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

Abstract

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

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.

Figure 2

Longitudinal and lateral aspects of representative fractured fragments from the RTL method (left two columns) and STL method (right two columns). Specimens from the RTL method show rare unwinding near the fracture area, while the specimens from UTL method show typical features of unwinding (white arrow). The specimens from the RTL method show irregular and catastrophic aspects with some cracks without unwinding. RTL: repetitive torsional loading; STL: single continuous torsional loading.