Aim and Objective: This investigation analyzed the cyclic fatigue resistance (CFR) of three controlled memory (CM) of (twisted files, Hyflex cm, Hyflex EDM and Edgefile X3) after immersion in sodium hypochlorite.

Material and Method: 160 files of a similar brand were arbitrarily allocated to four groups (n =40) and submitted to the immersion protocol: Group 1: no immersion (as control), Group 2: 3 minute in 2.5% NaOCl at room temperature, Group 3: 32.5 minutes in 2. 5% NaOCl at 121ºC and one cycle of autoclave sterilization, Group 4: To five cycles of the same test protocols as in Group 3. Cyclic fatigue test was performed on rotating/reciprocating all instruments. The number of cycles to failure (NCF) for every instrument was calculated by multiplying the quantity of rotations per minute (RPM) multiplied by total time taken for the instrument to fracture (minutes).

Result: EdgeFile X3 files showed most increased cyclic fatigue resistance, followed by Hyflex EDM files, Hyflex CM files and least resistance displayed by twisted files. Instruments immersed in NaOCl solution after five cycles of autoclave sterilization show increase in cyclic fatigue resistance for heat-treated instruments in comparison with the control group.

Conclusions: This study show that fracture resistance to cyclic fatigue was the most increased for the EdgeFile X3 group followed by Hyflex EDM, Hyflex CM and least for Twisted File group and instruments undergoing dynamic immersion in NaOCl solution for long time and to five cycles of autoclave sterilization increase CFR.