Dental Dentures

84 (Partial Dentures: Crown and Clasp)

If a tooth to be clasped is cariously destroyed or has an unsuitable form for retention, it must be crowned. The wax pattern for the required crown should be contoured using the parallelometer, or surveyor. This naturally assumes that even for single crowns, an impression of the entire arch is taken.

The path of insertion of the partial denture is determined prior to designing the wax patterns (Fig. 84).

85 (Partial Dentures: Crown and Clasp)

The contours of each wax pattern must be designed to satisfy the requirements set by the three elements of anchorage (dental rests, bracing, retention). To aid in bracing and guidance for the correct path of insertion, a parallel surface, or guiding plane, should be cut into the lingual and mesial and/or distal regions, depending on the location of the dental rest.

In reference to preparing the dental rest, see Chap. 5.1 and 9.3.

The undercut should be designed such that its angle of inclination from the path of insertion is approximately 25-30°. Optimal clasp retention is generated at this angle (Fig. 85).

86 (Partial Dentures: Crown and Clasp)

A short explanation of the preceding statements:

In its designed position, the clasp lies tension- free on the abutment tooth. It must first be moved toward the height of contour before retention occurs. The extent of retention is dependent upon the angle of the undercut and is defined by the functional equation: R = K • tan alfa (Fig. 86).

87 (Partial Dentures: Crown and Clasp)

We recognize the validity of this equation with respect to the existing intraoral findings when the extreme values are tested. The tangent of a 0° angle is 0. The retention is, therefore, also 0. On a surface without undercuts, no clasp retention occurs. The tangent of a 90° angle is not defined. If the clasp lies under a 90° overhang, the retention is also undefined (Fig. 87).