Retention strength of zirconia occlusal veneer restoration

Aim: The present study aimed to evaluate the retention strength of nonretentive zirconia occlusal veneers bonded to different bonding surfaces (enamel, enamel and dentin, and enamel with composite filling) using two adhesive resin cement systems that use either organophosphate carboxylic acid or organophosphate monomers as a ceramic primer.
Materials and methods: Sixty extracted mandibular molars were prepared to receive occlusal veneers as follows (n = 20): 1-mm reduction within enamel; 2-mm reduction within enamel and dentin; 1-mm reduction within enamel with composite filling. Each occlusal veneer was designed with an occlusal bar to aid in the retention test, then milled from a zirconia block, and sintered. Within each group, the zirconia occlusal veneers were bonded using either Duo-Link Universal or Panavia V5 (10 specimens each). All specimens were thermocycled for 5000 cycles. After the pull-off test, the retention strength was calculated for each specimen. Each specimen was examined under magnification to determine its mode of failure. Representative specimens were examined using a scanning electron microscope. Data were analyzed using the two-way analysis of variance (ANOVA) and Tukey HSD tests (P = 0.05).
Results: Both bonding substrate and cement type had a significant influence on retention strength values (P 0.05). The two-way ANOVA showed a significant interaction between bonding substrate and cement type (P = 0.003). There were significant differences in the retention strength between the cements in both the enamel and enamel and dentin substrates (P 0.05), but no significant difference between the cements in the enamel with composite filling substrate (P > 0.05). The predominant mode of failure was cement remaining principally in the restoration surface (adhesive failure), followed by cement adhesion to both the tooth and the restoration surface (mixed failure).
Conclusions: Among the studied substrates, enamel was the optimal dental bonding surface. However, bonding to dentin was not a limiting factor for the retention of zirconia occlusal veneers. The resin cement using an organophosphate (Panavia V5) provided superior retention strength compared with the cement using organophosphate carboxylic acid monomer (Duo-Link Universal).