الفهرس 
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Abstract
Effect of autoclave sterilization on cyclic fatigue resistance of NRT, ProTaper Universal and Revo-S was studied. Sixty rotary instruments were included. They were divided into 3 main groups. group 1: PTU#F3 and group 2: Revo-S#AS30, group 3: NRT#30*0.06. Each group contained 20 files divided into 4 subgroups (containing 5 files) according to the number of sterilization cycles (control, 1 cycle of sterilization, 2 cycles, 3 cycles). Sterilization of the files was done at 134° temperature and 2.1 bar. Then files were tested for cyclic fatigue resistance in stainless steel grooves that duplicate the size and taper of each instrument, utilizing a 60° angle of curvature and a 5 mm radius with a 4.3 mm length of curvature. Files were rotated at 250 rpm and torque was adjusted according to the manufacturer’s recommendation for each file. Handpiece lubricant was used to facilitate the sliding of the file inside the groove and to minimize friction while rotating. Each file was inserted for 20 mm length then rotated.
Separation of the file was monitored through magnifying loupes and time to failure was recorded in minutes using a stop watch. Then, number of cycles to failure was calculated (NCF = T x 250/60).
Results were tabulated and statistical analysis was carried out. The effect of sterilization cycle, system used and their interaction had significant effect on the mean NCF. The effect of sterilization was found to be system dependent, where it had no effect on PTU, while it had a variable significant negative effect on both of NRT and RS.
SEM analysis of the fractured files revealed the typical picture of cyclic fatigue fracture, with a single or multiple crack origin(s), slow crack propagation area and a fast fracture surface with dimpling and voids.