Factors influencing the retention and resistance of preparations for cast intracoronal restorations.

The many different aspects of preparation design that have been tested have been reviewed, and the following guidelines can be drawn: Retention is specifically related to a single path of insertion and the larger that path, the greater the retention. Intracoronal restorations transmit stress to the tooth structure, whether of the "slice" type or the proximal boxes/flares combination. Rounded internal angles will reduce the stress and cusps should be onlaid whenever possible. The slice preparation displays weakness at the lingual entrant angle, which should therefore be reduced to increase the bulk of gold. Proximal boxes aid retention in slice and flared preparations, and the occlusal isthmus also helps in flared preparations. By their nature, intracoronal restorations provide resistance to nonaxial dislodging forces, and features such as the occlusal isthmus and proximal boxes and grooves will obviously also increase this resistance. Thus it would seem that all intracoronal restorations should have rounded internal line angles, slice preparations should incorporate proximal boxes and have adequate bulk in weak areas, and cusps should be protected in all but the smallest of restorations.