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Cyclic fatigue fracture is caused by repeated compressive/tensile stresses that accumulate around the curved rotating file. Many efforts have been made to increase the fracture resistance of Nickel-titanium (NiTi) rotary instruments, such as the use of heat-treated alloy, modification of the instrument's cross section and the improvement of surface treatment. OBJECTIVES: The aim of this study was to compare the static flexural cyclic fatigue of TwoShape (Micro Mega, Besancon, France) rotary file system which is made of T-Wire heat treatment technology and OneShape (Micro Mega, Besancon, France) rotary file system which is made of conventional nickel-titanium alloy. MATERIALS AND METHODS: Eighteen files from each system were subjected to static cyclic fatigue test. Cyclic fatigue testing was performed in a device which allowed a reproducible position of the instrument inside a custom-made stainless-steel artificial canal. The artificial canal had a curvature of 60° angle and 5 mm radius. All instruments were allowed to rotate inside the artificial canal until fracture. Time to fracture (TTF) was recorded using a digital chronometer. The number of cycles to failure (NCF) was calculated. The length of separated fragments was recorded. Data were analysed. RESULTS: The NCF of TwoShape files (365.909±82.077) was statistically significantly higher when compared with the OneShape files (266.681±35.218) p < .05. While there was no significant difference in the length of separated segments between both systems. CONCLUSIONS:Within the limitations of this study, the cyclic fatigue resistance of heat treated TwoShape files was significantly higher than the OneShape files. [T-wire] heat treatment drastically enhanced the cyclic fatigue resistance of the file.

STATIC FLEXURAL CYCLIC FATIGUE OF TWO ROTARY FILE SYSTEMS (AN IN VITRO STUDY)