PRINCIPLES OF TOOTH

PRINCIPLES OF TOOTH

PREPARATION

DR SABZAR ABDULLAH DEPARTMENT OF PROSTHODONTICS

AMU ALIGARH

CONTENTS

• INTRODUCTION

• BROAD CATEGORIES OF PRINCIPLES

• FEATURES OF TOOTH PREPARATION

• MECHANICAL CONSIDERATION

• BIOLOGICAL CONSIDERATION

• ESTHETIC CONSIDERATION

• REFERENCES

INTRODUCTION

• Tooth preparation is defined as the mechanical alteration of a defective , injured or diseased tooth to best receive a restorative material that will restore the healthy state for the tooth , including aesthetic corrections where indicated along with normal form and function of tooth. (GPT 7)

BASIC PRINCIPLES

• PRESERVATION OF TOOTH STRUCTURE

• RETENTION AND RESISTANCE

• STRUCTURAL DURABILITY

• MARGINAL INTEGRITY

• PRESERVATION OF PERIODONTIUM

PRESERVATION OF TOOTH STRUCTURE

• The perpetual preservation of what remains is more important than the meticulous replacement of what is lost

• To prevent excess tooth removal

• Minimal reduction to obtain required characteristics

FERRULE EFFECT

• A ferrule , in respect to teeth , is a band that encircles the external dimension of residual tooth structure

• Sufficient vertical height of tooth that will be grasped by future crown is necessary to allow ferrule effect

❖ Provision of ferrule effect when restoring teeth with a post retained crown may optimize the strength of the restored tooth

❖ This effect binds the remaining tooth structure together

preventing root fracture during functions.

RETENTION AND RESISTANCE FORMS

• RETENTION defined as the ability of the preparation to prevent displacement of the restoration in a direction opposite to path of insertion

• RESISTANCE defined as the ability of prosthesis to resist displacement by forces directed in an apical or oblique direction

• Retention and resistance are reciprocative to one another

RETENTION

• The essential element of the retention is two opposing vertical surfaces of the same preparation

• May be external or internal

• External surfaces such as buccal and lingual walls of a full veneer crown (sleeve retention)

• Internal surfaces such as buccal and lingual wall of the proximal box of a inlay

Fig. 1 A crown depends primarily on external retention to resist removal Fig.2 An inlay depends on internal retention to hold it within its preparations

TYPES

• PRIMARY RETENTION

• Sleeve retention by opposing vertical surfaces

• Wedge retention seen in intracoronal restorations

• SECONDARY RETENTION

• Obtained by retentive features such as retentive pins , grooves , boxes .

F

• TAPER

• FREEDOM OF DISPLACEMENT

• LENGTH

• PATH OF INSERTION

• SUBSTITUTION OF INTERNAL FEATURES

TAPER

• Degree of taper inversely proportional to retention form

• Sum of degree of taper is degree of convergence

• Optimum retention 4 – 10 degree convergence

• Anterior teeth should have minimum of 10 degree taper and posterior should have maximum of 22 degree

• A tapering fissure diamond bur is ideal to produce taper

FREEDOM OF DISPLACMENT

• Limiting the freedom of displacement from torquing and twisting forces aid to increase the resistance

• A single path of insertion is best for retention

• There will be loss of retention in over tapered restoration due to multiple path of removal

• A proximal box with its wall at right angle to the pulpal wall prevents sliding of restoration

A) Retention is improved by limiting the paths of withdrawal.

B) A preparation with unlimited freedom of displacement is much less retentive.

LENGTH

• Greater the height of crown , better the retention

• Length of crown improves retention in two ways :

• firstly ,the height of prepared tooth should be greater than tipping arc of displacement to prevent displacement

• Secondly , increase in height increases area of cementation

A) Preparation with longer walls interfere with tipping displacement of the restoration

.

PATH OF INSERTION

• “It is an imaginary line along which the

restoration will be placed onto or removed from the preparation”

• It should be accurately determined using a surveyor

• Intra orally the preparation can be evaluated by using mouth mirror over preparation and with one eye

• A facial path of insertion is avoided as it produces a prominent unaesthetic

cervical margins

• The mesiodistal inclination should be parallel to proximal inclinations of

adjacent teeth

SUBSTITUTION OF INTERNAL FEATURES

• Internal features such as proximal grooves , boxes and retention pin holes can be incorporated to improve retention

• INDICATIONS

• Over tapered preparations

• Partial veneer crowns

• Absence of two opposite walls

Interproximal grooves for added retention

STRUCTURAL DURABILITY

• The ability of restoration to withstand destruction due to external forces

• Durability comes with the thickness of restoration

• Adequate reduction is required to obtain adequate thickness of restoration

• Amount of reduction depends on type of restorative material and design of

restoration

FEATURES FOR DURABILITY

OCCLUSAL REDUCTION

• Occlusal strength is most important as

most of the forces act directly on occlusal surface

• Inadequate clearance may lead to a weaker restoration that is prone to fracture

Inadequate occlusal reduction does not

provide the needed space for cast restoration of adequate thickness.

• OCCLUSION REDUCTION ACCORDING TO RESTORATION

• For gold alloys – 1.5 mm clearance for functional and 1 mm for non functional cusps

• Metal ceramic – 1.5 – 2 mm for functional and 1- 1.5 mm for non functional

• All ceramics - 2 mm throughout

FUNCTIONAL CUSP BEVEL

-To increase the thin

occluso – axial junction of the restoration

• Additional thickness is required as

functional cusp

undergoes maximum masticatory load

• Palatal cusp for maxillary and buccal cusp for mandibular

AXIAL REDUCTION

• Adequate axial reduction is required for structural durability

• Inadequate reduction may lead to over contoured proximal surface

• Axial reduction is done such that it aligns the abutment parallel to one another

• Over reduction may leads to loss of retention

AXIAL REDUCTION

❖Plays an important role in securing space for an adequate thickness of the restorative

material.

❖Inadequate axial reduction will have thin walls subject to distortion or result in over

contouring the axial surface which could lead to periodontal problems.

MARGINAL INTEGRITY

• Marginal adaptation and seating of restoration affect marginal integrity

• Poor marginal adaptation leads to marginal leakage and secondary caries

• Casting shrinkage may lead to marginal discrepancy

• INDICATIONS FOR SUB GINGIVAL MARGINS

• Teeth with short clinical crowns

• Sub gingival caries

• Cervical erosion

• Esthetics

• Additional retention

• Metal ceramic margins

FINISH LINE CONFIGURATIONS

❖ A finish line should have following characteristics :

• Shallow bevels parallel to cavosurface margin should be avoided

• There should be increased angulation of bevel

• The bevel should not make an acute angle at margins

• During finish line preparation , tooth

should not be reduced greater than half of the width of diamond

FINISH LINE CONFIGURATION

❖ Chamfer

❖ Heavy chamfer

❖ Shoulder

❖ Shoulder with bevel

❖ Knife edge

Margin configurations

CHAMFER

• Possesses a curved slope from the axial wall till the margin

• Produced by torpedo diamond point

• Finish line of choice for cast metal restoration

CHARACTERISTICS

• Less stress and good success rate

• Heavy chamfer is used to provide 90 degree cavosurface margin

• Internal angle

• Improper reduction will produce an undesirable fragile piece of enamel (LIPPING)

SHOULDER

• Possesses a gingival finish wall perpendicular to the axial surfaces

• Preferred for all ceramic restoration

• Used for anterior restoration where aesthetics is a prime concern

Shoulder with a bevel on the

occlusal shoulder of an MOD onlay

CHARACTERISTICS

• Finish line of choice for ALL CERAMIC RESTORATION

• Wide ledge gives resistance

• Requires more tooth reduction

SHOULDER WITH BEVEL

• Similar to shoulder , but an external bevel is created on the gingival margin

• Used on the proximal box of inlays and onlays

• Can be used as facial finish line for metal ceramic restoration

• ADVANTAGE OF BEVEL

• Aids in contouring the restoration

• Improves burnish ability

• Minimizes marginal discrepancy

• Prevents chipping off of unsupported enamel

KNIFE EDGE

• An extremely thin preparation

• Difficult to wax up and cast

• Difficult to produce a smooth margin

• Susceptible to distortion

INDICATIONS

• Lingual surface of mandibular posteriors

• Very convex axial surface

• Undercut in tipped teeth

FINISH LINE FEATURES

• Determines the outcome of restoration

• Placed in an accessible region , for easy maintenance

• Reproducible

• Ease of removal

• Preferably within enamel

• Most preferable supra gingival

PRESERVATION OF THE PERIODONTIUM

❖It has a direct bearing on the ease of fabrication and the ultimate success of the restoration

❖The best results can be expected from margins that are as smooth as possible and are fully exposed to cleansing action

❖Sub gingival finish line predispose to periodontitis

• Distance between finish line and alveolar crest must be 2 mm

• If decreased , may lead to gingival

inflammation , loss of alveolar crest and pocket formation

PRINCIPLES OF TOOTH PREPARATION

• Biological consideration

• Mechanical consideration

• Esthetic consideration

BIOLOGIC CONSIDERATION

• Conservation of tooth structure

• Avoidance of overcontouring

• Supra gingival margin

• Harmonious occlusion

• Protection against fracture

MECHANIC CONSIDERATION

• Retention form

• Resistance form

• Deformation : It should not undergo

structural changes under occlusal forces

ESTHETIC CONSIDERATION

• Minimum display of metal

• Maximum thickness of porcelain

• Sub gingival margins

SOFT TISSUE:

❖Damage to the soft tissues of tongue and cheeks can be prevented by careful retraction with an aspirator tip or a mouth mirror or a flanged saliva ejector.

Pulp

❖Great care is also needed to prevent pulp

injuries during fixed prosthodontic procedures especially complete crown procedures.

❖Selection of technique and material becomes important to prevent any pulpal damage.

Causes of pulp injury

❖Temperature i.e. heat generated by friction between rotary instrument and tooth surface being prepared.

❖Chemical action of certain dental materials ( bases, solvents, luting agents) can cause pulpal damage particularly when they are applied to freshly cut dentin.

❖Bacteria, that either were left behind or gained access to the dentin.

ARMAMENTARIUM

• End cutting burs

• Torpedo diamonds

• Torpedo carbide burs

• Flat end tapered diamonds

• Round end diamonds

• Abrasive paper discs

• Rubber dam

BUR DESIGNS

REFERENCES

• Penny RE, Kraal JH. J Prosthet Dent.1979;42: 34-38

• Jacobi R, Shillingburg HT, Duncanson MG. J Prosthet Dent.

1985;54:178-183

• Reynold JM. Abutment selection for fixed prosthodontics.

J Prosthet Dent. 1968;19:483-488

REFERENCES

• shillingburg fundamentals of tooth preparation

• crowns revision part 2 –tooth preparation guidelines

• textbook of prosthodontics.