Microscopic Features of Fractured Fragment of Nickel-Titanium Endodontic Instruments by Two Different Modes of Torsional Loading

This study compared the microscopic features of the fractured endodontic nickel-titanium (NiTi) rotary instruments by twodifferent torsional loadings: repetitive torsional loading (RTL) and single torsional loading (STL) based on the InternationalOrganization for Standardization (ISO). ProTaper Next, HyFlex EDM, and V-Taper 2 were compared in this study. In the STLmethod, the torsional load was applied after fixing the 3 mm tip of the file, by continuous clockwise rotation (2 rpm) untilfracture. In the RTL method, a preset rotational loading (0.5 N⋅cm) was applied and the clockwise loading to the preset torqueand counterclockwise unloading to original position were repeated at 50 rpm until the file fractured. Fractured fragments by twomethods were compared under a scanning electron microscope (SEM) to examine the topographic features of the fractured surfacesand longitudinal aspects. SEM examinations showed significantly different features according to the loading methods. Specimensfrom the RTL method showed ruptured aspects on cross sections, with multiple areas of initiated cracks while the STL methodshowed the typical features of torsional failure, such as circular abrasion marks and fatigue dimples. This study suggested a newrepetitive torsional loading method which is much more clinically relevant and may result in a different fracture feature from STLmethod