Influence of thermal expansion mismatch on residual stress profile in veneering ceramic layered on zirconia: Measurement by hole-drilling.

OBJECTIVES: Mismatch in thermal expansion coefficient between core and veneering ceramic (Deltaalpha=alphacore-alphaveneer, ppm/ degrees C) is reported as a crucial parameter influencing veneer fractures with Yttria-tetragonal-zirconia-polycrystal (Y-TZP) prostheses, which still constitutes a misunderstood problem. However, the common positive Deltaalpha concept remains empirical. The objective of this study is to investigate the Deltaalpha dependence of residual stress profiles in veneering ceramic layered on Y-TZP frameworks. METHODS: The stress profile was measured with the hole-drilling method in bilayered disc samples of 20mm diameter with a 0.7mm thick Y-TZP framework and a 1.5mm thick veneer layer. 3 commercial and 4 experimental veneering ceramics (n=3 per group) were used to obtain different Deltaalpha varying from -1.3ppm/ degrees C to +3.2ppm/ degrees C, which were determined by dilatometric analyses. RESULTS: Veneer fractures were observed in samples with Deltaalpha>/=+2.3 or </=-0.3ppm/ degrees C. Residual stress profiles measured in other groups showed compressive stresses in the surface, these stresses decreasing with depth and then becoming more compressive again near the interface. Small Deltaalpha variations were shown to induce significant changes in residual stress profiles. Compressive stress near the framework was found to decrease inversely to Deltaalpha. SIGNIFICANCE: Veneer CTE close to Y-TZP (+0.2ppm/ degrees C Deltaalpha) gived the most favorable stress profile. Yet, near the framework, Deltaalpha-induced residual stress varied inversely to predictions. This could be explained by the hypothesis of structural changes occurrence within the Y-TZP surface. Consequently, the optimum Deltaalpha value cannot be determined before understanding Y-TZP's particular behavior when veneered.