Thermal mismatch, fabrication procedures and deflection of layered ceramic discs

Objectives:, Residual thermal stress caused by mismatch in thermal contraction coefficients between ceramic core and veneering porcelain together with fabrication procedures might induces immediately fracture or distortion of the restoration. The aim of this study was to evaluate the acceptable amount of thermal mismatch between a ceramic core and veneering porcelain by measuring the deflection of layered discs. Methods:, Thirty-five heat-pressed ceramic core discs (13.0 mm x 1.0 mm) were made and divided in groups (n=5). One group of discs was used as control and subjected to washing, 1st, 2nd dentin and glaze simulation firings. The other groups of discs were veneered with 1.0 mm of porcelains that involved: washing firing, i.e. thin layer of watery porcelain mix, 1st dentin and 2nd dentin firings. Final the specimens were subjected to simulation glaze firing. The mismatch values were: -3.48, 0.52, 1.01, 1.35, 1.8, and 3.72 ppm/°C from 450 to 20°C. The deflection of the specimens was measured with a Profilometer device and after each processing steps. One and two-way analyses of variance together with Tukey post hoc tests were used to analyse the data. Results:, The specimens in the control group were not significantly deflected by fabrication procedures. Thermal mismatch values of -3.48, 0.52, and 3.72 did significantly deflected the specimens during washing, 1st dentin, and 2nd dentin firings while thermal mismatches of 1.01, 1.35 and 1.8 did not. The greatest deflection occurred during the 2nd dentine firing and with the larger mismatch either positive or negative. Conclusion:, The deflection test proved to be a simple and reliable method to evaluate thermal compatibility of not fractured layered ceramic discs. Significant change in deflection indicated that residual stress is present within the composite structure, but smaller changes are as well considered indicative of smaller overall state of stress.