Principles of tooth

preparation

Dr. Hsu Zenn Yew

Dr Jasmina Qamaruz Zaman

Department of Operative Dentistry

11/7/2012

Definition

Tooth preparation is the mechanical

alteration of defective, injured or diseased

tooth to receive a restorative material that re-

establishes a healthy state for the tooth;

√ normal form & function

√ esthetic (corrections where indicated)

√ Longevity

√ Comfort

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Indications

1. To provide restoration for

carious teeth (seen clinically

or radiographically)

2. To restore form and

function (eg: Malformed

teeth – due to fracture,

tooth wear, congenital

malformations

Amelogenesis imperfecta

Fractured tooth

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Indications

3. To improve

esthetics/appearance (eg:

diastema, discoloration)

Fluorosis

Diastema

4. Replace defective restorations

(secondary caries, improper

proximal contacts, overhang,

open margins, poor esthetics,

bulk fracture)

Defective amalgam margins

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Biological

•Maintain vitality of the tooth

and protect the pulp

•Eliminate defects and diseased

part of tooth

•Be as conservative as possible

•Prepare the tooth so that the

tooth and restoration

remain intact under

masticatory

forces

Produce a functional

restoration

•Retention form

•Resistance form

•Convenience form

•Finishing

•Debridement

Mechanical

Esthetic

Produce an esthetic result

•Minimum display of metallic

restoration

•Adequate esthetic material

thickness

Objectives of Tooth Preparation 11/7/2012

Terminology

Distal Buccal

Mesial

Lingual

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Terminology

BUCCAL

buccoaxial

buccoaxiogingival

Buccal wall

LINGUAL

Line angle = line

between 2 cavity walls

Point angle = point

where 3 point angles

meet

Margin = angle

between the prepared

cavity and outer

surface of the tooth

(aka cavosurface angle

cavosurface margin)

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Cavo-surface margin

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Cavo-surface angle

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DISTAL MESIAL

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Principles of tooth

preparation

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History: Traditional cavity

preparation GV Black (1836-1915)

Described 6 stages of cavity preparation

EXTENSION FOR PREVENTION – the restoration is extended into sound fissures to prevent caries from occurring: destructive to the tooth.

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Reasons for Extension for

Prevention

Prevent recurrence of decay in the surface of

the enamel next to the restoration

Self-cleaning benefit of the embrasures with

saliva and fluids of the diet.

Properties of amalgam

adequate strength in bulk but brittle in small

sections

does not adhere to tooth structure

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Features of cavities with Extension

for Prevention

Occlusal surface extends through pits and fissures

Proximal line angles extended

Bucally and lingually through embrassures

Cervically below the gingival margin

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Features of cavities with Extension

for Prevention

Margins of the restoration placed on line angles of the

tooth Mesiobuccal, distobuccal, mesiolingual, distolingual line angles of

the tooth

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GV Black’s principles of cavity preparations

Steps:

1. Outline form

2. Resistance form

3. Retention form

4. Convenience form

5. Removal of remaining caries

6. Defining and finishing enamel margins

7. Debridement of the cavity (Cavity toilet)

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GV Black’s principles of tooth preparations

Very precise preparations

Flat walls

Flat floors

Uniform depth

Applied to

Amalgam

Cast Gold

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Biological

•Prevent recurrence of decay in

the surface of the enamel next

to the restoration

•Self-cleaning benefit of the

embrasures with saliva and

fluids of the diet

1. Outline form

2. Resistance form

3. Retention form

4. Convenience form

5. Removal of remaining

caries

6. Defining and finishing

enamel margins

7. Debridement of the

cavity (Cavity toilet)

Mechanical

??Esthetic

Extension for prevention 11/7/2012

Reasons why extension for prevention

is no longer relevant.

Natural and fluoride-induced remineralization Effect of calcium and phosphate from saliva

Fluoride introduction through water, toothpastes, mouth rinses, restorative materials, ect.

Advancements in instrumentation Smaller and more precise instruments

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Reasons why extension for prevention

is no longer relevant.

Introduction of new restorative materials Composite resin requirements v. conventional amalgam

Design of more conservative preparations Slot preparations and PRR’s

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Modern Restorative Concept

Minimal Intervention

1. Reduces cariogenic bacteria

2. Uses preventive measures

3. Early lesions remineralised

4. Minimal cavity preparation on cavities /

Conservative preps

5. Repair of defective restorations.

Arrested lesion

Early lesion

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Why conservative preparations?

d. Preservation of Tooth Structure

Preserve tooth structure

Enamel doesn’t regenerate

Minimize pulpal insult

Tooth retains strength

Less effect on surrounding teeth/tissues

Minimizes esthetic problems

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How to achieve conservative preps?

1. Minimal extension of

preparation

2. Supragingival margins

3. Rounded internal line

angles

Modified tooth preparation

design

GV Black’s

Modified cavity

preparation

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Modified preparations

Applied to CR, GIC and

other bonded materials

non uniform depths

shallower preps

marginal angles of 90

degrees or more

enamel bevels (for CR)

less need for retention and

resistance form

preparation features (eg:

locks, slots, pins and

grooves)

Traditional Black’s cavity

design

Modified tooth preparation design

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Fundamental concepts for tooth

preparation

Applies for all tooth preparation (conventional

and modified):

1. No weak tooth structure left (e.g undermined

enamel)

2. The fault, defect or caries is removed

3. The remaining tooth structure is left as strong as

possible

4. The underlying pulp tissue is protected

5. The restorative material is retained in a strong,

esthetic (in some cases)and functional manner. 11/7/2012

Steps for cavity preparation

1. Access and outline form

2. Caries removal

3. Cavity preparation - Resistance form,

Retention form, Convenience form,

Finishing

4. Cavity debridement

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ACCESS AND

OUTLINE FORM

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Access and Outline form

The outline form is the shape that the cavo-

surface angle will assume after the cavity has

been prepared.

Determined by:

Extent of caries

It is obtained by removing all enamel

undermined by caries.

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Access and Outline form

Principles:

1. All weakened enamel must be removed

2. All faults should be included

3. Margins ideally placed in areas which is easy

to finish.

Undermined/

weakened enamel

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Access and Outline form

Specifications:

Must be able to see the DEJ

Extend preparation to

sound tooth structure

Enamel

DEJ

Maintain limited pulpal or

axial depth

Caries in dentin is left at

this stage Caries spread laterally at the DEJ

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Access and Outline form

Enamel wall / margin

must rest on

sound dentine

correct

Incorrect

Access cavity needs to be wide enough to enable

you to see all the carious dentine esp at the DEJ

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Access and Outline form

Features: 1. Preserve cusp strength – avoid

terminating margins on cusp

heights or ridge crests

2. Preserve marginal ridge strength

(maintain at least 1.5mm, may be

slightly less for bonded

restorations)

3. Minimize bucco-lingual

extensions

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Access and Outline form

Features:

4. Apply fissure widening

(enamel biopsy/

enameloplasty) where

necessary

5. Restricting depth into dentine

6. Connecting 2 close

(<0.5mm apart) tooth

preparations

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Access and Outline form

Features:

7. Gingival margins of proximal cavities extended apically to

provide clearance of 0.5mm between gingival margin and

adjacent tooth.

Enhance placement of matrix, insertion of material, finishing of restorative

material and defining gingival margin of prep)

8. Buccal and lingual margins of proximal cavities extended into

embrasures to provide clearance. However, if patient’s caries risk is low and oral hygiene is good, it is

permissible to leave the buccal and lingual margins in contact with adjacent

tooth)

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CARIES EXCAVATION

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Caries excavation

Determined by:

Location and size of demineralized area/zone

Proximity of carious lesion to pulp

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Principles:

1. All infected caries must be removed

2. All decalcifications at cavosurface margin

must be removed

Caries excavation

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Infected dentin

Characterized by irreversibly denatured collagen

Bacteria present

Not able to be remineralized

Must remove during tooth preparation

Affected dentin

Reversibly denatured collagen

Capable of remineralization

Not to be removed during tooth preparation

*Fusayama T. Two layers of carious dentin: diagnosis and treatment, Oper Dent 4:63-70, (1979).

Caries excavation

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Caries – soft to explorer, may be yellow,

orange or brown stain. If hard (calcified)

leave stain.

Leave calcified (hard) dentin. Colour

does not matter.

Caries excavation

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BREAK

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CAVITY

PREPARATION

•Retention form

•Resistance form

•Convenience form

•Finishing

•Debridement

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RESISTANCE FORM

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Resistance form

Definition: Resistance form is the design of a

cavity which enables the remaining tooth

substance and the restorative material to

withstand masticatory stress.

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Resistance form

Determined by:

Biological and physical

properties of enamel and

dentine

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Resistance form

Determined by:

Biological and physical properties of

restorative material

Magnitude and direction of occlusal forces

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Resistance form Create adequate resistance form

Principles:

1. Preserve adequate bulk of tooth (or protect it)

2. Provide adequate bulk of restorative material

(Amalgam need at least 1.5mm. This principle is less

important for CR and GIC/RMGIC)

3. Create shapes that resist or diffuse stress.

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Resistance form

Features:

1. Relatively flat floors

2. Box shape

3. Rounded internal line angles

4. Preservation of cusps and marginal ridges (ie restrict extension

of prep)

5. Inclusion of weakened tooth structure

6. Adequate thickness of restorative material

7. Reduction of cusps for capping when indicated.

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Resistance form Create adequate resistance form

Features:

1. Box shape with relatively flat floor.

Help tooth resist occlusal loading

because it is at right angles to the

force of mastication.

A. Flat floor prevent restoration movement

B. Rounded floor – may allow non bonded

restoration to rock/ rotate and dislodge or

wedge tooth apart – tooth fracture.

However, sound tooth tissue should, not be

removed simply to obtain a flat pulpal floor.

Place a base

A

B

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Resistance form Features

2. Rounding of internal line angles to

reduce stress concentration in line

angles. External outline form should

also be rounded for the same reason.

Stress concentrated in sharp line angles

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Resistance form Features:

3. Restrict extension of preparation (as small as possible).

• Allow strong cusps and ridges to remain with sufficient

dentine support.

• A marginal ridge which is too weak during prep of

occlusal cavity - Class II cavity may have to be prepared

instead, so as to eliminate the weak marginal ridge.

• Especially where the ridge is only of enamel thickness

and unsupported by sound dentine.

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Resistance form

Features: 4. Cap weak cusps

Rule for cusp capping:

a. If cavity margin is less than ½ the

distance form the primary groove to

cusp tip – NO CAPPING

b. If cavity margin is ½ - 2/3 distance

from primary groove to cusp tip –

CONSIDER TO CAP

c. If extension > 2/3 distance from

pimary groove to cusp tip – CUSP

CAPPING

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Resistance form Note fracture line on distal surface. This further weakens

already weaken cusps. Prevent by cusp capping.

THE RESULT

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Resistance form

Features:

5. Enough thickness of restorative material to prevent

fracture under occlusal load.

Min thickness amalgam = 1.5mm

Min thickness CR = 1mm

(Bonded restorations (eg: CR material to tooth structure

may increase strength of weakened tooth structure and

reduce fracture)

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RETENTION FORM

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Retention form

Definition: Shape of

preparation that resists

displacement or removal

of restoration by tipping

or lifting forces.

Retention from is less

critical for adhesive

materials (GIC / CR).

Occlusal

proximal

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Retention form

Determined by:

General shape of prep

Surface area and position of

axial walls

Magnitude and direction of

(para)functional forces

Type of material used

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Retention form

Principle:

Create a shape that provides macro or micro-

mechanical locks or frictional resistance to

dislodgement of the restoration along any path

of withdrawal.

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Retention form Features:

Related to restorative material used:

Amalgam

Macromechanical interlocking

1. Undercut (external walls that converge

occlusally). Must not overdo– can leave

unsupported enamel rods on cavosurface

margin

2. Box form

X

Friction

at walls

X

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Retention form

Feature:

Amalgam

3. Retentive groove / slots

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Retention form Feature:

Amalgam

4. Pins

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Retention form Features:

Amalgam

5. Dovetail

Extension of the occlusal portion of a

Class II cavity into carious pits and

fissures which resists the displacement

of the Class II cavity in proximal

direction.

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Retention form

Feature:

Composite resin –

Macromechanical and

micromechanical interlocking

– between bonding resin and

tooth.

Micromechanical porocities

created by acid etching

procedure

Macromechanical interlocking

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Retention form

Feature:

Glass ionomer cement–

chemical adhesion by

material to enamel

and dentine.

Macromechanical

interlocking

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Retention form

Features:

Feature How does it work?

Walls oppose and parallel Friction

Wall oppose and converge (undercut) Macro lock

Floors flat and parallel occlusion Anti-slip

Walls 90º or less to floors Anti-torque and shear

Adequate surface area of retention

walls

Friction

Dovetails Macro lock

Accessory retention (Pins, amalgapins,

grooves, bonding)

Macro and micro lock

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After you have created the cavity to receive the

material, check again the preparation prior to

finishing, cleaning the cavity and insertion of

the restorative material.

Consider convenience form

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CONVENIENCE FORM

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Convenience form

Definition: Shape or form of the preparation that

allows adequate observation, accessibility and

and ease of operation in preparing and restoring

the tooth.

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Convenience form

Features:

1. Cavity is wide enough to admit instruments used in operative procedure.

2. Margins of restoration placed in areas accessible for proper finishing.

3. Outline wide enough to allow visualization of the DEJ and other areas of the cavity.

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FINISHING

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Finishing

Determined by:

1. Biological and physical properties of enamel and dentine at tooth restoration interface

- Direction of enamel rods

- Support for enamel rods at DEJ

- Location of the margin

-Degree of smoothness/roughness required.

2. Biophysical properties of restorative material

at tooth restoration interface.

- Type of restorative material to be placed

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Finishing

Principles:

Margin of tooth and restoration must be

fracture resistant

Margin of restoration can be properly finished

Objectives

1. Create best marginal seal between tooth and restoration

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Finishing

Features:

1. Create smooth and accessible margins

2. Margin shape approprite for material (see following slides)

3. Unsupported enamel removed

4. Preparation outline must be smooth curves , line angles rounded off

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Finishing

Type of Cavosurface Margins.

Amalgam –

- All unsupported enamel must be removed.

- Margins must be smooth.

- Bevel - ? Gingival cavosurface margin

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Finishing

Type of Cavosurface Margins.

Composite resin – bevel in some areas

- Buccal enamel (eg Class III cavity): Acceptable to

leave unsupported enamel in the anterior teeth.

- Unsupported enamel in the posterior teeth, (esp on

occlusal margin) must be removed.

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Finishing

Type of Cavosurface Margins.

Composite resin – bevel in some

areas

Function:

1. Improves retention – increase

surface area, enamel rods are cut

perpendicularly – better etching

pattern compared to enamel rods

cut longitudinally.

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Finishing

Cavosurface Margins.

Composite resin – bevel in some areas

Function:

2. Adjacent minor defects may be included with the

bevel

3. Esthetics may be improved by creating a gradual

transition between restoration to tooth

4. Marginal seal may be enhanced.

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CAVITY

DEBRIDEMENT

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Cavity debridement

Wash debris away with water spray.

The cavity is dried with air spray.

Examine the cavity from all aspects to check

for defects.

Errors must be corrected.

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Cavity debridement

Feature:

The preparation is clean and dry but not

dessicated.

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References

Studervant’s ART AND SCIENCE OF

OPERATIVE DENTISTRY pg: 283-319

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