Bending properties of rotary nickel-titanium instruments

Objective. We sought to compare the bending properties of different rotary nickel-titanium instruments and to investigate the correlation between their bending moments and their cross-sectional surface areas. Study design. Resistance to bending was determined according to International Standards Organization publication 3630-1. The sample size was 10 files for each type, taper, and size. The cross-sectional surface area of all instruments was determined by using scanning electron microscope photographs of the cross section. The images were scanned and the area was calculated by using special software. Data were analyzed by using analysis of variance and the Student test and the Newman-Keuls test for all pairwise comparisons. The strength of the correlation between the bending moment and the cross-sectional area was determined by computing the Pearson product moment correlation. Results. Bending moments were significantly lower for ProFile and RaCe files than for all other files (P < .05). K3 files were significantly less flexible than all other instruments (P < .05). The correlation between stiffness and crosssectional area was highly significant (r = 0.928; P < 0001). Conclusion. Nickel-titanium files with tapers greater than .04 should not be used for apical enlargement of curved cabals because these files are considerably stiffer than are those with .02 or .04 tapers.