Influence of framework design, contraction mismatch, and thermal history on porcelain checking in fixed partial dentures.

The objective of this study was to characterize the relative influence of contraction mismatch, framework design, furnace type, cooling rate, and multiple firings on immediate or delayed checking in fixed partial dentures. Frameworks for 60 anterior bridges (three-unit fixed partial dentures) were cast from a low-expansion Au-Pd alloy (O) and a high-expansion Pd-Ag alloy (J). A high-expansion porcelain (B) was applied to each of three framework designs. Firing was performed at heating rates of 56 degrees C/min and 180 degrees C/min. Specimens were cooled at two rates after each of five glazing cycles. For O-B specimens which exhibited a negative thermal contraction mismatch between 600 degrees C and 25 degrees C, 60% of the bridge specimens failed when they were subjected to slow cooling preceded by either fast or slow heating. When J-B specimens (which exhibited a smaller negative contraction mismatch) were heated and cooled rapidly, no failures occurred through all of the firing cycles. However, cracks were observed in 13.3% of the J-B bridges which were slowly heated and rapidly cooled. Delayed cracks (after the fifth glaze cycle) developed over periods of up to two years only in bridges which were slowly cooled in the furnace chamber. The results of this study suggest that checking in conventional feldspathic porcelains can be promoted by slow cooling rates and an excessive number of firing cycles.