MICROCRACKS IN DENTAL PORCELAIN AND THEIR BEHAVIOR DURING MULTIPLE FIRING

Dental porcelains rely on the high-thermal-expansion mineral leucite t o elevate their bulk thermal expansion to levels compatible with denta l PFM alloys. The microcracks that form around these leucite particles when cooled during porcelain manufacture are a potential source of ch ange in bulk porcelain thermal expansion during fabrication of porcela in-fused-to-metal crowns and bridges. The purpose of the present study was to determine whether multiple firings of commercial dental porcel ains could produce changes in microcrack density. Specimens of six com mercial porcelains and the ''Component No. 1'' of the Weinstein patent were fabricated and subjected to 1, 2, 4, 8, and 16 firings. The micr ocrack densities were determined by quantitative stereology, whereby i ntersections of microcracks were counted with a test grid. The microcr ack data were subjected to linear regression analysis and analysis of variance. The microcrack densities of four of the six porcelains and t he Component No. 1 frit were not significantly affected by the number of firings (p > 0.05). One porcelain exhibited a weak but highly signi ficant positive correlation between microcrack density and multiple fi rings (r(2) = 0.24, p = 0.0003), while the remaining porcelain exhibit ed a weak but statistically significant negative correlation between m icrocrack density and multiple firings (r(2) = 0.15, p = 0.006). The r esults of this study indicate that even for porcelains that exhibit a measurable change in microcrack density as a function of multiple firi ngs, the magnitude of the increase or decrease in microcrack density a fter several firings is sufficiently small to cause only negligible sh ifts in porcelain bulk thermal expansion.