Fundamentals of extracoronal tooth preparation. Part I. Retention and resistance form. 1974.

The differential selection and application of retention and resistance form in tooth preparation depend upon the individual clinical situation. Since the ‘‘textbook ideal’’ is not often encountered clinically, knowledge of basic principles and factors is mandatory for the successful preparation of extracoronal restorations. Black astutely recognized and stressed the importance of these two basic characteristics. He stated: ‘‘Resistance form is that shape given to a cavity intended to afford such a seat for the filling as will best enable it to withstand the stress brought upon it in mastication. Retention form is the provision for preventing it from being dislodged.’’ More recent concepts tend to emphasize the duplication of a preconceived design of a preparation rather than the assessment of biomechanical requirements of individual retainers. The modification of an ideal design for a preparation entails judgment in the application of the basic principles of retention and resistance (Table I). An analysis of these principles and factors should enable the dentist to effectively apply them during the design of any preparation. The ability to cope with atypical clinical situations is markedly enhanced by such organized application. All preparations require the incorporation of factors to prevent the dislodgment of the restoration by functional stresses. Retention form counteracts tensile stress, and resistance form counteracts shearing stress (Fig. 1). A preparation with good resistance form will be retentive; the opposite is not necessarily true. The same method is often used when incorporating the factors of both retention and resistance form. Thus, for practical purposes, these factors can be considered together (Fig. 2).