Fatigue Testing of Controlled Memory Wire Nickel-Titanium Rotary Instruments

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Introduction To improve the fracture resistance of nickel-titanium (NiTi) files, manufacturers have introduced new alloys to manufacture NiTi files and developed new manufacturing processes. This study was aimed to examine the fatigue behavior of NiTi instruments from a novel controlled memory NiTi wire (CM Wire). Methods Instruments of ProFile, Typhoon (TYP), Typhoon CM (TYP CM), DS-SS0250425NEYY (NEYY), and DS-SS0250425NEYY CM (NEYY CM) (DS Dental, Johnson City, TN) all size 25/.04 were subjected to rotational bending at the curvature of 35° and 45° in air at the temperature of 23° ± 2°C, and the number of revolutions to fracture (N f) was recorded. The fracture surface of all fragments was examined by a scanning electron microscope. The crack-initiation sites, the percentage of dimple area to the whole fracture cross-section, and the surface strain amplitude (ε a) were noted. Results The new alloy yielded an improvement of over three to eight times in N f of CM files than that of conventional NiTi files ( P < .05). The vast majority of CM instruments (50%-92%) showed multiple crack origins, whereas most instruments made from conventional NiTi wire (58%-100%) had one crack origin. The values of the fraction area occupied by the dimple region were significantly smaller on CM NiTi instruments compared with conventional NiTi instruments ( P < .01). The square (NEYY CM) versus the triangular (TYP CM) configuration showed a significantly different lifetime on CM wire at both curvatures ( P < .01). Conclusions The material property had a substantial impact on fatigue lifetime. Instruments made from CM Wire had a significantly higher N f and lower surface strain amplitude than the conventional NiTi wire files with identical design.