Torsional bond strength and failure pattern of ceramic brackets bonded to composite resin laminate veneers

The objectives of this study were to investigate the torsional bond strength of ceramic brackets bonded to composite resin veneer laminates and to human enamel. Microfilled resin veneers were bonded directly to prepared, etched bovine teeth embedded in epoxy resin. Brackets [Allure IV (NSB), Fascination, Starfire TMB, and Transcend 2000] were bonded to abraded, acid-etched resin veneers with a light-cured or a chemically-cured adhesive. Brackets were also bonded to human teeth with light-cured and chemically-cured adhesives for comparison purposes. After 24 hours storage in water, specimens were subjected to torsional stress and the maximum shear stress tau(max), was calculated. The debonded brackets, the veneer, and enamel surfaces were examined under a stereomicroscope and a SEM to study the failure modes. Three-way ANOVA with a Tukey multiple comparisons test revealed significant differences in bond strengths among bracket types and bonding substrate at P=0.05 level. Highest bond strength was observed in brackets with a combination of micromechanical retention and chemical adhesion. Significant interactions among bracket, substrate (enamel or resin) and mode of cure of adhesive were observed. Analysis of the failure pattern of brackets revealed adhesive and/or cohesive resin failures in all brackets studied, while cohesive bracket failures occurred in the single-crystal Starfire TMB bracket and the polycrystalline Transcend 2000 bracket. Debonding ceramic brackets under a steady torsional load caused no substrate surface alterations regardless of adhesive used.