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UBC Theses and Dissertations

The effect of cyclic fatigue on the hardness of new NiTi endodontic files : a nanoindentation study Hieawy, Ahmed


Objective: The purpose of this study was to compare the flexibility and cyclic fatigue of ProTaper Universal (PTU; Dentsply Tulsa Dental Specialties, Tulsa, OK) and ProTaper Gold (PTG; Dentsply Tulsa Dental Specialties, Tulsa, OK) in relation to their phase transformation behavior, as well as to determine the effect of cyclic fatigue on the nanohardness with a nanoindentation method. Hypotheses: PTG and PTU have similar flexibility and fatigue resistance. Cyclic fatigue has no effect on the hardness of both PTG and PTU NiTi rotary endodontic instruments. Methodology: PTU and PTG instruments were subjected to rotational bending at a curvature of 40° and a radius of 6mm. The number of cycles to fracture (NCF) was recorded. According to the ISO 3630-1 specification, 45° bending tests was used to determine the flexibility. Unused and fractured instruments were studied by differential scanning calorimetry. The hardness and modulus of elasticity of new files, fractured instruments and instruments stressed to 50% of the NCF for sizes S1, F1 and F2 were measured with the use of a nanoindenter. Results: PTG had a cyclic fatigue resistance superior to PTU in all sizes (P <.001). The fractured files of both PTU and PTG showed the typical fracture pattern of fatigue failure. Bending load results for PTG were significantly lower than that for PTU (P <.05). The differential scanning calorimetry analyses showed that PTG instruments had a higher austenite finish temperature (50.1°C ± 1.7°C) than the PTU instruments (21.2°C ± 1.9°C) (P <.001). There were no significant differences in the austenite finish between unused files and instruments subjected to the fatigue process. There were statistically significant differences in nanohardness and elastic modulus between PTU and PTG groups (P < 0.05). Conclusions: Within the limitation of this study, PTG files were significantly more flexible and resistant to fatigue than PTU files. PTG exhibited different phase transformation behavior than PTU, which may be attributed to the special heat treatment history of PTG instruments. The fatigue process had no significant effect on the hardness and elastic modulus of both NiTi instrument. PTG may be more suitable for preparing canals with sudden curvature.

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