The influence of resin cement thicknesses on shear bond strength of the cement-zirconia

Introduction
Zirconia is a polycrystalline ceramic with no glassy phase like other glass ceramics. To gain a suitable retention, zirconia present some inherent problems. Surface treatment of etched zirconia with hydrofluoric acid does not adequately roughen the surface for the purpose of retention, while grinding zirconia to create surface roughness is often used as an option to improve its’ mechanical bonding.

Objectives
The study examined the influence of resin cement thicknesses on shear bond strength of zirconia treated with a universal adhesive.

Material and methods
Forty zirconia specimens were prepared, surface-treated with universal adhesive, and divided into 4 groups according to resin thicknesses (50, 80, 160, and 240 μm). All samples were stored in 37°C distilled water for 24 hours. Shear bond strength was performed using a universal testing machine at a 0.5 mm/min crosshead speed, until failure. Failure modes were analyzed using stereomicroscope at a magnification of ×50. Results were statistically analyzed by one-way ANOVA and Tukey’s multiple comparison tests.

Results
The shear bond strength for each group was 30.28 ± 3.09, 26.86 ± 2.21, 25.98 ± 2.96, and 18.22 ± 1.71, respectively, and 50 μm significantly showed highest bond strength than in other groups (p < 0.05). There were no significant differences in bond strength between 80 μm and 160 μm (p > 0.05), while 240 μm significantly showed lowest bond strength than in other groups (p < 0.05). Adhesive failure was mostly found in all groups.

Conclusions
Resin cement thicknesses had influence on zirconia shear bond strength treated with a universal adhesive. The thinner resin cement showed higher shear bond strength than the thicker resin cement thicknesses.