DOI: 10.18363/rbo.v74n1.p.23 23 Original Article/Endodontic Rev. Bras. Odontol., Rio de Janeiro, v. 74, n. 1, p. 23-6, jan./mar. 2017 Effect of autoclave cycles on cyclic fatigue resistance of control memory NiTi files observed by scanning electron microscopy Victor Talarico Leal Vieira, 1 Hélio Pereira Lopes, 2 Carlos Nelson Elias, 3 Leticia Chaves de Souza, 4 Emmanuel João Nogueira Leal da Silva, 1 Daniel de Brito Prata, 1 Edson Jorge Lima Moreira, 1 Marcia Valeria B. Vieira 2 1 Department of Endodontics, Faculty of Dentistry, Grande Rio University - UNIGRANRIO, Rio de Janeiro, RJ, Brazil 2 Department of Endodontics, Faculty of Dentistry, Estácio de Sá University -UNESA, Rio de Janeiro, RJ, Brazil 3 Department of Material Science, Military Institute of Engineering - IME, Rio de Janeiro, RJ, Brazil 4 Department of Endodontics, Faculty of Dentistry, Rio de Janeiro Federal University, Rio de Janeiro, RJ, Brazil • Conflicts of interest: none declared. AbstrAct Objective: Hyflex CM instruments were developed for debridement of the root canal system. The control-memory alloy recovers its shape on sterilization, thereby allowing instrument reuse. This study investigated the effect of shape recovery by sterilization on the fatigue life and crack formation of Hyflex CM NiTi instruments. Material and Methods: the cyclic fatigue test was performed in two groups: In the first, it was continuously performed to establish the fatigue life (group 1, n = 10); in the second, it was performed with outages of 30s to evaluate cracks’ nucleation and propagation (group 2, n = 10). The test was performed using a stainless steel canal that was 1.5mm in diameter and 3.5mm in depth, with an arc of 9.42mm and radius of curvature of 6mm. The instruments were driven at 300 rpm. The fatigue time was recorded using a digital stopwatch (Technos). An electronic microscope (Quanta 250) was used to observe the crack formation and growth during the cyclic fatigue. Results: the autoclaved instruments endured 1196.67+214.63 cycles, and the non-autoclaved endured 1000+199.42 cycles until the occurrence of fracture. The t-student test (α=5%) revealed a significant difference. The cyclic fatigue test with pauses demonstrated a significantly longer cyclic fatigue life than the continuous cyclic fatigue test (p<0.05). Conclusion: as revealed by SEM images, cracks grow with an increase in the number of cycles to fracture (NCF). It can be concluded that the shape of Hyflex CM instruments recovered by autoclaving withstood signifi- cantly more NCF than non-autoclaved instruments (p<0.05), demonstrating an improvement of approximately 16%. Keywords: Endodontics; Autoclavage; Electron microscopy; Fatigue resistence; Control memory NiTi files. Introduction A common complication inherent in the use of nickel-tita- nium instruments is their fracture, which can compro- mise the prognosis of canal treatment. 1 Diferent NiTi al- loys have been proposed to increase the fexibility and resistance to fatigue fracture of endodontic instruments. 2,3 Te Hyfex CM is one system that uses a controlled-memory alloy technology. Tese instruments are produced by thermomechanical treatment, resulting in more fexibility and greater resistance to fatigue. 2,4-6 Stress and temperature infuence the mechanical properties of endodontic instruments manufactured using the NiTi alloy; they are sensitive to thermal cycles, which modify the microstruc- ture. 7 Some studies have investigated these efects in the M-wire, Phase-R, and controlled-memory NiTi alloys. 8-10 Te latter alloy, which was recently introduced in endodontics, has shown prom- ising results. 2,11 More studies are needed to investigate how these efects afect the controlled-memory NiTi alloy. Te Hyfex CM manufacturer claims that the sterilization method enables reuse. However, further studies reporting the efects of mechanical cy- cling (fatigue) and heat (autoclaving) are needed to ensure that instruments are not re-used empirically. Terefore, the aim of this study was to evaluate the infuence of shape recovery by autoclaving on the number of cycles to frac- ture (NCF) and on the crack propagation of Hyfex CM endodon- tic instruments. Te null hypothesis tested was that there are no diferences in the cyclic fatigue fracture resistance between the instruments deformed and non-deformed plastically. Material and Methods • Instruments and Sterilization Twenty Hyfex CM (Coltene Whaledent, Cuyahoga Falls, OH, USA) instruments with a nominal size of 0.35 mm at D0, length of 25 mm, and constant taper of 0.04 mm/mm along the working length were used in this study. Te instruments were randomly distributed with a free computer algorithm program (http://www. random.org) into two experimental groups (n = 10 in each group). Prior to the tests, the instruments were sterilized by autoclaving using a Cistofoli Vitale 12 (Cristofoli Biosafety equipment LTDA, Campo Mourão, PR, Brazil). Cycles had a holding temperature between 134°C and 137°C for 3 min. 4 Te instruments were steril- ized before use, according to the manufacturer’s instructions, and afer each 30-s cycle of the cyclic fatigue test. Tese parameters comply with the requirements of the American National Stan- dard/American Dental Association Specifcation No. 101. 2001, Root Canal Instruments: General Requirements. • Cyclic Fatigue Tests Te cyclic fatigue test adopted the following procedures: 1. Group 1: ten instruments were subjected to the continuous cyclic fatigue test until fracture occurred, establishing the fatigue life without allowing instrument shape recovery. 2. Group 2: ten instruments were subjected to the cumulative cyclic fatigue test. Te test was stopped afer 30s, and the instru- ments were autoclaved to recover their shape before a new test. Tis sequence of tests was performed until fracture occurred in all instruments.