Influence of Continuous Rotation and Optimal Torque Reverse Kinematics on the Cyclic Fatigue Strength of Endodontic NiTi Clockwise Cutting Rotary Instruments

Abstract

Objectives: The objective of the present study was to evaluate the cyclic fatigue strength of clockwise cutting rotary endodontic instruments when subjected to two different kinematics: continuous clockwise rotation and clockwise reciprocation movement under optimum torque reverse (OTR) motion. Methods: New ProTaper Next X1 (n = 20) and X2 (n = 20) instruments were randomly divided into two subgroups (n = 10) based on kinematics (continuous rotation or OTR). The specimens were tested using a custom-made device with a non-tapered stainless-steel artificial canal measuring 19 mm in length, featuring a 6 mm radius and an 86-degree curvature. All instruments were tested with a lubricant at room temperature until a fracture occurred. The time to fracture and the length of the separated fragment were recorded. Subsequently, the fractured instruments were inspected under a scanning electron microscope for signs of cyclic fatigue failure, plastic deformation, and/or crack propagation. The subgroup comparisons for time to fracture and instrument length were performed using the independent samples t-test, with the level of statistical significance set at 0.05. Results: When using OTR movement, the ProTaper Next X1 increased the time to fracture from 52.9 to 125.8 s (p < 0.001), while the ProTaper Next X2 increased from 45.4 to 66.0 s (p < 0.001). No subgroup exhibited plastic deformations, but both showed dimpling marks indicative of cyclic fatigue as the primary mode of failure. Additionally, OTR movement resulted in more metal alloy microcracks. Conclusions: The use of OTR motion extended the lifespan of the tested instruments and resulted in a higher number of metal microcracks. This suggests that OTR motion helped to distribute the mechanical stress more evenly across the instrument, thereby relieving localized tension. As a result, it delayed the formation of a single catastrophic crack, enhancing the overall performance of the instruments during the experimental procedures.

Keywords: cyclic fatigue; endodontics; kinematics; reciprocation; root canal therapy; scanning electron microscope.

Figure 1

Representative photos of the separated surfaces of ProTaper Next X1 files: (a) axial view of the separated surface from the rotation group; (b) axial view of the separated surface from the OTR group; (c) lateral view of the separated area from the rotation group; (d) lateral view of the separated area from the OTR group. The dimpling areas visible on the whole instrument’s axial view surfaces and the absence of plastic deformations on the tips indicate features consistent with cyclic fatigue failure.

Figure 2

Representative photos of the fractured surfaces of the ProTaper Next X2 instruments: (a) axial view of the fractured surface from the rotation group; (b) axial view of the fractured surface from the OTR group; (c) lateral view of the separated area from the rotation group; (d) lateral view of the separated area from the OTR group. The dimpling areas visible on the whole instrument’s axial view surfaces and the absence of plastic deformations on the tips indicate features consistent with cyclic fatigue failure.

Figure 3

Representative images of the OTR groups showing metal alloy microcracks (red arrows) in both the ProTaper Next X1 (a) and ProTaper Next X2 (b) instruments.