The prognosis of root canal treatment, and therefore the survival of the treated tooth, depends on the asepsis achieved inside the root canal system prior to root canal filling. The separation of rotary instruments inside the root canal system prevents root canal disinfection, predisposing to the persistence of primary endodontic infections.A better knowledge about the behavior of NiTi endodontic instruments will provide information about their resistance to fracture against different variables related to cyclic fatigue and will contribute to avoid intraoperative complications.