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Three-dimensional photoelastic stress analysis was used to investigate the design of Class I cavity preparations. Enlarged, epoxy resin models of the mandibular first molar were modified with variations of Class I preparations. The models with stresses "frozen" into them were sectioned on predetermined planes. The resulting fringe patterns demonstrated that significant stresses exist in planes remote from loads, and that rounding internal line angles reduces stresses significantly.For many years, the design of cavity preparations has been the result of clinical experience and empiricism, with the teachings of Black1 gaining almost universal acceptance. Nevertheless, restorations (particularly those of amalgam) would occasionally fail even when careful operative procedures were observed. Only recently has research been done on the design of cavity preparations. Because teeth are irregular three-dimensional bodies that defy mathematical analysis, experimental methods such as photoelasticity have been used. Evaluation of cavity design based on an analysis of stress concentrations, using the two-dimensional photoelastic method, was begun by Noonan. Mahler and Peyton,3 as well as several other investigators,4'3 used the same method for further investigation. Granath67 has produced the most recent work in this field.

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