PRINCIPLES OF PREPARATION IN FIXED PROSTHODONTICS

*Teeth don’t possess regenerative ability found in most other tissues. Therefore, once enamel & dentine are lost as a result of caries ,trauma or wear restorative material must be used, to reestablish form & function.*Teeth require preparation to receive restoration & these preparations must be based on fundamental principles from which basic criteria can be developed to help predict the success of prosthodontic treatment.

Restoration which covers all or part of surfaces of clinical crown. It maybe made from various material (metal, metal-acrylic, metal-ceramic, ceramic, zirconium) .

Crown

Full metal crown All ceramic crown

Metal-ceramic crownGold crown

Bridge replaces one or more missing teeth (named as a pontic) and supported on adjacent remaining teeth (named as abutment). Bridge covers surfaces of abutments to which the bridge is cemented.

Missing teeth ( pontic )

Adjacent remaining teeth (abutment)

Bridge(fixed prosthodontic appliance)

Missing teeth ( pontic )

Adjacent remaining teeth (abutment)

Fixed bridge

Replaces one missing tooth in posterior area which is bonded to occluso-lingual surfaces , and replaces maximum two teeth in anterior area which is bonded to lingual surfaces. The restoration bonds to the prepared surfaces by a resin luting cement (etching enamel technique).

resin-bonded bridge (MAryland bridge)

Posterior resin bonded bridge Anterior resin bonded bridge

Cover only the facial surface of the tooth for esthetic purpose. They are fabricated from porcelain (ceramics) and bonded (micro-retention) to enamel by resin luting cement (etching enamel technique).

laminates (facing)

Principles of tooth preparation:1.Biologic consideration(affects health of oral tissues).2.Mechanical consideration (affects integrity and durability of restoration).3. Esthetic consideration (affects appearance of the patient).

Mechanical principles

- minimal reduction

- partial veener crown or laminate (partial coverage) versus full veener crown (full coverage).

- Supragingival margin versus subgingival margin .

Supra-ging At crest Sub-ging

1- preservation of tooth structures

2- Path of insertion

It’s the specific direction in which a prosthesis is placed onto the abutment or removed from the abutment . Any deviation (undercut) from this line will result in distortion of wax pattern and failure of restoration seating.

Axial reduction must be with Taper - - facio-lingual reduction

{ height of contour must be removed to eliminate any divergent (undercut) with taper 5 – 10 degrees }

- mesio-distal reduction (proximal)

{ removal of divergence in occlusal third to eliminate any undercuts , and

mesio-distal inclination (taper 5-10 degrees) of the path must be parallel to adjacent

proximal contact of adjacent teeth }

Taper

its defined as a convergence between two opposing axial walls , in occluso-axial junction .

Two opposing surfaces , each with a 3 degree inclination would give the preparation a total of 6 degrees taper.

Taper must be 5- 10 degrees of inclination.

Why tapered ?

1- To visualize preparation walls

2- To prevent the undercuts

3- To compensate for inaccuracies in the fabrication process

4- To permit complete seating of the restoration during cementation

Taper

tilting bur slightly toward the tooth creates taper 5 – 10 degrees

More tilting toward the tooth creates excessive convergence(Taper)

Weak

retention

Undercut

Its defined as a divergence between opposing axial walls , in a cervical –occlusal direction . If the cervical diameter of tooth preparation at the margin is narrow than at the occluso-axial junction (reverse taper) , it will be impossible to seat a complete crown.

Undercuts can be present where two axial walls face in opposite directions .

For example :- the mesial wall of a complete crown preparation can be undercut relative to the distal wall , and the buccal wall can be undercut relative to the lingual wall

Tilting the bur away from tooth creates undercut.

3- Retention and resistance

It’s the quality of the preparation to prevent removal of the restoration along the axis of the tooth ( its path of insertion )

Retention

Retention depends on the factors :-

1 - Magnitude of the dislodging forces.

2 - Geometry of preparation:

- taper preparation.

- duplicating anatomical form of occlusal surface preparation.

- type of preparation (full crown more retentive than partial crown).

3 - Roughness of the internal surface of the restoration

( air-abrasion – oxide layer -- micro retention ).

4 - Type and film thickness of luting agent (cement).

Resistance

It’s the ability of the preparation to prevent dislodgement of the restoration by forces directed in apical , oblique or horizontal directions.

Factors affecting resistance :-

1- Length of preparation

The more occluso-gingival dimension, the more resistance .

Resistance is increased due to the amount of tooth structure interfering with (lateral or oblique force) ---------------- the arc of rotation .

2- Taper:

The resistance is increased by decreasing the taper of the preparation

3 – Ratio of diameter to length of preparation:

There is a balance ratio between the diameter and height ,which provides more resistance .

4 – Occlusal irregularity of reduction:

Duplicating the anatomical form of occlusal surface during preparation increases the resistance (flat reduction of the occlusal surface of the tooth may decrease the resistance ).

4 –Adequate Reduction of the tooth structure

Steps of tooth reduction (preparation)

Occlusal (incisal) reduction Axial reduction

- buccal (facial)

- lingual (palatal)

- mesial

proximal

- distal

1- Occlusal (incisal) reduction

Occlusal and incisal reduction is preformed to provide adequate clearance between he prepared surface and opposing teeth .

The occlusal and incisal surfaces must be reduced by minimum 1.5 mm clearance for (full metal crown) and 2 mm clearance for (full ceramic and metal ceramic crowns).

To allow:-

- adequate thickness of restorative material (metal – ceramic) to be enough strong (to prevent fracture of crown).

- To form anatomical morphology in occlusal surface of the restoration to be more esthetic.

Adequate clearance more strength of restoration

Adequate clearance adequate anatomical form more esthetic and more chewing effect

- Occulsal depth cuts Occlusal cut acts as a guide for reduction ,and placed using round bur No.2 (1.5 mm) or by round ended-taper bur on the grooves and cusps.

Depth cuts

To complete occlusal reduction the remaining tooth structures between the depth cuts is then uniformly removed by (round ended- taper bur ) or by (wheel-shaped bur ).The reduction of occlusal surface should be uniform and follows anatomical contour ,which permits increase of retention and resistance of the restoration.

Note : flat plane reduction of the occlusal surface may be acceptable in older patients with attrition on the occlusal surface

- Functional cusp bevel It’s provided in the functional cusp area (deep contact of occlusion) to increase the thickness of the restoration in occluso-axial junction

The functional cusp is reduced more than nonfunctional cusp by 0.5 mm

Note:

Functional cusp bevel in the mandibular molars buccal cusps

Functional cusp bevel in the maxillary molars palatal cusps

Funct. Cusp bevel

Lack of functional cusp bevel may produce several problems :-

1- can cause a thin area or perforation

2- may result in over- contouring and poor occlusion

- Evaluation of the clearance To evaluate clearance between the prepared and opposing teeth , the mandible and maxilla must be closed in centric occlusion . 1.5 – 2 mm of interocclusal space must be present; this space can be measured by using the strip of utility red wax with the patient in bite or by using reduction gauge which should pass through the occlusal surface without obstruction.

Complete reduction of occlusal surface

Axial Reduction

Axial reduction commonly includes the entire circumference of the tooth ( buccal – lingual – mesial & distal surfaces )

Buccal and lingual reduction

The buccal and lingual surfaces must be reduced so that the restoration can reestablish tooth contours. `

- Depth cuts

for adequate & uniform reduction to be achieved, depth cuts in buccal and lingual surfaces (1-1.5 mm) can be made as a guide to preparation by using taper round-ended diamond bur making 2 -3 depth cuts equally placed along the mesio-distal inclines of buccal and lingual surfaces in the posterior teeth .

Placing depth cuts

To complete the preparation of buccal and lingual surfaces in posterior teeth , use the taper round – ended diamond bur to remove the remaining structures between depth cuts to obtain uniform reduction . The bur should be tilted toward the surface during reduction to obtain taper .

The reduction of buccal and lingual surfaces should be with a taper of 5 – 10 degree occluso-gingivally with finish line (chamfer or knife edge).

tilting bur slightly toward the tooth creates taper 5 – 10 degrees.

In the buccal and lingual surfaces, the height of contour and any divergence should be removed to eliminate the undercuts , the over reduction of the axial surfaces will lead to loss of retention .

More tilting toward the tooth creates excessive convergence.

Tilting the bur away from tooth creates undercuts.

Hieght of contour and undercut

-Making or creating “flanges or enamel lip” on the buccal and ligual surfaces during their reduction , which serves as a guide to avoid the contact with adjacent teeth and help to complete the preparation of the proximal surfaces .

Using the matrix band – matrix band can be placed around the adjacent tooth to protect it from any damage during the preparation.

Note :- Tapered flame shaped (needle) diamond burs are used in reduction of proximal surfaces.

break the contact with adjacent teeth

Preparation from buccal or lingual to proximal

Flange or enamel lip

Inter-proximal reduction not less than 0.6 mm

Anterior crown preparation features

Crowntype

PorcelainJacket

ResinbondedporcelainMetalceramic

10/30/2007

Occlusal reduction

2mm occlusally1mm lingual side

2mm occlusally0.5-1.0mm lingual side

2mm occlusally0.5-1.0mm lingual side

C&B 3

Finish line

0.8-1.0mm shoulder

>0.4mm chamfer

1.2mm labial shoulderor heavy chamfer0.5mm lingual chamfer

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Posterior crown preparationfeatures

Crowntype of

Full metal

Highstrengthporcelain

Metalceramic

10/30/2007

Occlusal reduction

1mm non-functional cusp1.5mm functional cusp

2mm non-functional cusp2.5mm functional cusp

2mm non-functional cusp2.5mm functional cusp

C&B 3

Finish line

0-1mm chamfer knife-edge shoulder orshoulder with bevel0.8-1.0mmShoulder or heavychamfer1.2mm labial shoulder0.5mm lingual champer

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Smoothing the preparation and rounding line angles

All prepared surfaces should be smoothed by using fine diamond bur or by carbide bur to remove any rough areas and undercuts.

The sharp line angles should be rounded , this procedure reduces the stress that develops when occlusal forces are applied to the restoration .

When the wax pattern is invested, less possibility for air to be trapped that can cause internal nodules on the cast .

The smoothing of the prepared tooth surfaces maybe achieved without using water spray to maximize visual observation . Using the air as a coolant and short intermittent contact of abrasive instrument can avoid pulpal irritation .

Smoothing all surfaces and rounding all sharp lines .

Smoothing surfaces and rounding incisal edge and lingual lines

Biologic considerationIncludes three main points a.Prevention of damage during tooth preparationb.Conservation of tooth structurec.Consideration affecting future dental health

a.Prevention of damage during tooth preparation1.Adjacent teeth2.Soft tissue 3.Pulp

a.Adjacent teethIatrogenic damage to adjacent teeth is common error in dentistry . Even if damaged proximal contact area is carefully reshaped and polished it will be more susceptile to dental caries than original undamaged surface

Causes of injury to pulp 1.TemperatureConsiderable heat generated by friction between rotary instrument and tooth surface being prepared .Excessive pressure , higher rotational speed ,and type , shape and condition of cutting instrument may all increase generated heat With high speed hand piece, light intermitted touch, sharp cutting instrument and water spray are efficient for removal of tooth material with minimal heat generation

2.Chemical actionCertain dental materials(bases, restorative resins, solvents and luting agent ) can cause pulpal damage particularly when they are applied to freshly cut dentin.Cavity varnish and dentine bonding agent form effective barrier in most instances but their effect on retention of cemented restoration is controversial Use of chemical cleaning agent is contraindicated because it cause irreversible pulp damage

3.Bacterial action Ether due to bacteria left behind or micro-leakage *Removal of carious tooth structure is must before placing restoration

Advantages &disadvantages of different margin design

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