[Effect of sodium ascorbate and calcium hydroxide on shear bond strength of composite resin to bleached enamel]

Application of sodium ascorbate as an antioxidant and calcium hydroxide as a buffering agent has been recommended after bleaching .The aim of this study was to investigate the effect of these two materials on shear bond strength of composite resin attached to bleached enamel. This in-vitro investigation used 60 human sound premolars. They were randomly divided into five groups [n=12]. Buccal enamel surfaces were flattened by using a trimmer machine and silicon carbide paper. The samples in negative control [NC] group were not bleached, while all others were bleached with 35% hydrogen peroxide gel for 5 days. Composite cylinders were bonded in positive control [PC] group immediately after bleaching; in delayed bonding [DB] group after one week; in sodium ascorbate [SA] and calcium hydroxide [CH] groups after 40 hours of treatment with these two materials, respectively. The specimens were thermocycled [5-55°C, 500 cycles] 24 hours after bonding and they were subjected to shear bond testing by using universal machine. Statistical analyses were carried out to identify group differences and Kruskal-Wallis and Mann-Whitney tests were used to detect the significance level. Group differences revealed that, the lowst mean of bond strength belonged to the calcium hydroxide group the difference of which was significant compared with the other groups [P<0.006]. The positive control group had lower bond strength in comparison with the negative control, sodium ascorbate and delay bonding groups [p<0/001] .But the difference among other groups was not significant. Both one week delay and using sodium ascorbate treatment could significantly increase the bond strength of composite resin to bleached enamel, while the use of calcium hydroxide had an adverse effect on composite resin bond strength