The aim of this paper was to evaluate the effect of torsional stress preloading angle on fatigue resistance of Typhoon (TYP) CM instruments. For this, TYP NiTi 25/.04, TYP NiTi 40/.04, TYP CM 25/.04 and TYP CM 40/.04 were rotated until fracture to obtain the mean angular deflection according to the +-ISO 3630-1 standard. Files were pre-torqued to 25, 50, and 75% of their elastic limit and then subjected to cyclic loading in a three-point binding device until fracture. The fatigue life was recorded for each file. The fracture surface of each fragment was examined with a scanning electron microscope. The study bore the following results – the angle of rotation at fracture of TYP CM was significantly higher than that of TYP instruments (P < 0.05). However, there was no significant difference between size 40 and size 25 in all types of files. The fatigue resistance of TYP CM was significantly higher than that of TYP instruments (P < 0.05). Size 25/.04, TYP and TYP CM files in all three preloading groups had a significantly lower fatigue life than files with no preloading (P < 0.05). Size 40/.04 TYP CM files in the 50% and 75% preloading groups had a significantly lower fatigue life than files in the groups with no preloading (P < 0.05). The fractured files in the preloading groups showed the typical pattern of fatigue failure. Conclusions: TYP CM files have a higher fatigue resistance than conventional TYP NiTi files, irrespective of the amount of previous torsional stress. Fatigue resistance of TYP CM and TYP instruments was reduced after torsional stress preloading. Size 25/.04 file fatigue life was affected by preloading at lower distortion angles than was size 40/.04 fatigue life.

Keywords: Nickel-titanium endodontic instruments, Angular deformation, Flexural fatigue resistance, Controlled memory