Photoelasticity as a research technique for analyzing stresses in dental structures.

THE primary purpose of the masticatory process is to apply forces to the nutritional substances which are taken into the mouth, thereby causing a reduction in their size. The mechanism for this action includes the medium of tooth structure which not only applies these forces directly to the food masses, but is itself subject to the effects of these forces. In general, the effects of forces applied to a body are manifest in the development of internal stresses which are distributed in accordance with the direction of the applied forces, the manner in which the body is supported, and the shape of the body. Furthermore, these stresses are accompanied by internal deformations or strains within the body, and if their magnitudes are sufficiently large, permanent deformation and, ultimately, failure will ensue. Tooth structure and dental restorations are not exceptions to these rules. The structural design of normal tooth structures is such that they withstand the effects of masticatory forces. However, when tooth structure is removed and replaced by a dental restoration, the normal internal stress distribution is altered and the new stress situation will depend on the design of the cavity preparation. Furthermore, the shape of the restoration, which is also determined by the cavity preparation, is of considerable importance as to whether the restorative material will undergo permanent deformation or fracture. In general, then, an evaluation of the internal stresses in both dental restorations and tooth structures should yield valuable criteria for successful design. The consideration of stresses in dental structures is not a new concept. There has been considerable reflection concerning the design of dental restorations so as to avoid failure caused by internal stresses. Most of these efforts have resulted in generalities deduced from the theory of elasticity as applied to uniformly shaped structures and, therefore, leave much to be desired when confronted by the irregular shapes of dental structures. Empirical rules or criteria have developed over the years on the basis of clinical evidence and these have served to avoid the deleterious effects of internal stress magnitudes. However, since both the design and size of dental restorations are extremely limited by the biologic aspects of tooth structure, an exacting analysis is the only solution to the determination of an optimum structural configuration.