[Shrinkage stress developed by polymerization of composite resin]

The effects of the shrinkage stress on the various cavity forms, were examined. The shrinkage stress was obtained from the displacement of the brass mold developed by polymerization of the composite resin. The shrinkage stress developed in the cavity wall, was analyzed by the finite element method, and the crack of the cylindrical silica tube filled with composite resin, was observed. The shrinkage stress 30 minutes after mixing the composite resin was reduced with the increase in the marginal angle as well as the diameter or the depth of cavity. Preparation of the bevel at the cavity margin reduced the shrinkage stress. In the case of reverse bevel, reduction of the stress was not observed within 24 hours, but in the other cases, the stress immediately decreased after hardening. The analysis by the finite element method reveoled the maximum stress at the margin, with less stress for the round, flat and reverse bevel in this order. For the observation of the cylindrical silica tube filled with composite resin, the crack was observed in the cylindrical silica tubes prepared round, flat or without bevel, it was not observed in the tube prepared reverse bevel that the crack was developed in this tube and composite resin was detached from this tube within 24 hours.