Low-cycle fatigue of rotary NiTi endodontic instruments in hypochlorite solution

Objective. To examine the low-cycle fatigue (LCF) behaviour of NiTi rotary endodontic instruments in aqueous sodium hypochlorite solution. Methods. Four brands of NiTi rotary instrument (n=179) were subjected to a rotational-bending fatigue test at a rate of 250 rpm until fracture, the instrument being immersed in 1.2% sodium hypochlorite solution. The surface strain amplitude, calculated from the curvature (from a pre-test photograph) and diameter of the fracture cross-section (from SEM photomicrograph) was plotted against the total number of revolutions to failure. An apparent fatigue-ductility exponent was determined from a regression line fitted to the LCF lives for each brand of instrument. All detached fragments were examined fractographically for crack initiation and the extent of crack extension into the cross-section. The values were compared using one-way analysis of variance (ANOVA) or chi(2), where appropriate, at alpha=0.05. Results. A linear strain-life relationship was obtained for all groups; the apparent fatigue-ductility exponent was similar between various brands (ANOVA, P>0.05), but not for the number of crack origins (chi(2), P<0.05). There was an inverse, linear relationship between the square root of the extension of the fatigue-crack and the strain amplitude. Significance. NiTi rotary instruments fatigued in hypochlorite shows a strain-life relationship with low-cycle and high-cycle fatigue regions. The LCF behaviour is not affected by the cross-sectional shape of the instrument. There appears to be a critical extent of crack propagation for various surface strain amplitudes leading to final, catastrophic fracture of the instrument. (C) 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.