Abstract
In this study the marginal fit changes that occurred during the porcelain firing cycles of palladium–silver alloy (Pd–Ag) and nickel–chromium alloy (Ni–Cr) coping both with shoulder and heavy chamfer facial finishing lines were investigated with light microscope. Forty copings were fabricated which were divided into four groups according to alloy type and facial finishing line, each group consisted of ten copings: Group I (Pd–Ag) alloy copings with shoulder finishing line; group II (Pd–Ag) alloy copings with heavy chamfer finishing line; group III (Ni–Cr) alloy copings with shoulder finishing line; and group IV (Ni– Cr) alloy copings with heavy chamfer finishing line. Five control metal copings (non veneered) from each group were subjected to exactly the same firing cycles without the application of porcelain. Measurements were made during five stages of crown fabrication: (1) Before degassing, (2) after degassing, (3) after opaque application, (4) after body porcelain application, and (5) after glazing. Changes in the marginal fit of the coping after the various firing stages were calculated for each coping. The statistical analysis of the results showed that the mean marginal fit changes during porcelain firing cycles of the base metal alloy copings (Ni–Cr) were significantly greater than those of noble metal alloy copings (Pd–Ag). The copings with heavy chamferfinishing line showed significantly greatermarginal fit changes during porcelain firingcycles than those with shoulder finishingline.The greatest amount of marginal fitchanges occurred during the degassing stageof porcelain firing cycle followed by bodystage of porcelain firing cycles while therewere no significant marginal fit changes inthe other stages of porcelain firing cycles.
