Abstract:

Objective： To evaluate the influence of sodium hypochlorite (NaOCl) solution used during root canal therapy on dentin bond strength. Methods: In the study, 15 freshly extracted human third molars with complete dental crowns, caries and filling-free were selected. The occlusal enamel was removed perpendicular to the long axis of the tooth to expose middle flat surfaces of sound dentin. The occlusal dentin surfaces were then polished using 600-grit silicon papers for 1 min and rinsed with deio-nized water for 1 min. The teeth were randomly divided into three groups according to the treatment received: group A (negative control group), the samples were immersed in deionized water for 20 min; group B, the dentin surfaces were immersed in 2.50% NaOCl solution for 20 min, with the solution being renewed every 5 min; group C, the dentin surfaces were immersed in 5.25% NaOCl solution for 20 min, with the solution being renewed every 5 min. All the treated dentin surfaces were bonded using a self-etching adhesive system (SE bond) with a 5 mm in height resin composite (AP-X). After storage in deionized water at 37 ℃ for 24 h, the adhesive samples were sectioned longitudinally to produce 1.0 mm×1.0 mm stick specimens(n=45) for micro-tensile bond strength testing (MPa). Failure modes (adhesive failure, cohesive failure or mixed failure) at the dentin-resin interface were observed using a stereomicroscope. The micro-tensile bond strength data among the three groups were analyzed by a one-way ANOVA, then the Post-hoc test（LSD）was employed for pairwise comparison. The distribution of failure modes among the groups were analyzed by chi square test. Results:  Significant decreased bond strength values were found for the 2.50% NaOCl-treated group (26.04±5.74) MPa and 5.25% NaOCl-treated group (24.46±3.77) MPa when compared with the strength of negative control group (48.71±7.77) MPa，P=0.000. Compared with the negative control group, themicro-tensile bond strength of the 2.50% NaOCl-treated group and 5.25% NaOCl-treated grouphad dropped by 46.5% and 50.2%. However, there was no significant difference of bond strength between the 2 NaOCl-treated groups （P=0.214）. The distribution of failure modes showed significant difference in all the three groups （2=56.324, P=0.000）. The mixed failure (68.9%) was the most mode of fracture in the negative control group, followed by adhesive failure(24.4%), and the cohesive failure was least（6.7%）. The proportion of adhesive failure mode was higher in NaOCl-treated groups than in negative control group  (P=0.000)． There was no significant difference of the distribution of failure modes between the 2.50% NaOCl-treated group and 5.25% NaOCl-treated group（P=0.197）, and there was no cohesive failure mode detected in the two groups. Conclusion： The micro-tensile bond strength of dentin to composite resin was lower after exposure to NaOCl solution.

Key words: Dentin, Sodium hypochlorite, Microtensile bond strength