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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