8: THE COMPLETE CAST CROWN PREPARATION

8 THE COMPLETE CAST CROWN PREPARATIONimage

Although esthetic factors may limit its application, the all-metal complete cast crown should always be offered to patients requiring restoration for badly damaged posterior teeth. The complete cast crown has the best longevity of all fixed restorations. It can be used to rebuild a single tooth or as a retainer for a fixed dental prosthesis. It involves all axial walls, as well as the occlusal surface of the tooth being restored (Fig. 8-1).

Preparation for a complete cast crown requires that adequate tooth structure be removed to allow restoration of the tooth to its original contours. Tooth structure should be preserved when possible, but reduction should produce a crown of acceptable strength.

ADVANTAGES

Because all axial surfaces of the tooth are included in the preparation, the complete cast crown has greater retention than a more conservative restoration on the same tooth (e.g., a seven-eighths or three-quarter crown [see Fig. 7-34]).

Normally, a complete cast crown preparation also has greater resistance form than does a partial-coverage restoration on the same tooth. For a partial veneer crown to rotate off the tooth, only the tooth structure immediately lingual to the occlusal portion of the proximal groove or box need fail. However, if the axial walls of a complete cast crown have been prepared with the proper degree of taper or convergence, a significant amount of tooth structure must fail before the crown can be torqued off.

The strength of a complete cast crown is superior to that of other restorations. Its cylinder-like configuration encircles the tooth and is reinforced by a corrugated occlusal surface. Just as an O-shaped link in a chain resists deformation better than a C-shaped link, this restoration is less easily deformed than its counterparts, which are more conservative of tooth structure.

A complete cast crown allows the operator to modify axial tooth contour. This can be of special significance with malaligned teeth, although the extent of possible recontouring is limited by periodontal considerations. Similarly, it is possible to allow better access to furcations for improved patient oral hygiene through recontouring of buccal and lingual walls (Fig. 8-2). When special requirements exist for axial contours, such as when retainers are needed for partial removable dental prostheses, a complete crown is often the only restoration that allows the necessary modifications for the creation of properly shaped survey lines, guide planes, and occlusal rests (Fig. 8-3) (see Chapter 21).

The restoration permits easy modification of the occlusion, which is often difficult to accomplish if a more conservative restoration is made. This is especially important when supraerupted teeth are present or when the occlusal plane needs to be reestablished.

CRITERIA

The occlusal reduction must allow adequate room for the restorative material from which the cast crown is to be fabricated: type III or IV gold casting alloy or their low–gold content equivalent. Minimum recommended clearance is 1 mm on nonfunctional (noncentric) cusps and 1.5 mm on functional (centric) cusps. The occlusal reduction should follow normal anatomic contours to remain as conservative of tooth structure as possible. Axial reduction should parallel the long axis of the tooth but allow for the recommended 6-degree taper or convergence, which is the angle measured between opposing axial surfaces.

The margin should have a chamfer configuration and is ideally located supragingivally. Sometimes crown lengthening is indicated to obtain a supragingival margin, rather than risk future periodontal disease (see Chapter 6). The chamfer should be smooth and distinct and allow for approximately 0.5 mm of metal thickness at the margin. Typically, it is an exact replica of half the rotary instrument that was used to prepare it. (The recommended dimensions for reduction are shown in Fig. 8-4.)

Special Considerations

Jan 17, 2015 | Posted by in Prosthodontics | Comments Off on 8: THE COMPLETE CAST CROWN PREPARATION
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