Effects of different concentrations and exposure time of sodium hypochlorite on the structural, compositional and mechanical properties of human dentin

This study evaluated the effects of sodium hypochlorite (NaOCl) with different concentrations and exposure time on the structural, compositional and mechanical properties of human dentin in vitro. Sixty dentin slabs were obtained from freshly extracted premolars, randomly distributed into four groups (n=15), and treated with 1%, 5%, 10% NaOCl and distilled water (control group), respectively, for a total of 60 min. Attenuated total reflection infrared (ATR-IR) spectroscopy, Raman spectroscopy and X-ray diffraction (XRD) were carried out before, 10 min and 60 min after the treatment. Scanning electron microscopy (SEM) and flexural strength test were conducted as well. The results showed that dentins experienced morphological alterations in the NaOCl groups, but not in the control group. Two-way repeated-measures analysis of variance revealed that the carbonate:mineral ratio (C:M), Raman relative intensity (RRI), a-axis, c-axis length and full width at half maximum (FWHM) with the increase of time and concentration in the NaOCl groups were not significantly different from those in the control group (P>0.05). Nevertheless, the mineral:matrix ratio (M:M) increased and the flexural strength declined with the increase of concentration and the extension of time in the NaOCl groups (P<0.05). Additionally, it was found that the M:M and the flexural strength remained unchanged after 1% NaOCl treatment (P>0.05), and the morphology changes were unnoticeable within 10 min in 1% NaOCl group. These results indicated that NaOCl has no significant effects on the inorganic mineral of human dentin; but it undermines and eliminates the organic content concentration- and time-dependently, which in turn influences the flexural strength and toughness of dentins. In addition, an irrigation of 1% NaOCl within 10 min can minimize the effects of NaOCl on the structural and mechanical properties of dentin during root canal treatment.