Impact of Thermal Misfit on Shear Strength of Veneering Ceramic/zirconia Composites

Objectives: Thermal misfit is discussed as one reason for chipping of veneered zirconia restorations. The aim of the investigation was to assess the effect of thermal misfit on the shear strength of zirconia/veneering ceramic composites.

Methods: Shear strengths of 12 different veneering ceramic/zirconia composites were measured (n=10). The veneering ceramics were fired onto polished Y-TZP. In order to create a strong thermal mismatch, one of the veneering ceramics was intended for use on alumina and one for the metal-ceramic technique. The glass transition temperatures of the veneering ceramics and the coefficients of thermal expansion of all ceramics were measured (n=6). Statistical analysis was performed with one-way ANOVA and a post hoc Bonferroni test (p<0.05).

Results: Shear strength ranged from 21.9+/-6.2 to 31.0+/-7.1 MPa. The ceramic for the metal-ceramic technique showed spontaneous debonding. The differences in the coefficients of thermal expansion of core and veneer (Delta alpha) were calculated. In addition the differences between glass transition temperatures of the veneering ceramics and room temperature (Delta T) as the effective temperature range for stress formation were calculated. Highest shear strength was observed when Delta alpha Delta T approximately 1000 x 10(-6).

Conclusions: Thermal expansion and glass transition temperature of the veneering ceramic have an impact on the shear strength of veneer/zirconia composites.