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

Int Endod J. 2007 Aug;40(8):626-32. doi: 10.1111/j.1365-2591.2007.01257.x. Epub 2007 Apr 24.

Abstract

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.

Publication types

  • Comparative Study

MeSH terms

  • Dental Alloys / chemistry*
  • Equipment Failure
  • Nickel / chemistry*
  • Root Canal Therapy / instrumentation*
  • Rotation
  • Stress, Mechanical
  • Surface Properties
  • Titanium / chemistry*

Substances

  • Dental Alloys
  • titanium nickelide
  • Nickel
  • Titanium