Abstract: AIM OF THE STUDY: The aim of this study is to evaluate and compare the effect of cyclic preloading on cyclic fatigue resistance in two rotary file systems. METHODOLOGY: Rotary files – One Shape and KOMET F360 were used for instrumentation in simulated root canals. Artificial canals were simulated measuring 1.5mm in width, 20mm in length, and 2. 5mm in depth with U-shaped cross-section, were machined into 316L stainless steel blocks by computer-assisted milling and hardened with polished chrome plating. A straight cervical segment measuring 10.58mm with an arc length of 9. 52mm and a curvature radius of 6mm, and the arc located in the apical portion of the canal. All the instruments were operated with a torque-controlled electric motor. The time to fracture was recorded using a digital clock. The length of the time between the beginning of the procedure until instrument fracture occurred (Ncf) was recorded. The mean value of number cycles to fracture (Ncf) was calculated for each group of files. Then each file system was subjected to preloading of 25 % of mean Ncf and then subjected to cyclic fatigue test. The experimental procedure was performed by a single operator to avoid inter-operator variability. CONCLUSION: It was concluded that cyclic preloading decreases the cyclic fatigue resistance of One Shape and KOMET F360 rotary file systems. Comparatively, KOMET F360 had higher cyclic fatigue resistance than the One Shape rotary file system.
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