The effects of thermal mismatch and fabrication procedures on the deflection of layered all-ceramic discs

Objectives Residual thermal stress caused by thermal incompatibility of ceramic core and veneering porcelain in all-ceramic restorations might induce fracture or distortion of the restoration. The aim of this study was to evaluate the thermal compatibility between a ceramic core and veneering porcelain by measuring the bending of layered discs and to compare the result with the thermal mismatch values. Methods Thirty-five heat-pressed ceramic core discs were made and divided into seven groups (n=5). One group of discs was used as control and subjected only to firing cycles, while the other groups of discs were veneered with porcelains. The mismatch of thermal contraction coefficient values of the Empress 2 Core and veneer porcelains used ranged from −3.7 to 3.8 ppm/°C. Results The Empress 2 Core disc without a veneer (control) was not significantly deflected by the fabrication procedures. The deflection of layered discs was strongly influenced by the degree of thermal mismatch during fabrication procedures. The greatest deflection occurred during the 2nd dentin firing and with the largest mismatch either positive or negative. Significance The deflection test proved to be a reliable test method for the evaluation of thermal compatibility of porcelains in all-ceramic systems.