Torsional load can be limited during intra-canal rotary instrumentation by the torque-controlled endodonticmotor: torque settings can be selected to prevent excessive torsional load on the instruments. According to this specification, the 3 mm file tip must be fixed with brass and a rotational speed of 2 rpm applied to create a continuous torsional load until fracture occurs. 3 The main method of testing for static rotational fracture is the comparison of the torsional resistance of the instruments as described by ISO 3630-1. 2 In contrast, there is less information available on torsional fracture resistance tests. 1Ĭyclic fatigue resistance of NiTi instruments has been assessed extensively. 1 Cyclic fatigue fracture is caused by repetitive compressive and tensile stresses on the outermost fibres of a file rotating in a curved root canal, and torsional failure occurs when the tip of the instrument binds to the canal wall, even in a straight root canal. The former contributes to a significant proportion of failures. The main mechanisms of nickel-titanium (NiTi) endodontic instrument fracture have been revealed as two modes of failure, one being torsional failure and the other cyclic fatigue. Moreover, torsional fracture occurs extremely rapidly when an instrument’s tip becomes blocked. This study explores how heat treatment less significantly influences (increasing or decreasing) torsional resistance when compared to the high increase in flexibility and fatigue resistance reported in many published articles.
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