Resistance form in tooth preparation.

Theoretical and clinical studies of resistance form are reviewed, supporting the basic prosthodontic principle that resistance form is an essential element in preparation design. Concepts of the"on-off" nature of resistance form, its application in guidelines for minimally acceptable preparation taper, and the controversy over whether there is a relationship with clinical success or failure are discussed. It is more challenging to obtain resistive molar preparations than resistive anterior preparations, and uneven preparation margins can make parallel 0 degree taper preparations lack resistance form. The usefulness of grooves, especially in molar preparations with uneven margins, is illustrated. The Lewis and Zuckerman techniques for evaluating resistive areas of a preparation are shown to be consistent with each other but not with the Weed method.