The effect of repeated porcelain firings on the marginal fit of millable and conventional casting alloys

The durability of dental restorations is highly determined by an accurate marginal fit, which is in turn affected by the high temperature of porcelain firing. Information is inadequate about the marginal adaptation of metal-ceramic restorations fabricated by soft metal milling technologies after repeated firings. This study aimed to compare the effect of repeated ceramic firings on the marginal fit of copings fabricated from cobalt-chromium through soft metal milling and a conventional nickel-chromium casting alloy. A single standard brass die was designed, machined, and scanned, based on which, 20 frameworks were designed and fabricated through either soft metal milling or conventional casting (n = 10 per group) and porcelain veneered. The vertical marginal fit of the metal copings was measured after 3, 5, and 7 firing cycles by using a digital microscope on 16 points around the finish line of the metal die at x80 magnification. The data were analyzed through repeated measures ANOVA and independent t-test (alpha = 0.05). The marginal fit of neither metal group was significantly affected by the number of firing cycles (P = 0.747). However, the marginal discrepancy was statistically lower in the soft metal milling group than that in the casting group (P<0.001). Repeated porcelain firings did not significantly affect the marginal fit of either alloy and remained within the clinically acceptable range after firings. However, the milled alloy had superior marginal fit regardless of the number of firing cycles.