Marginal Distortion of Thermally Incompatible Metal Ceramic Crowns with Overextended Margins

Purpose: The present study tested the hypothesis that metal ceramic crowns with a varying axial height are moer susceptible to marginal distortion during mechanical and thermal processing treatments than crowns with a uniform axial height. Materials and Methods: Copings of Pd-Cu-Ga alloy with buccal margin extensions of 0, 1.5, and 3.0 mm were prepared. Oxidized copings were veneered with experimental opaque porcelain with a mean thermal contraction coefficient (25 degrees C to 500 degrees C) that was either 2.1 ppm/degrees C below (a = + 2.1 ppm/degrees C) or 0. 1 ppm/degrees C above (a = - 0.1 ppm/degrees C) that of the alloy. Nine groups of six specimens each were prepared for analysis. Eighteen copings from these 54 specimens were used as porcelain-free controls. All specimens were subjected to a 10-step procedure including grinding, oxidation, firing of four opaque porcelain layers (O1: 0.15 mm; O2: 0.15 mm; O3: 0.5 mm; O4: 0.5 mm), glazing, abrasive blasting for 15 seconds, removal of ceramic by dissolution in hydrogen fluoride, and a postannealing treatment. The control specimens were also subjected to this procedure with the exception of the firing of four layers of porcelain, which were not applied. Marginal gap width was determined using a measuring microscope at a magnification of 30X. Results: Analysis of variance revealed a significant difference in mean gap width as a function of axial length. Th e largest gap change was associated with a 3.0-mm buccal extension and the negative mismatch condition (a 0). Conclusion: Marginal distortion of crowns decreases as the axial length becomes more uniform. Analysis of crown distortion based on differences in the mean contraction coefficients of metal and porcelain alone is not recommended because it ignores the effects of metal grinding, metal sandblasting, and transient stress.