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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The purpose of this double blind clinical trial was to determine the anticaries activity of a dentifrice containing 0.78% sodium monofluorophosphate in a silica gel abrasive base compared with a placebo under conditions of supervised brushing. 1154 schoolchildren, ages 9-12, were recruited in a non-fluoridated semi-rural area of northeastern Connecticut. Subjects were stratified according to school, grade and sex, and then randomly divided into two groups. Each school day, children brushed their teeth for 1 min under supervision by project personnel. Weekend and vacation usage was ad libitum. Caries examinations and radiographic readings were performed by the same examiner (J.R.). After 12 months, the 996 subjects examined showed that the group using the test dentifrice had significantly (less than 0.05) lower DMFT (25.0%) and DMFS (19.1%) increments than the group using the placebo. After 24 months the 876 subjects examined showed that the test group continued to have significantly lower DMFT (24.5%) and DMFS (24.7%) increments than the placebo group. Surface protection after 24 months ranged from 22.1% for occlusal to 37.1% for interproximal surfaces.

Does the price of a ticket entitle hockey fans to see more injuries? That's the result of excusing paid gate junior hockey players from wearing facial protection.