PRINCIPLES OF TOOTH PREPARATION (LECTURE-1)

CONTENTS- Tooth preparation is governed by the principles of tooth preparation:- 1) Preservation of tooth structure 2) Retention & Resistance 3) Structural durability 4) Marginal integrity 5) Preservation of the periodontium (According to Shilinburg) Definition: The process of removal of diseased and/or healthy enamel and dentin and cementum to shape a tooth to receive a restoration

1. Preservation of Tooth Structure While replacing a lost tooth structure, a restoration must preserve the remaining tooth structure as much as possible. Tooth structure is conserved by using the following guidelines:- 1. Use of partial coverage rather than complet coverage restorations. 2. Preparation of teeth with minimum practical convergence angle (taper) between axial walls. 3.  Preparation of the occlusal & labial surface so reduction follows the anatomic planes to give uniform thickness in the restoration.

4. Preparation of the axial surfaces so that the tooth structure is removed evenly: if necessary, teeth should be orthodontically repositioned. 5.  Selection of a conservative margin compatible with the other principles of tooth preparation. 6.  Avoidance of unnecessary apical extension of the preparation.

Factors for –Resistance- 2. Retention & Resistance RETENTION:- Resistance to removal of restoration in the path of insertion. RESISTANCE:-Prevention of dislodgement of a restoration from apical, oblique and horizontal forces. Factors for –Retention- Degree of taper Freedom of displacement Length of preparation Substitution of internal features Factors for –Resistance- Degree of taper Freedom of displacement Length & width of preparation Leverage action from the oblique forces. Rotation around the vertical axis

TAPER:- The axial walls of the preparation must taper slightly to permit the restoration to seat.i.e. 2 opposing external walls must gradually converge – ANGLE OF CONVERGENCE. 2 opposing internal surfaces of the tooth structure must diverge occlusally – ANGLE OF DIVERGENCE.

The more nearly parallel the opposing walls of a preparation, the greater should be the retention. Most retentive preparation should be one with parallel walls, but the parallel walls are impossible to create in the mouth without producing preparation undercuts. An UNDERCUT - is defined as a divergence between opposing axial walls, or wall segments, in a cervical- occlusal direction.

FREEDOM OF DISPLACEMENT:- Retention is improved by geometrically limiting the numbers of paths along which a restoration can be removed from the tooth preparation. Maximum retention is achieved when there is only one path. Limiting the freedom of displacement from torqueing or twisting forces in a horizontal plane increases the retention of a restoration.

LENGTH:- Occlusogingival length is an important factor in both retention & resistance. Longer preparations will have more surface area & therefore will be more retentive . The length must be great enough to interfere with the arc of casting pivoting about a point on the margin on the opposite side of the restoration.

SUBSTITUTION OF INTERNAL FEATURES:- It may not be possible always to use opposing walls for retention. Therefore, internal features such as the groove, the box form, & the pin hole can be substituted for an axial wall or for each other.

PATH OF INSERTION:- It is an imaginary line along which the restoration will be placed onto or removed from the preparation. It is of special importance when preparing teeth to be fixed partial denture abutments, since the paths of all the abutment preparations must parallel each other. Surveying visually- it is the primary means of insuring that the preparation is neither undercut nor over-tapered.

Leverage Action From The Oblique Forces Rotation around the vertical axis Width of the preparation

REFERENCES Fundamentals of Fixed Prosthodontics, Herbert T. Shillingburg Contemporary Fixed Prosthodontics ,Stephen F.Rosensteil