Effect of incompatibility stress on the fit of metal-ceramic crowns

The interactive effect of coping thickness and a positive thermal contraction mismatch between metal and porcelain on the fit of metal-ceramic crowns has not yet been experimentally determined. Previous studies have suggested that marginal distortion may be due to contraction differences, although finite element analyses indicate that these distortion effects should be negligible. The marginal gap between metal-ceramic crowns and prepared dies was determined under conditions designed to exaggerate distortion effects. These included the use of thin metal copings (0.1 and 0.2 mm), a chamfer preparation, an alloy with relatively poor creep resistance, and a large thermal contraction mismatch between the alloy and porcelain layers. Gap changes which resulted during porcelain firing cycles were relatively small, but larger marginal discrepancies developed in crowns prepared with a compatible porcelain during grinding and abrasive blasting procedures. This study conclusively demonstrates that incompatibility stress induced by a positive contraction mismatch is not a primary cause of marginal or generalized distortion of metal-ceramic crowns and suggests that external grinding and internal abrasive blasting of crowns are more likely causes of this effect.