Effect of porcelain veneering technique in bilayered zirconia on bond strength and residual stress distribution

Abstract

Background: The use of zirconia in dentistry has increased tremendously in recent years. However, porcelain chipping remains a common problem, indicating that the bond between the zirconia and veneering porcelain may be influenced by various factors including veneering techniques and residual stress distribution. Hence, this study aims to investigate the effects of porcelain veneering techniques, specifically hand-layering and heat-pressing on bonding capabilities of two zirconias and three different veneering ceramics. Methods: 42 cylindrical disc samples of zirconia (n=7/group) were fabricated in the dimensions of 10x1.2mm (diameter x height). The zirconia specimens were milled from IPS e.max® ZirCad [Z] block and Luxen Zr [L] block(n=21/zirconia). The zirconia cores were layered with IPS e.max® Zirliner and heat-pressed with IPS e.max® ZirPress (P) to produce a final veneer dimension of 5x3mm. Conventional layering was done for the rest of the zirconia cores using IPS e.max® Ceram (C) and Shofu Vintage Zr (V). The final study groups were L-V, L-C, L-P, Z-V, Z-C and Z-P. Five samples from each group were subjected to Shear Bond Testing (SBS) with a Universal Testing Machine with a load cell of 5kN and a crosshead speed of 0.5mm/min and loaded to failure. Representative samples were analysed using FESEM/EDX. One-way ANOVA was used to run the statistical analyses of the shear bond strength test (SBS). Results: Statistical differences were found between ZV with LC and LP(p<0.05). On the same zirconia, all three types of porcelain showed no statistical differences in their bonding. Z had better bonding in all three groups compared to L respectively but did not achieve statistical significance. FESEM imaging of the porcelain interface showed minimal defects in the feldspathic porcelain groups and some defects in fluorapatite porcelain groups. EDX analysis reveals cohesive or mixed failure patterns in all representative samples. Tensile residual stress was estimated to be higher in the middle of the porcelain compared to the surface and the interface. Conclusion: Heat-pressed veneering showed comparable bond strength but not superior results to conventional hand-layered veneering. Samples with high bond strength showed predominantly cohesive failure. Heat-pressed veneering produced similar stress distribution profiles compared to handlayered veneering.