Fracture Resistance and Bending Stiffness of Heat Treated (oneflare)and Non-Heat Treated (oneflare) Nickel titanium Rotary Instruments with Same Geometric Features

Abstract

Background: The main objective of this study was to compare the cyclic fatigue resistance, torsional resistance and bending stiffness in commercially available heat treated NiTi rotary instruments (OneFlare-H) and non-heat treated NiTi instruments (OneFlare-O) of similar geometric features, customized for the present study.

Methods: Ninety OneFlare were analyzed with torsional resistance, cyclic fatigue resistance and bending stiffness tests. The torsional resistance evaluated at 3 mm from the file tip by plotting the torsional load changes until fracture by rotational loading of 2 rpm. The cyclic fatigue resistance was compared by measuring the number of cycles to failure. The bending stiffness was measured in accordance with the American National Standard/American Dental Association Specification No. 28 and ISO Specification 3630-1. Many fractured fragments have been observed under a scanning electron microscope (SEM). Data analyzed: nonparametric Mann-Whitney U-test and parametric t-test were used.

Result: No significant difference in torsional resistance was found between two groups of files when compared in terms of ultimate strength, fracture angle and toughness. Heat treated Oneflare showed superior resistance to cyclic fatigue compared to non-heated Oneflare files. The examination of fracture’s surfaces from Cyclic Fatigue test with SEM showed topographic differences between the two groups of files. Heat treated Oneflare showed lower Bending stiffness than conventional Oneflare-O.

Conclusion: The Oneflare-H instruments using T-wire heat treatment technology showed improved mechanical properties in cyclic fatigue resistance and bending stiffness without any effect on torsional resistance.