Microcracks in dental porcelain and their behavior during multiple firing

Dental porcelains rely on the high-thermal-expansion mineral leucite to elevate their bulk thermal expansion to levels compatible with dental PFM alloys. The microcracks that form around these leucite particles when cooled during porcelain manufacture are a potential source of change in bulk porcelain thermal expansion during fabrication of porcelain-fused-to-metal crowns and bridges. The purpose of the present study was to determine whether multiple firings of commercial dental porcelains could produce changes in microcrack density. Specimens of six commercial porcelains and the "Component No. 1" of the Weinstein patent were fabricated and subjected to 1, 2, 4, 8, and 16 firings. The microcrack densities were determined by quantitative stereology, whereby intersections of microcracks were counted with a test grid. The microcrack data were subjected to linear regression analysis and analysis of variance. The microcrack densities of four of the six porcelains and the Component No. 1 frit were not significantly affected by the number of firings (p > 0.05). One porcelain exhibited a weak but highly significant positive correlation between microcrack density and multiple firings (r2 = 0.24, p = 0.0003), while the remaining porcelain exhibited a weak but statistically significant negative correlation between microcrack density and multiple firings (r2 = 0.15, p = 0.006). The results of this study indicate that even for porcelains that exhibit a measurable change in microcrack density as a function of multiple firings, the magnitude of the increase or decrease in microcrack density after several firings is sufficiently small to cause only negligible shifts in porcelain bulk thermal expansion.