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

Figure 1

Cross-sectional aspects of representative fractured fragments from the RTL method (two left columns) and STL method (two right columns) (A, a, B, and b: V-Taper 2; C, c, D, and d: HyFlex EDM; E, e, F, and f: ProTaper Next). Specimens from the RTL method show ruptured aspects with multiple levels of fracture surfaces and gross catastrophic features (white arrows in A, C, and E) without any circular abrasion marks. Magnified dotted box (a, c, and e) shows atypical features which were not seen in two right columns of STL method. The specimens from STL method show typical features of torsional failure, such as a circular abrasion mark (arrow circle) and fatigue dimple at the center of rotation (white circle). RTL: repetitive torsional loading; STL: single continuous torsional loading.