Purpose: To evaluate the effects of different treatment time of 5.25% Sodium hypochlorite (NaOCl) on the microshear bond strength (μSBS), attenuated total reflection Fourier transform infrared (ATR-FTIR) and etching pattern in mild and moderate fluorotic enamel. Study design: Forty-eight fluorotic molars were divided into two groups: mild and moderate fluorotic enamel which were classified by a Thylstrup and Fejerskov index (TFI). Based on the application time (0s, 60s, 120s, 180s) of 5.25% NaOCl, each group was sectioned into four parts. Then the etched enamel was bonded with resin and tested to acquire μSBS. The statistical method was two-way ANOVA and Least Significant Difference (LSD) test at α = 0.05. Besides, fracture modes were observed under a stereo microscope. SEM was used to evaluated the enamel-etching pattern and organic content on the fluorotic enamel surface were investigated by ATR-FTIR. Results: Duration of 5.25% NaOCl at 60s or 120s significantly increased the μSBS of fluorotic enamel compared to 0s (p<0.05). Fracture modes indicated that dominating failures were set in the bonding interface but whose proportion decreased when 5.25% NaOCl was applied. The enamel-etching pattern in 180s was deepest under SEM. Spectra of enamel samples manifested an obvious and gradual removal of its organic phase after duration of NaOCl increased. Conclusion: The maximal μSBS is acquired by using 5.25% NaOCl at 60s for mild fluorotic enamel but 120s for the moderate. The prolonged application time of 5.25% NaOCl prior to phosphoric acid etching improves enamel-etching pattern. Treatment of 5.25% NaOCl decreases proteins on the fluorotic enamel surface.

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