‫جابر فائز االبراهيمي‬.‫د‬
Lec .6
Principles of cavity preparation
DEFINITION OF CAVITY PREPARATION
Cavity preparation is the mechanical alternation of a tooth to receive a restorative
material, which will return the tooth to proper anatomical form, function, and esthetics.
The procedure of the preparing the tooth is the removal of the defective or friable tooth
structure. Any remaining infected or friable tooth structure may result of further
carious progression, sensitivity or pain or fracture of the tooth and restoration.
OBJECTIVES OF CAVITY PREPARATION
1. To remove diseased tissue as necessary and at the same time provides the protection
to the pulp. (caries removal)
2. To locate the margins of the restoration as conservative as possible.
3. Form the cavity, so that both the restoration and tooth can withstand the load of
mastication.
4. To allow the esthetic and
functional placement of the restorative
material.(restoration replacement)
Indication of Restoration
1. Caries.
2. Malformed , discolored or fractured teeth.
3. Restoration replacement.
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STEPS IN THE CAVITY PREPARATION (Given by G V Black)
Initial cavity preparation stage
Step 1- Obtaining Outline Form
Step 2- Obtaining Primary Resistance Form
Step 3- Obtaining Primary Retention Form
Step 4- Obtaining Convenience Form
Final cavity preparation stage
Step 5 - Removal of Remaining Carious Dentin
Step 6 -Obtaining Secondary Resistance & Retention Form
Step 7- Providing Pulp Protection
Step 8- Finishing of Enamel Walls & Margins
Step 9- Performing the Toilet of the Cavity
Step 1: Outline form
Outline form of the cavity is the shape which the cavosurface line angle of the cavity
assumes after preparation.
Principles:
The three general principles on which outline form is established regardless of the type
of tooth preparation being prepared are as follows:
(1) All unsupported or weakened (friable) enamel usually should be removed,
(2) All faults should be included, and
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(3) All margins should be placed in a position to allow finishing of the margins of the
restoration. The third principle has ramifications that differ for pit-and-fissure
preparations compared with smooth-surface preparations.
Access:
1. Gain initial access via the most carious part of the tooth.
2. Remove all unsupported enamel to expose the entire extent of caries.
Class I outline form
Class II outline form
Step 2: Primary Resistance Form
Definition
Primary resistance form may be defined as the shape and placement of the preparation
walls that best enable the remaining tooth structure and the restoration to withstand,
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without fracture, masticatory forces delivered principally in the long axis of the tooth.
The relatively horizontal pulpal and gingival floors prepared perpendicular to the
tooth's long axis help resist forces in the long axis of the tooth and prevent tooth
fracture from wedging effects caused by opposing cusps.
Principles
(1) using a box shape with a relatively horizontal floor, which helps the tooth resist
occlusal loading by virtue of being at right angles to the forces of mastication that are
directed in the long axis of the tooth;
(2) Restricting the extension of the external walls to allow strong cusp and ridge areas
to remain with sufficient dentin support.
(3) Having a slight rounding of internal line angles to reduce stress concentrations in
tooth structure.
(4) Reducing and covering (capping) weak cusps and enveloping or including enough
of a weakened tooth within the restoration in extensive tooth preparations to prevent or
resist fracture of the tooth by forces in the long axis and obliquely (laterally) directed
forces
(5) Providing enough thickness of restorative material to prevent its fracture under
load.
(6) Bonding the material to the tooth structure, when appropriate. Conventional and
beveled preparation designs provide these resistance form principles.
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Features
• To achieve resistant form, the prepared cavity should possess the following:
1. Relatively horizontal, flat floor at right angles to the line of stress :
 Make the floor of the prepared cavity flat
 Make the floor of the cavity at right angles to direction of the occlusal forces.
 No sound tooth structure should be removed to achieve this.
2. Box-like shape.
3. Cavosurface line angle should be sharp 90 degree.
4. Width of the cavity In practice one-quarter (1/4) of the intercuspal distance at
isthmus area.
5. Mesial &distal wall shoud be with the direction of enamel rods to prevent
unsuported enamel formation ( diverge occlusaly) .
6. Occlusal cavity margins in the areas not subjected to excessive occlusal trauma.
 Occlusal margins of the cavity should not be subjected to heavy stress.
 Place them about 1/4 of intercuspal distance.
7. Inclusion of weakened tooth structure
8. Preservation of cusps and marginal ridges
9. Rounded internal line angles
10. Adequate thickness of restorative material .The depth should be enough to take
adequate bulk of the restorative material which can withstand forces of
mastication.Amalgam thickness should be 1.5 — 2 mm at least.
11. Reduction of cusps for capping, when indicated.
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Step 3: Primary Retention Form
Primary retention form is the shape or form of the conventional preparation that
prevents displacement or removal of the restoration by tipping or lifting forces
for nonbonded restorations.
Retention form
1. Undercuts :(converge buccal & lingual wall)
2. Dovetail lock
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3. Flat floor and definite internal line angles.
4. Grooves ,Holes and boxes (in class 2 restoration )
5. Acid etch technique(in composite restoration )
6. adhesives(in composite restoration )
7. Increase the surface area of the enamel available for bonding by a beveled or
flared (>90 degrees) enamel marginal configuration. (in composite restoration).
Occlusal convergense
Dovetail design
Step 4: Convenience Form
Convenience form is the shape or form of the preparation that provides for
adequate observation, accessibility, and ease of operation in preparing and
restoring the tooth. Occasionally, obtaining this form may necessitate the
extension of distal, mesial, facial, or lingual walls to gain adequate access to
the deeper portion of the preparation.
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Features
• To achieve convenience form, the prepared cavity should possess the
following:
1. Make the cavity wide enough to admit the instruments you are using.
2. Extending proximal preparations beyond proximal contacts. (Preparing the
proximal walls to obtain clearance with an adjacent proximal surface affords
better access to finish the preparation walls and the restorative material and
to place a matrix). In class II direct amalgam restoration .
3. The occlusal divergence of vertical walls of tooth preparations for Class II
cast indirect restoration.
Step 5: Removal of Any Remaining Enamel Pit or Fissure,
Infected Dentin, or Old Restorative Material, If Indicated
 Elimination of any infected carious tooth structure or faulty restorative
material left in the tooth after initial tooth preparation.
 Caries left in the pulpal/ axial floor is excavated thoroughly making the
cavity deeper. 0.75- 1 mm of dentin should cover the pulp. All the
infected/ soft dentin should be removed.
 Placement of cavity margins in areas of lessened caries susceptibility
 It is not necessary in low caries experience patients.
 It is advisable to include the pits and fissures in an amalgam cavity in
cases of high caries rate patients (Extension for prevention).
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Step 6: Pulp Protection, If Indicated
The placement of liners and bases , it is a step in adapting the preparation for
receiving the final restorative material. The reason for using liners or bases is to
protect the pulp or to aid pulpal recovery or both.
Pulpal irritation that occurs during or after operative procedures may result from
(1) Heat generated by rotary instruments,
(2) Some ingredients of various materials,
(3) Thermal changes conducted through restorative materials,
(4) Forces transmitted through materials to the dentin,
(5) Galvanic shock.
(6) The ingress of noxious products and bacteria through microleakage.
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Step 7: Secondary Resistance and Retention Forms
Additional resistance and retention features may be deemed necessary for the
preparation. Many compound and complex preparations require these additional
features.
The secondary retention and resistance forms are of two types: (1) mechanical
preparation features and (2) treatments of the preparation walls with etching,
priming, and adhesive materials.
1. Mechanical Forms:
A variety of mechanical alterations to the preparation enhance retention form,
and these alterations require additional removal of tooth structure. This include
the following:
1. Retention grooves, Coves, Slot, steps
2. Preparation extensions.
3. Beveled Enamel Margins
4. Pins.
2. Placement of Etchant, Primer, or Adhesive on Prepared Walls.
Certain alterations to the preparation walls by actions of various materials also
afford increased retention and resistance to fracture. Enamel and dentin surfaces
may be treated with etchants or primers or both for certain restorative
procedures.
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(A) Lock, (B) Slot, (C) Coves
Step 8: Procedures for Finishing the External Walls of the
Tooth Preparation
Finishing the preparation walls is the further development, when indicated, of a
specific cavosurface design and degree of smoothness or roughness that
produces the maximum effectiveness of the restorative material being used.
Objectives
The objectives of finishing the prepared walls are to (1) create an optimal
marginal junction between the restorative material and the tooth structure, (2)
afford a smooth marginal junction, and (3) provide maximal strength of the
tooth and the restorative material at and near the margin.
The following factors must be considered in the finishing and correction of
enamel walls and margins:
(1) The direction of the enamel rods, no undermined enamel rods.
(2) The support of the enamel rods at the DEJ and laterally (preparation side),
(3) The type of restorative material to be placed in the preparation,
(4) The location of the margin,
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(5) The degree of smoothness or roughness desired.
(6) Cavosurface angle 900.
(7) No beveling for amalgam.
(8)
Composite needs a bevel to provide greater enamel surface area for
etching and micromechanical retention.
Composite (A), amalgam (B)
Step 9: Cleaning of the cavity
•
The prepared cavity should be free from all debris
•
No disinfectant should be used to clean the cavity.
•
Don’t desiccate it.
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