Effect of sodium hypochlorite on mechanical propel-ties of dentine and tooth surface strain

Aim The aim of this study was to test the null hypothesis that sodium hypoc hlorite irrigation of root canals dues not alter the properties of dentine and contribute to the weakening of root-treated teeth. Methodology The effect of two concentrations (0.5%, 5.25%) of sodium hypoch lorite (NaOCl) and saline on ii) the elastic modulus and flexural strength of machined dentine bars, and (ii) changes in strain of 'whole' extracted h uman teeth were evaluated. One hundred standardized piano-parallel dentine bars (> 11.7 x 0.8 x 0.8 mm) were randomly divided into the three groups, i mmersed for 2 h in the respective solutions and then subjected to a three-p oint bend test. Changes in strain of each of 10 teeth on cyclical nondestru ctive occlusal loading were measured using electrical resistance strain gau ges bonded to the cervical aspects. Each tooth had its crown and enamel red uced and root canal prepared. These were irrigated sequentially in a series of four separate, 30-minute regimes; initial-saline, 0.5% NaOCl, 5.25% NaO Cl and final-saline. The changes in strains after each irrigation regime we re compared. Results There was a significant decrease in elastic modulus of the dentine bars immersed in 5.25% NaOCl compared with the saline group (P < 0.01). The re was also a significant decrease in flexural strength of the dentine bars in the 5.25% NaOCl group compared to both the saline and 0.5% NaOCl groups (P < 0.01). The strain data from the nondestructive tooth loading tests re vealed significant increases in tensile strain between the initial-saline a nd the final-saline stages (P<0.01). Significant increases in compressive s trains were also found between initial-saline and 5.25% NaOCl; and between 0.5% NaOCl and 5.25% NaOCl stages (P < 0.01). Conclusions The null hypothesis was rejected, 5.25% NaOCl reduced the elast ic modulus and flexural strength of dentine. Irrigation of root canals of s ingle, mature rooted premolars with 5.25% NaOCl affected their properties s ufficiently to alter their strain characteristics when no enamel was presen t.