evaluation of repeated firing effect on shear bond strength between zirconia core and different porcelain types

Abstract

Background and aim: Multiple firings are needed to reach better contour, color and esthetics, but there is no scientific data about the exact number of firing cycles to achieve a perfect ceramic restoration. The aim of this study was to evaluate the effect of multiple firing cycles on shear bond strength between zirconia core and different veneering porcelains. Materials and Methods:This study was performed in a laboratory. Fifty-four specimens consisting of zirconia discs(Rainbow, Dentium, Gennossco, Korea) were prepared and divided into 3 groups for applying different porcelain types, including Vita VM9,Vita Zahnfabric, Germany, GC Initial Zr-Fs, Japan and Noritake CZR, Japan on each group. Then each group was divided into 3 subgroups(n=6) according to number of firing cycles: one(1-firing), three(3-firings), five(5-firings). Then shear bond test was done by the load applied at zirconia-porcelain interface using a universal testing machine. The load required for debonding was recorded in N. We reported the results of this study using descriptive statistical methods. In order to compare the shear bond strength between zirconia core and studied porcelain types, at different firing cycles, one-way analysis of variance or its nonparametric equivalent of Kruskal-Wallis test was used. Two-way analysis of variance was used to compare the shear bond strength between all groups. P-value was considered less than 0.05 and statistical analysis was performed using version 24 of SPSS software. Results: There was a statistically significant difference between mean shear bond strength values of different firing cycle groups. Specimens submitted to a single firing cycle presented the lowest bond strength values, three firing cycles provided intermediate and five firing cycles showed the highest bond strength values.There was a statistically significant difference between mean shear bond strength values of different porcelain groups. Specimens veneered with GC Initial porcelain presented the highest bond strength values, where zirconia disks veneered with Noritake CZR showed the lowest. Regarding mode of failure; most of the specimens in each group presented mixed failures rather than adhesive or cohesive failures. Conclusion: Increase in firing cycles provided higher bond strength between zirconia core and veneering porcelain.Moreover, using different porcelain types can affect the shear bond strength of zirconia-porcelain.