Assessment of shear bond and failure surface of bonded and rebounded brackets using conventional acid and micro-etching techniques

To compare the mean shear bonding force and mode of bond failure of metallic brackets bonded and rebounded (using new and pre-used brackets) to sandblasted and acid-etched enamel is described. Materials and methods: The buccal surfaces of 40 extracted human premolars were sandblasted for 5 seconds with 50 μ alumina at 4mm distance and the buccal surfaces of a further 40 human premolars were acid etched with 37% phosphoric acid for 20 seconds. Following storage for 24 hours at 37°C in distilled water, then each group was divided into four subgroups of ten teeth, shear bonding force and the bond failure were measured for bonded and rebounded new and pre-used brackets using a Univer-sal Testing Machine with a cross-head speed of 10 mm/minute. Results: The mean shear debonding force was significantly lower for brackets bonded to sandblasted enamel compared to acid etched ena-mel (P < 0.000), the same thing for the rebounded brackets, with the lowest shear bonding forces for brackets bonded to sandblasted enamel in the first and second step while the shear bond strength was higher for brackets bonded to acid etched enamel what ever the first treatment of enamel weither con-ventional acid etching or microetching. Statistical analysis showed that at a given stress the probability of failure was significantly greater for brackets bonded to sandblasted enamel. This is in the first step, the same thing in the second step was greater probability of bond failure for enamel treated in the first and second step with Microetcing and still greater in teeth treated with Microetching opposite to that is for teeth treated in second step with acid etching and the significantly lower probability of bond failure for brackets bonded to acid etched enamel in first and second step. Brackets bonded to acid etched enamel showed a mixed mode of bond failure (adhesive and cohesive failure) , whereas following sandblasting, failure was adhesive at the enamel/composite interface (p < 0.000). Conclusions: What-ever the first step of treatment, higher shear bond in the second step could be obtained using acid etch-ing with new and old brackets, but in cases treated with acid etching in first step and microetching in second step a suitable shear bond strength could be obtained insist with new brackets. Key Words : Shear bond, failure surface, bonded and rebounded brackets. الأهداف : الكشف المختبري لمقارنة معدل قوة الالتصاق و مقياس كمية المادة اللاصقة للحواصر المعدنية الجديدة و المعادة باست