Effect of metal design on marginal distortion of metal-ceramic crowns.

An analysis of residual stress and marginal distortion due to thermal contraction mismatch between metal and ceramic is presented for metal-ceramic crowns. Using dilatometric data, finite element stress analyses were performed on pre-molar crowns designed with chamfer-knife edge, chamfer with collar, shoulder with collar, and shoulder-bevel with collar geometries and a maxillary central incisor crown with a chamfer-knife edge geometry. Calculations were made using combinations of a Ni-Cr alloy or a Au-Pd alloy with each of three porcelain products. Calculated marginal distortions due to crown design and metal-porcelain thermal contraction incompatibility were found to be well below experimental values found in the literature. For the cases studied, the calculated marginal distortions due to metal-porcelain thermal contraction mismatch depend primarily on the metal-porcelain combination and are insensitive to the coping design. However, this study excluded copings which have been extensively ground to thicknesses of 0.1 mm or less, and such copings may be more susceptible to localized or generalized distortion.