ratio of number of total fractures to number of partial fractures(total fracture =, partial fracture =)]. Group 1 = water storage only, Group 2 = preloaded, Group 3 = thermocycled and preloaded.

For the 18 patients in the second clinical part of the thesis (IV), 20 three–

five-unit FPDs with a zirconia framework were fabricated according to the

DCS® Precident System. For details of FPD placements and dimensions of the inter-

dental connectors, see Table 10.

IV

45

Tab

le 1

0.

46

47

All FPDs were in use and none had fractured at the 12-month follow-up. No chip-off

fractures or clinical wear could be observed. Marginal integrity was rated Alpha at 46

abutments and Bravo at 10 abutments. No margins were rated Charlie. All patients

were fully satisfied with their FPDs. Initial inter-examiner agreement was 91%.

At the 24-month follow-up, all FPDs were still in use and none had fractured or showed

any clinical wear. In three cases, however, minor chip-off fractures were observed on

FPD #2, #9, and #12. These units were opposed by a natural tooth with an occlusal

amalgam filling in one case, a PFM crown in the second case, and a gold crown in the

third case. Marginal integrity was rated Alpha at 45 abutments and Bravo at 11

abutments. No margins were rated Charlie. All patients were fully satisfied with their

FPDs, and none of the three patients where chip-off fractures had occurred had

noticed the fracture. Initial inter-examiner agreement was 89%.

Complications

During the fabrication period, one patient (FPD #5) developed symptoms of pulpitis on

the second molar. The tooth was subsequently treated endodontically with sufficient

remaining tooth substance to allow reconstruction without a post and core. The same

patient developed symptomatic acute apical periodontitis during the first 6 months after

FPD cementation on the mesial abutment tooth (first premolar). Endodontic treatment

was performed subsequent to trepanation through the occlusal surface of the FPD. No

further symptoms were registered at the 12- or 24-month follow-up. Also during the

fabrication period, one FPD had to be remade due to unacceptable esthetics.

(mean = 604 N, SD=184 N) than the FPDs loaded on simulated teeth (mean

= 378 N, SD=152 N). The result was significant at the P = 0.003 level. All fractures

were total fractures (through both the core in the area of the fuse and the veneer) that

occurred in one of the connector areas (in the fuses between an abutment crown and

the pontic) (Table 11).

V

Table 11. Loads at fracture in the two groups:fixed partial dentures (FPDs) supported by simulated teeth and FPDs supported by implants.

Load at fracture (N)FPDno. Simulated teeth Implants1 178 2932 230 4483 234 4614 246 5005 297 5296 314 5427 380 5938 435 6259 465 692

10 551 76511 567 90112 634 903

Mean 378 604SD 152 184

48

In this study (V) the FPDs loaded on implants resisted 60% higher loads

Results from the long-term follow-up examinations (II, IV).

The outcomes of the 2- and 3-year follow-ups of the zirconia FPDs were similar. All

FPDs were still in service and showed no signs of fractures other than the minor chip-

off fractures discovered at the 2-year follow-up.

The survival rate of the alumina FPDs, however, decreased dramatically. The last

follow-up was made when the FPDs had been in service for 11±1 years (mean 11). Of

the original 20 patients, 3 dropped out: 2 died and 1 was untraceable. At this follow-up,

11 (65%) of the remaining 17 FPDs were still in service. The surface condition of 10 of

these 11 was excellent. The one that was not excellent had an excessively rough

occlusal surface. Ten (59%) of the FPDs were considered successful (no signs of

secondary caries, no excessive clinical wear, no cracks, or no structural flaws).

No abutment teeth were lost during the period (Table 12).

In the patient interview, all patients with FPDs still in service stated that they were

satisfied with their prostheses. The patients who had their FPDs removed were unable

to clearly explain why.

Table 12. Results of long-term follow-ups of 20 alumina and 20 zirconiafixed partial dentures (FPDs). Dropouts: 15% (alumina) and 0% (zirconia)

Material Observation FPD Remainingperiod

(yr)Successrate (%)

Survival(%)

Removed(%)

abutmentteeth (%)

Alumina 11 ± 1 59 6 35 100Zirconia 3 100 0 0 100

49

50

51

DISCUSSION

Several different material systems intended for all-ceramic FPDs are available in

dentistry; some of them since more than 15 years. Despite this length of time, few

results from clinical studies are available to support (or refute) the use of all-ceramic

FPDs. Although in-vitro studies are reporting promising results concerning the flexural

strength and fracture toughness of ceramics in the oxide ceramic group—and in

particular zirconia—long-term clinical trials are needed to establish the advisability of

such procedures 22,25,40,43,52,59,60,64. In-vitro studies can, on the other hand, give useful

information, and together with data from actual in-vivo measurements, they can be

used to determine indicative values for occlusal loads to be resisted by prosthetic

reconstructions, basic information that is valuable for other researchers in the design

of clinical trials16,25. The second (II) and the fourth (IV) studies in this thesis are clinical

(in-vivo) trials of all-ceramic FPDs. The others (I, III, V) are in-vitro studies that focus

on important steps in the clinical procedures, which must be considered when

designing an FPD.

The clinical studies in the thesis (II, IV)

The time, complexity, and expense that are entailed in fixed prosthetic treatment are

justified only if the lifetime of a restoration is lengthy23. It is not evident, however, how

“a long time” or “failure” should be defined. The failure criteria used to evaluate FPDs

differ substantially between published studies. The clinical definitions vary from

“loosening of one retainer” to “endodontically treated” and “unacceptable esthetics”,

while for the individual patient the criterion of failure may be extremely subjective10.

52

Scurria et al.51 describe three categories of FPD failure: 1) the prosthesis had been

removed, 2) the prosthesis had been removed or had technically failed, necessitating

replacement, and 3) one or more abutments were lost. The failures in the present

thesis are technical failures if the follow-ups are restricted to 5 years (II). Meta-

analyses of the outcome of treatment with conventional FPDs show survival rates of

95%–98.5% after 5 years, compared to 90% in the present material, which is

considered acceptable10,23,51. When the material was followed up after 11 ± 1 years,

comparisons become more uncertain because the reasons for FPD replacement were

unknown. In comparisons with results after 10 years or longer on PFM FPDs, the

failure rate of the alumina FPDs in this thesis is higher than published failure rates for

high-gold alloy PFM reconstructions.

Studies on conventional FPDs report survival rates of 90%–92% after 10 years and

67.5% after 15 years10,23,51. It has been stated that half-life close to or slightly more

than 15 years must be considered both a satisfactory outcome of a complex

restorative treatment and a sound economic investment for the patient23. Based on the

assumption of a linear relationship, a 65% survival rate after 11 ± 1 years (II) is close

to a half-life of 15 years. If this assumption holds, the clinical suitability of alumina-

based all-ceramic FPDs could be considered acceptable. Such linearity, however,

cannot always be assumed, especially in the case of ceramics that are susceptible to

time-dependent fatigue. Conclusively, the clinical performance of alumina is not as

good as that of comparable high-gold alloy based PFM FPDs. This conclusion,

however, is based only on study II, which comprised 20 FPDs.

Zirconia, on the other hand is more than 3 times stronger and tougher than alumina.

The fracture strength of glass-infiltrated alumina is approximately 400 MPa and the

53

fracture toughness approximately 3 MPa m½. The corresponding values for zirconia

are 1200 MPa and 9 MPa m½, which are substantially higher than for alumina17,19,50.

Furthermore, CAD-CAM is being used in many new material systems to produce the

FPD cores under optimized industrial conditions by milling a substructure from a blank.

In this way, it can be assumed that the population of intrinsic flaws in the new

materials is reduced in both number and size compared to traditional ceramics. Thus,

it makes it possible to produce a core with enhanced integrity and strength5.

The results of the 3-year zirconia study (IV) are promising when compared with results

from clinical studies on alumina-based FPDs. Pröbster reported in 1993 that 13 of 15

anterior and posterior In-Ceram® FPDs were still in use after a clinical trial period of 2–

35 months. The cumulative survival rate was calculated to be 93.3% for a 12-month

observation period. One anterior FPD fractured due to improper dimensioning of a

connector and one posterior FPD was removed because of periodontal

complications44. In a 1-year perspective, the 20 zirconia FPDs (IV) had a 100%

success rate.

Another study reported an 82.5% success rate for posterior alumina FPDs after 3

years of service57; the success rate in our alumina study was 95% after 3 years (II). In

our study on zirconia (IV), the success rate after 3 years was 100%. Other authors

have had similar success with zirconia. Molin reported a 100% survival rate for 18 all-

ceramic zirconia-based FPDs after 2 years in service39. Thus, it might be assumed that

in this short term perspective, clinical results for zirconia-based all-ceramic FPDs are

comparable to those of high-gold alloy PFM FPDs. Long-term follow-up studies,

however, are needed to establish their longevity.

54

The minor occlusal chip-off fractures experienced in study IV were not a cause for

replacing any of the reconstructions, especially since the patients were unaware of

them until the clinical examination. That the chip-off fractures occurred is important to

discuss since a fracture is always undesirable and in one way a failure, even though

such fractures are insignificant. The fracture pattern is similar to that found in another

study (III) in this thesis where two types of fractures occurred: total fractures, through

both the core and the veneer, and partial fractures, through the veneer only. Veneer

fractures were more frequent in the zirconia group in this in-vitro study.

One reason for chip-off fractures could be that the strength of the veneering porcelains

is insufficient, as was the case with the early veneer porcelains used in titanium-PFM

FPDs and crowns that were susceptible to chip-off fractures. The survival probability of

ceramic-veneered titanium FPDs in a 3-year follow-up study was only 59% after 30

months28. One of the criteria for failure in that study was the presence of cracks or

chipping of the veneering porcelain, failures which did not result in the replacement of

the entire construction.

Other reasons for chip-off fractures can be flaws emanating from the fabrication of the

porcelain powder or the build-up of the porcelain at the laboratory33. A ceramic

laminate will always form a constant strain system because of the mismatch of elastic

moduli across the core-veneer interface. Furthermore, the interface is an important

source of structural flaws29 due to wettability factors and difficulties to build up the

green porcelain prior to firing densely and homogenously over the core surface without

trapping air bubbles. All chip-off fractures in the present clinical study, however, were

superficial. The shape of the high strength inner construction normally serves to

support the less strong veneer material. In study IV, where the design of the

55

framework was processed in the computer by "true" CAD/CAM, there is an inherent

risk that the occlusal shape is insufficient regarding veneer support. Mechanically

defective microstructural regions in the porcelain, including areas of porosities,

agglomerates, inclusions, and large-grained zones, are other possible reasons for

veneer fractures31,33.

The try-in procedure (I)

There are several possible mechanisms that, after try-in in the mouth and subsequent

glaze firing, could decrease the strength of the veneering porcelain. One is that the

strength of the ceramics is directly related to the number of firings; additional firings

have been shown to decrease ceramic strength. The advantages of glaze firing with

respect to flaw-healing are not achieved if the glaze firing itself weakens the material14.

The specimens in the first part of study I (I a) were all fired twice, irrespective of which

group they represented, to avoid any influence that different numbers of firings might

have had. Polishing, on the other hand, can strengthen a material by eliminating

surface flaws and the development of residual compressive stresses in the porcelain

surface14,21,30.

Other explanations for the differences found in the first part of study I (I a) can be

saliva molecule residues on the surface or in cracks and porosities of the specimens

as well as inadequate cleaning procedures that leave behind saliva molecules which

could react with the porcelain during firing and thus decrease strength. Finally, residual

moisture after clinical and cleaning procedures may expand during firing and act to

decrease the strength. The results in the first part of study I (I a) lead to the conclusion

that short-term exposure to saliva prior to the final firing could have a negative effect

56

on the strength of porcelain. With respect to the limitations of an in-vitro study,

however, this phenomenon must be investigated further.

The cervical shape of the preparation

The question of how strong a ceramic reconstruction needs to be to withstand loads in

the oral cavity during service is still unclear. Insufficient clinical data often led

manufacturers and dentists to place great emphasis on the data for the strength of a

material to define clinical indications. Hence, data from laboratory studies are used to

extrapolate strength and toughness values to promote novel materials and processing

technologies25. Survival probability analyses assume that the maximum biting forces

on anterior crowns rarely exceed 900 N and the maximum force on posterior crowns

rarely exceeds 2200 N whereas most patients generate typical bite forces between

400 N and 800 N3. To withstand such loads, it is important not only to select materials

with suitable material properties, but to design the prosthesis to reduce stresses in the

beam of the prosthesis.

The cervical shape of a supporting abutment can be critical to the strength of all-

ceramic crowns. It has been shown that fracture resistance is related to cervical shape

since chamfer preparations induce higher levels of stresses in crowns when loaded

than do shoulder preparations. Thus, it has been emphasized that all-ceramic crowns

should be supported by shoulder preparations to resist loading12,55.

However, no studies have been conducted on whether the stress that occurs when

loading an FPD supported by chamfer preparations also affects the connector areas.

Together with the complexity of the stress pattern that occurs during loading of

57

FPDs24,27, regardless of the cervical shape, the type of preparation could influence

failure mechanisms and be a factor to consider when trying to reduce stresses.

The other results in study I (I b) show that there are dissimilarities between the fracture

patterns in FPDs supported by chamfer preparations and the fracture patterns in FPDs

supported by shoulder preparations. All fractures in the latter were fractures through

both core and veneer—total fractures—while the former exhibited two modes of

failures: total fracture or partial fracture through the veneer only. This might be

explained by differences in stress distribution in FPDs supported by the two

preparations as discussed above.

The characteristic strength and elasticity modulus of the alumina core is much higher

than that of the veneer porcelain. Kelly et al. described an interfacial mismatch in

properties between the core and the veneer material29. This could imply that when

FPDs supported by chamfer preparations are loaded, tensile and shearing stresses

reach critical levels in the veneer before they do in the core ceramic, resulting in

different kinds of failures depending on the cervical shape of the preparation. This,

together with our finding that FPDs on shoulder preparations resisted higher loads

than FPDs on chamfer preparations, supports earlier findings concerning crowns, that

all-ceramic restorations should be made on shoulder preparations; this

recommendation is also valid for all-ceramic FPDs.

The abutment support

Other sources of stress in an FPD beam could be differences in support provided by

the different abutments of a prosthesis. An FPD will splint the abutment teeth, but

resultant bending forces will be absorbed in the beam of the prosthesis, especially in

58

the connector areas58,66. The biomechanical support provided by implants differs,

however, and the biomechanical situation is similar to that of tooth ankylosis where no

periodontal ligament exists13.

The results of study V suggest that the support provided by implants might be

favorable compared to that provided by natural teeth since implants give a more solid

support than teeth. Because all-ceramic FPDs are more susceptible to bending forces,

implant support could be especially favorable compared to natural support. Implants,

on the other hand, if well integrated, do not allow for sensory response or movements.

This could be detrimental to the veneering porcelain if it is subjected to excessive

loads that exceed the load bearing capacity of the veneer. This could more easily

result in chip-off fractures in a reconstruction that is supported by implants compared

to one supported by teeth. Concerning the framework, however, the situation is

different. A more resilient attachment, such as the periodontal ligament, may respond

to loadings with different amounts of movement occurring at each end of the FPD7.

The resultant bending forces absorbed in the beam of the FPD might then be

proportional to the different amounts of movement. Thus will a solid support be

favorable compared to a more resilient one. Conclusively, these results (V) suggest

that all-ceramic FPDs might be used in combination with implants.

Aspects of the methods used

When analyzing results obtained from in-vitro studies, it is important to keep in mind

that masticatory system are highly complex and that the actual clinical situation is

impossible to mimic to more than a limited extent. It has been concluded that only

simple surveys should be carried out and that rigid die stone models can provide

experimental data of the same quality as more complex ones15,45. Less complicated

models than those used in this thesis (I, V) could have been used, but the approach

59

chosen was considered valid since a more rigid model could have been resulting in

unrealistically (high) fracture strength values17,63. Comparisons between the results

should, furthermore, be made only intra-individually to compare the methods or

techniques under study. The fatigue tests used in the thesis (I,III, V) are recognized as

valid for ceramic testing and have been used in previous studies8,67. Finally, the clinical

parts of the study could preferably have included control groups. Clinical studies are,

however, complicated for several reasons. One reason is that it is very difficult to find

patients with indications for the reconstructions that are to be studied. If patients only

with indications for both the studied reconstruction and an intra-individually control

should be included, this would result in few patients or an unacceptable time gap

between the first and the last patient in the study. The approach chosen was therefore

considered valid and acceptable.

60

61

CONCLUSIONS

Within the limitations of the in-vitro parts of this thesis, the following conclusions can be drawn:

Short-term exposure to saliva, before the final firing, could have a negative

effect on the strength of dental porcelain. The mechanisms behind this

phenomenon are, however, not fully known and further studies are needed to

confirm this finding. The hypothesis that dental porcelain which is glazed prior

to saliva exposure will resist higher loads than equivalents that are subjected to

saliva prior to the final firing, is thus confirmed.

All-ceramic FPDs cemented with non-adhesive cementation techniques should

be supported by shoulder preparations in order to resist extensive loading

whenever this is expected. The hypothesis that all-ceramic FPDs supported by

abutments cut with circumferential shoulders will resist higher loads than

equivalents that are supported by abutments cut with circumferential chamfers

is thus confirmed.

There is no difference in fracture strength between crowns made of zirconia

cores and crowns made of alumina if they are subjected to loading without any

previous cyclic preload or thermocycling. There is, however, a significant

difference in the fracture mode. This conclusion is restricted to the crown design

used in this study.

Crowns made with zirconia cores have significantly higher fracture strengths

after preloading compared with crowns made with alumina cores. The

hypothesis that zirconia-based reconstructions veneered with dental porcelain

and subjected to fatiguing and stress-corrosion can resist higher loads than

alumina-based equivalents is thus confirmed.

The hypothesis that all-ceramic FPDs supported by dental implants will resist

higher loads than equivalents that are supported by natural teeth is confirmed.

62

Hence, all-ceramic FPDs might be used in combination with dental implants.

Clinical studies, however, are needed to confirm these findings because other

factors also influence the final clinical outcome.

The results of the clinical (in-vivo) parts of this thesis justify the following

conclusions:

The In-Ceram Alumina slip-cast technique, properly employed, is acceptable

for three-unit FPDs in the posterior region in a 5-year perspective. The clinical

performance, however, is not as good as that of comparable high-gold alloy

based PFM FPDs concerning fracture resistance. Longer restorations than the

three-unit FPDs used in this study cannot be recommended.

The DC-Zirkon technique is excellent for up to five-unit FPDs in all regions in

the mouth in a 3-year perspective, and the properties of the core material equal

those found in previous clinical trials of high-gold alloy PFM FPDs. Those

findings, however, are limited to the design adopted in the present thesis,

especially concerning the dimensioning of the connector area which was a

minimum diameter of 3 mm. Furthermore, special attention must be paid to

avoid chip-off fractures. Further studies must be performed before the material

system can be recommended for more extended restorations, or restorations

with smaller dimensions, than the FPDs in the present thesis.

The hypothesis that oxide ceramics veneered with dental porcelain can be used

for FPDs with extensions of up to five units if based on zirconia and three units

if based on alumina can not be rejected.

63

ACKNOWLEDGEMENTS

To the many people who made this thesis possible I wish to extend my sincere gratitude. They include:

Professor Krister Nilner for his advice, invaluable support and personal involvement in this work.

Professor Per-Olof Glantz for his valuable advice and comments on this thesis.

Professor Tore Dérand for his valuable advice and comments on the first part of the study.

Associate professor Erik Strandman for many valuable hints and stimulating discussions and for always giving a helping hand.

Professor Björn Söderfeldt for advice and stimulating discussions.

Dr Yuji Kokubo, Tsurumi University, Japan, for fruitful collaboration and stimulating discussions.

Mr Stig A Svensson for his most skilful technical assistance and help with technical illustrations.

Mrs Gail Conrod-List for valuable linguistic advice and revision of the text.

Mrs Solweig Näsström-Nilsson for her support and clinical assistance.

Mr Sten Ahrne for assistance with the technical illustrations.

Mr Bertil Rohlin for the financial support that initiated this work.

My coauthors (if not mentioned above) for making this thesis possible; in order of appearance:

Dr Ola Jönsson Dr Asim Al-Ansari Mrs Katarina White Miss Sandra Ebbesson Miss Jenny Holmgren Dr Per Haag Dr Per Carlsson

The staff of the Department of Prosthetic Dentistry and Dental Technology, Faculty of Odontology, Malmö University.

64

The staff of the faculty library for their valuable help

Nobel Biocare AB, Gothenburg, Sweden, for material support

Mr Bengt Hoffman, Stockholm, Sweden, for material support

Swedish Dental Society for financial support

Last and most profoundly I extend my gratitude and love to my wife and my family for their love and never ending patience and support.

65

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