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Comparison of alternative adhesive cementation concepts for zirconia ceramic: glaze layer vs zirconia primer.

PURPOSE
Zirconia-based ceramics offer strong restorations in dentistry, but the adhesive bond strength of resin cements to such ceramics is not optimal. This study evaluated the bond strength of silane/adhesive/resin cement and zirconia primer/resin cement combinations on non-glazed and glazed zirconia surfaces before and after aging.


MATERIALS AND METHODS
Disk-shaped zirconia ceramic specimens (diameter: 8 mm; thickness: 2 mm) (N = 80, n = 10 per group) were randomly divided into 2 groups. While half of the specimens received one coat of glaze and were later finished by grinding, the other half was only ground using 1200-grit silicone carbide abrasives under water. The glazed specimens were then conditioned with 9.5% HF acid gel for 60 s, rinsed with water for 90 s, and neutralized. The glazed and non-glazed specimens were further divided into two groups. Two resin cements, namely, Variolink II and Multilink Automix were adhered onto the zirconia surfaces with their corresponding adhesive systems. In the Variolink II group, zirconia surfaces were silanized (Monobond-S), and adhesive resin (Heliobond) was applied and photopolymerized. In the Multilink Automix group, one coat of Metal/Zirconia Primer was applied with a microbrush, left to react for 180 s, and dried using oil-free air. Half of the specimens in each cement group were subjected to 5000 thermocycles (5°C to 55°C) and the other half was kept in the dark for 24 h at 37°C prior to testing. Specimens were mounted in the jig of the universal testing machine, and force was applied to the ceramic/cement interface until failure occurred (1 mm/min). After evaluating all debonded specimens under SEM, the failure types were defined as either "adhesive" with no cement left on the zirconia (score 0) or "mixed" with less than half of the cement left on the surface with no cohesive failure of the substrate (score 1). Data were analyzed using three-way ANOVA and Dunnett-T3 post-hoc tests.


RESULTS
Application of a glaze layer significantly improved the bond strength in the silane/adhesive/Variolink II group (p < 0.05), but no significant effect was found in the zirconia primer/Multilink Automix group (p > 0.05) (three-way ANOVA). Interaction terms were also significant (p < 0.05) (Dunnett-T3). Thermocycling did not decrease the results significantly in any of the groups (p > 0.05). Failure analysis revealed exclusively adhesive failures (score 0: 40 out of 40) in the non-glazed groups, but predominantly mixed failures (score 1: 34 out of 40) in the glazed groups.


CONCLUSION
The silane (Monobond S)/adhesive (Heliobond)/Variolink II resin cement combination benefitted from glazing the zirconia surface, but the zirconia primer/Multilink Automix resin cement combination alone also provided sufficient bond strength to zirconia.