19 The extent of the bevel may also be decreased if the cavity is increased in length occluso-apically. This creates more room for the flat “wall proper”. Te junction between the occlusal bevel and the secondary or primary flares proximally, and also, the junction between the primary or secondary flares proximally and the gingival bevel should be very rounded and smooth.
Modifications for Class IV and some Class III materials Although the general shape, location of margins, and most of the internal anatomy of preparations for cast alloys in the Class IV (and sometimes Class III) category are similar to those to be described for Class I and II alloys, certain specific modifications must be enumerated. Although the preparation will still contain internal boxed portions (buccal, lingual, and proximal “wall proper”) occlusally and proximally, the internal line and point angles should be more rounded. Surrounding walls should be more parallel to one another.
All
circumferential tie constituents (primary or secondary flares, occlusal or gingival bevels) should be hollow-ground to improve the capability of these alloys to replicate marginal details during casting. Tooth preparation should be deeper axially and pulpally to compensate for the loss of retention that results from the relatively poor castability of these alloys.