Special Considerations

Functional (Centric) Cusp Bevel. Proper tooth preparation for a complete cast crown will result in the reduction being directly beneath the cusps of the crown (see Fig. 7-43). This is important for ensuring optimum restoration contour with maximum durability and conservation of tooth structure. Proper placement of the functional cusp bevel will achieve it. Because additional reduction is needed for the functional cusps (to give 1.5 mm of occlusal clearance), the bevel must be angled flatter than the external surface (Fig. 8-5). On most teeth the functional cusp bevel will be placed at about 45 degrees to the long axis.

Nonfunctional (Noncentric) Cusp Bevel. All complete crown preparations should be assessed for adequate reduction at the occlusoaxial line angles of the nonfunctional cusps. A minimum of 0.6 mm of clearance is needed here for adequate strength. Maxillary molars in particular often require an additional reduction bevel in this area (Fig. 8-6). Without it, an overcontoured restoration that does not follow normal configuration may result. Such additional reduction is often unnecessary for mandibular molars, however, because they are lingually inclined and their profile is relatively straight.

Fig. 8-5. The functional cusp bevel is prepared by slanting the bur at a flatter angle than the cuspal angulation. This will ensure additional reduction for the functional cusp.
Maxillary Teeth Angulation
Fig. 8-6. The configuration of the facial wall of the maxillary molars may require slight additional reduction in the occlusal third to prevent an overcontoured restoration.

Chamfer Width. Increasing the faciolingual width of a complete crown is a common fault in practice and is a leading cause of periodontal disease associated with restorations. Adequate chamfer width (minimum 0.5 mm) is important for developing optimum axial contour. However, on small premolars it may be advantageous to prepare a slightly narrower chamfer to conserve tooth structure and retention form. This requires increasingly careful manipulation of the wax pattern during fabrication of the restoration and careful assessment to ensure that the crown is not excessively contoured.

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