Objectives:The aim of this study was to test the effect of post-treatment time on the shear bond strength of a composite to enamel treated by a 10% carbamide peroxide (CP) bleaching system. Methods: Seventy-five noncarious human mandibular central incisor teeth were used. Flat buccal enamel surfaces were obtained with 600-grit SiC paper. The teeth were then randomly divided into four experimental groups and one control group (n=15) and embedded in acrylic resin. Specimens of control group were not bleached. The specimens of experimental groups were exposed to one daily application of CP for eight hours for 14 days and comprised the evaluation of four time intervals after bleaching (immediately, 24 h, 1 or 2 weeks). After each incubation, standardized cylinders of composite were bonded to bleached buccal enamel using a bonding agent. Shear bond strength test was carried out 24 h after bonding-composite application. Data were recorded in MPa. Results: Statistical analysis showed that the bond strengths were significantly different (p<0.05). Composites bonded to the enamel immediately and 24 h after bleaching, showed significantly lower bond strengths values (p<0.05) than control group. However, bond strength values of groups restored after 1 and 2 weeks following bleaching were similar with that of untreated control samples (p>0.05). Conclusions: Bleaching with 10% CP had significant influence on the shear bond strength of composite resin bonded to enamel.Therefore, at least 1 week delay of resin composite bond to bleached enamel surfaces is suggested because the elapsed time following application of CP bleaching agents on enamel affects the bond strength of composite to enamel.

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