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 st rength 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 [Allu re IV (NSB), Fascination, Starfire TMB, and Transcend 2000] were bonde d to abraded, acid-etched resin veneers with a light-cured or a chemic ally-cured adhesive. Brackets were also bonded to human teeth with lig ht-cured and chemically-cured adhesives for comparison purposes. After 24 hours storage in water, specimens were subjected to torsional stre ss and the maximum shear stress tau(max), was calculated. The debonded brackets, the veneer, and enamel surfaces were examined under a stere omicroscope 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 le vel. Highest bond strength was observed in brackets with a combination of micromechanical retention and chemical adhesion. Significant inter actions among bracket, substrate (enamel or resin) and mode of cure of adhesive were observed. Analysis of the failure pattern of brackets r evealed adhesive and/or cohesive resin failures in all brackets studie d, while cohesive bracket failures occurred in the single-crystal Star fire TMB bracket and the polycrystalline Transcend 2000 bracket. Debon ding ceramic brackets under a steady torsional load caused no substrat e surface alterations regardless of adhesive used.