After the post space has been prepared, the coronal tooth structure is reduced for the extracoronal
1. Ignore any missing tooth structure (from previous restorative procedures, caries, fracture, or endodontic access) and prepare the re maining tooth as though it were undamaged (i.e., if a porcelain labial margin restoration is planned, a facial shoulder and lingual chamfer are placed).
2. Be sure that the facial structure of the tooth is adequately reduced for good esthetics.
3. Remove all internal and external undercuts that will prevent withdrawal of the pattern.
4. Remove any unsupported tooth structure, but preserve as much of the crown as possible. Because tooth structure has been removed internally and externally, the remaining walls often are thin and weakened. Defining absolute measurements for the dimensions of the residual coronal walls is difficult, but ideally they should be at least 1 mm wide. Wall height is reduced proportionally to the remaining wall thickness because tall, thin walls have a tendency to fracture when the provisional restoration is removed and during try-in and seating of the casting.
5. In addition, be sure that part of the remaining coronal tissue is prepared perpendicular to the post (see step 4 in Fig. 12-8), because this will create a positive stop to prevent over-seating and splitting of the tooth. Similarly, rotation of the post must be prevented by preparing a flat surface parallel to the post (see step 5 in Fig. 12-8). If insufficient tooth structure for this feature remains, an antirotation groove should be placed in the canal (see Fig. 12-22).
6. Complete the preparation by eliminating sharp angles and establishing a smooth finish line.
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