PRINCIPLES OF TOOTH PREPARATION

DONE BY

DR. WOROOD HASHIM AL-AMEEDY

​

​

​

Objectives of Cavity Preparation

​

• Remove all defects and give the necessary protection to pulp.
• to Locate the margins of the restoration as conservative as possible.
• Form the cavity so that both the restoration and tooth can withstand the load of mastication.
• To allow for the esthetic and functional placement of a restorative material

Need of Restoration

• Caries

​

​

​

• Malformed, discolored or fractured teeth

​

​

​

​

• Restoration replacement

​

​

​

FACTORS AFFECTING TOOTH PREPARATION

1. General Factors

• Pulpal & periodontal status
• Occlusal relationship

​

2.Dental anatomy

• Direction of enamel rods
• Thickness of enamel /dentin
• Size and position of pulp
• Relationship of tooth to its supporting tissues

​

3.PATIENT FACTORS

• Age
• Esthetic consideration
• Economic status
• Patients with high risk caries

​

4. Restorative material factors.

Stages and Steps in Cavity preparation

​

​

• Initial cavity preparation stage

​

• Final cavity preparation stage

​

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

Definition

• The shape of the cavosurface line angle of the cavity after preparation.

​

​

​

Principles:

• all friable and/or weakened enamel should be removed
• all faults should be included
• all margins should be placed in a position to afford good finishing of the margins of the restoration.
• Margins of the preparation will be located on self –cleansing area.

​

Features:

• preserving cuspal strength
• preserving marginal ridge strength
• minimizing facio-lingual extension
• using enameloplasty
• connecting two close faults or cavities
• restricting the depth of the preparation into dentin.

​

Step 2: Primary Resistance Form

​

• Definition :

The shape and placement of the preparation walls that best enable the remaining tooth structure and the restoration to withstand, without fracture, masticatory forces delivered principally in the long axis of the tooth.

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.

Features:

• Box shape
• Relatively flat floors
• Preservation of cusps and marginal ridges
• Rounded internal line angles
• Adequate thickness of restorative materials
• Inclusion of weakened tooth structure
• Reduction of cusps for capping if indicated
• Mesial &distal wall should be with the direction of enamel rods to prevent unsuported enamel formation ( diverge occlusaly) .

​

​

STEP 3: PRIMARY RETENTION FORM�

​

Definition:

​

The shape or form of the prepared cavity that resists displacement or removal of the restoration from tipping or lifting forces for non bonding restoration.

​

Retention form

• Undercuts :(converge buccal & lingual wall)
• Dovetail lock
• Flat floor and definite internal line angles.
• Grooves ,Holes and boxes (in class 2 restoration )
• Acid etch technique(in composite restoration )
• adhesives(in composite restoration )
• Increase the surface area of the enamel available for bonding by a beveled or flared (>90 degrees) enamel marginal configuration. (in composite restoration).

​

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.

Principles :

1. Make the cavity wide enough to admit the instruments you are using.

2. Extending proximal preparations beyond proximal contacts.

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�

Definition :

Elimination of any infected carious tooth structure or faulty restorative material left in the tooth after initial tooth preparation.

​

​

​

STEP 6: PULP PROTECTION, IF INDICATED

​

• Using liners or bases to protect the pulp or to aid pulpal recovery or both.
• If the infected dentin more than 2 mm deeper from the initial pulpal or axial wall then a Liner is indicated.

​

STEP 7: SECONDARY RESISTANCE AND RETENTION FORMS�

• Most compound and complex cavity preparations require additional resistance and retention form.

​

• Mechanical forms
• 1. Retention grooves, Coves, Slot, steps
• 2. Preparation extensions.
• 3. Beveled Enamel Margins
• 4. Pins.
• Cavity wall conditioning form

treatments of the preparation walls with etching, priming, and adhesive materials.

​

​

​

​

Coves

Lock

Slot

STEP 8: FINISHING THE EXTERNAL WALLS �

​

• The design of the cavosurface angle.
• The degree of smoothness of the wall.

Objectives:

• To create the optimal marginal seal possible between the restorative material and tooth structure;
• To afford a smooth marginal junction;
• To provide maximum strength of both the tooth and the restorative material at and near the margin.

​

The design of the cavosurface angle depending on the material:

​

• Amalgam: 90°
• Composite: beveling 30°~ 40°

​

Step 9 : Final procedures

​

• cleaning
• inspecting
• varnishing
• conditioning

​

Thank you