Principles Of Tooth Preparation Introduction
The success of fixed prosthodontics depends on the design of tooth preparation. Tooth preparation to receive cast restoration is based on principles of tooth preparation.
Table of Contents
Rationale Of Tooth Preparation
Tooth preparation is done to receive the prosthesis.
It is done to make the walls parallel or tapered and to devise the path of insertion for a prosthesis.
Tooth preparation is done to achieve adequate retention and resistance form.
To establish an adequate finish line as the endpoint of restoration.
Read and Learn More: Preclinical Prosthodontics Notes
Principles Of Tooth Preparation
The principles governing the tooth preparation to receive fixed restorations are discussed as under.
Preservation of Tooth Structure:
The fixed prosthesis must replace the lost tooth structure, but at the same time, it should not excessively reduce the remaining tooth structure.
Making depth orientation grooves, before actual reduction prevents excessive tooth preparation.
Prefer partial veneer crowns to save tooth structure wherever possible.
Tooth preparation should be done under adequate coolant.
Supragingival finish lines required less tooth preparation compared to subgingivally finish lines.
Retention and Resistance:
Retention:
It is the ability of the preparation to prevent the removal of restoration along its path of insertion (vertical forces).
Resistance:
It is the ability of the preparation to prevent dislodgement of the restoration by forces directed in an apical/oblique/horizontal direction and to prevent any movement of the restoration under occlusive forces.
Factors Affecting Retention:
Degree of taper: The prepared opposite walls should have a tapered configuration. The preparation of opposite walls has a taper of 6° (3° on either side). To achieve the desired amount of taper on the wall, a diamond point of known taper is used.
If the given taper is more, there will be a reduction in the retention. The total surface area of the cement film: Retention is directly proportional to the available cementing area. If more tooth surfaces are available, the retention achieved will be more compared to lesser tooth surfaces. Molars will have greater retention than anterior and premolars due to more surface area available for cementation on molars.
The roughness of the surface: A rough surface gives more retention compared to a smooth surface. This is due to an increase in surface area and micromechanical retention as a result of surface roughness. The prepared tooth cannot be made rough, so the tissue surface of the crown or the prosthesis is made rough to get the advantage of increased retention.
Factors Affecting Resistance:
Length of the preparation: During tooth preparation, a minimum height of the tooth should be maintained. The minimum length of abutment tooth should be 3–3.5 mm for adequate resistance and retention. If less crown height is available, then the operator should place the finish line subgingivally to attain more length of the prepared tooth.
Prevention of rotation around the vertical axis: Resistance to rotation around the vertical axis is a core feature to achieve resistance. To prevent rotation of the crown around the vertical axis grooves, retentive boxes, and retentive pins can be incorporated in tooth preparation.
Structural Durability:
The preparation should be so designed that the restoration will have an adequate bulk of metal or porcelain to withstand occlusal forces. The contours should be such that they prevent periodontal and occlusal problems.
The functional cusp (palatal cusp in the maxillary arch and buccal cusp in the mandibular arch) should be reduced more compared to the nonfunctional cusp to provide more bulk of the material. A functional cusp bevel is indicated for this purpose. If functional cusp bevel is not given, then the resultant casting or restoration will be weaker.
If reduction is inadequate, it results in weak restoration, occlusal discrepancy, and easy perforation of restoration. The occlusal surface preparation should follow the tooth anatomy. The inclined planes of the cusp should be maintained. Cusp tips should be rounded after preparation.
Marginal Integrity:
The success of a restoration depends upon the adaptation of the margins to the finish line. For the success of restoration, the
margins should extend up to the tooth line of preparation. It should closely adapt to the can surface of the finish line. It should maintain continuous contact with the finish line. An acute angle should be given at the margin for better adaptation. The finish line should follow the contour of the gingival margin.
Preservation of Periodontium:
During tooth preparation, there should be no trauma to periodontal tissue. Preserve gingival integrity during tooth preparation. If the finish line is to be placed subgingivally, it should not encroach upon gingival tissue.
Finish Lines:
The finish line is the termination of prepared tooth on which the restoration sits. Finish lines can be classified according to their shape and according to their location of placement.
Finish Lines According to Location:
The finish lines according to location are given in Table.
Finish Lines According to Shape:
The finish lines according to shape are described in Table.
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