EFFECTS OF FIRING CYCLE AND SURFACE FINISHING ON DISTORTION OF METAL-CERAMIC CASTINGS

The ''as-cast'' fit of metal ceramic restorations has been reported to deteriorate during the high-temperature firing cycles used for applic ation of porcelain veneer. In this study, thermocycling and surface fi nishing or cold working were examined for their effects on marginal ad aptation of metal ceramic castings. Methods for minimizing the loss of marginal adaptation were evaluated, and casting variables were elimin ated by construction of acrylic resin measuring dies directly in the r estorations, Thermocycling of metal ceramic restorations resulted in i ncreased marginal openings, and all of the loss of marginal fit occurr ed during the first thermocycling of the alloy. The restorations that were cold worked and then oxidized by conventional manipulation had su bstantially more marginal opening than any other group, A fourfold, st atistically significant improvement (p < 0.001) in the marginal adapta tion of a metal ceramic restoration was observed when the initial ther mal cycle was completed before the specimens were finished.