Low‐cycle fatigue of NiTi rotary instruments of various cross‐sectional shapes

AIM: To compare the low-cycle fatigue (LCF) behaviour of some commercial NiTi instruments subjected to rotational bending, a deformation mode similar to an engine-file rotating in a curved root canal, using a strain-life analysis, in water. METHODOLOGY: A total of 286 NiTi rotary instruments from four manufacturers were constrained into a curvature by three rigid, stainless steel pins whilst rotating at a rate of 250 rpm in deionized water until broken. The number of revolutions was recorded using an optical counter and an electronic break-detection circuit. The surface strain amplitude, calculated from the curvature (from a photograph) and diameter of the fracture cross-section (from a scanning electron micrograph), was plotted against the number of cycles to fracture for each instrument. A regression line was fitted to the LCF lives for each brand; the value was compared with that of others using one-way analysis of variance (ANOVA). The number of crack origins observed on the fractographic view was examined with chi-square for differences amongst various groups. RESULTS: A linear strain-life relationship, on logarithmic scales, was obtained for the LCF region with an apparent fatigue-ductility exponent ranging from -0.40 to -0.56. The number of crack-initiation sites, as observed on the fracture cross-section, differed between brands (chi(2), P < 0.05), but not LCF life (one-way ANOVA, P > 0.05). CONCLUSIONS: The LCF life of NiTi instruments declines with an inverse power function dependence on surface strain amplitude, but is not affected by the cross-sectional shape of the instrument.