Cyclic Fatigue Resistance of Three Different Nickel-Titanium Instruments after Immersion in Sodium Hypochlorite Eugenio Pedull  a, PhD, DDS,* Nicola M. Grande, PhD, DDS, † Gianluca Plotino, PhD, † Alfio Pappalardo, DDS,* and Ernesto Rapisarda, MD, DDS* Abstract Introduction: The purpose of this study was to assess the resistance to cyclic fatigue of three nickel-titanium (NiTi) files after the immersion in sodium hypochlorite (NaOCl) solution in conditions similar to those used in clinical practice. Methods: A total of 150 new Twisted Files (SybronEndo, Orange, CA), Revo S SU files (Micro Mega, Besancon, France), and Mtwo files (Sweden and Martina, Padova, Italy), size 25.06, were tested. Fifty files of the same brand were randomly assigned to five groups (n = 10) and submitted to the following immersion protocol in 5% NaOCl at 37  C for 16 mm: no immersion (control), 5 minutes statically, 1 minute statically, 5 minutes dynamically (300 rpm/min), and 1 minute dynamically. Resistance to cyclic fatigue was determined by counting the numbers of cycles to failure in a 60  curve with a 5-mm radius. Data were analyzed by two-way analyses of variance. Results: Resistance to cyclic fatigue of the same NiTi file was not significantly affected by immersion in NaOCl (P > .05). The Twisted File showed a higher resistance in all groups than Revo S SU (P < .001). The comparison between the same groups of Twisted Files and Mtwo files or between Mtwo and Revo S files did not show significant differ- ences (P > .05) except for two cases: group 2 of the Twisted Files and Mtwo files and group 5 of the Mtwo and Revo S SU files (P < .05). Conclusions: Static or dynamic immersion in NaOCl for 1 minute or 5 minutes did not reduce the cyclic fatigue resistance of NiTi signif- icantly. However, the type of instrument influences cyclic fatigue resistance. In our study, Twisted Files were more resistant followed by Mtwo and Revo S SU files. (J Endod 2011;37:1139–1142) Key Words Corrosion, cyclic fatigue, nickel-titanium, rotary instru- ments, sodium hypochlorite T he goals of root canal treatment are to achieve a high standard of disinfection of the root canal system through chemomechanical instrumentation and fill the canal to prevent reinfection (1, 2). Canal preparation requires a continuous and progressively tapered shape so as to allow irrigants to be delivered to the apical section of canal and perform its bactericidal action (3, 4) and to dissolve organic substances (5). Over the last 2 decades, nickel-titanium (NiTi) rotary files have become increasingly popular for this task (6). There is clear experimental and clinical evidence that the use of NiTi files, compared with stainless steel K-files, promotes canal shapes that follow the original canal path with less procedural errors (6, 7). On the other hand, in vitro studies have suggested that NiTi rotary files fracture more often than stainless steel files (8). The fracture of instruments used in rotary motion occurs in two different ways: torsion and flexural cyclic fatigue (9, 10). Torsional fracture occurs when an instrument tip or another part of the instrument is locked in a canal while the shank continues to rotate (11). Fracture caused by fatigue through flexure occurs because of metal fatigue. The instrument does not bind in the canal, but it rotates freely in a curva- ture, generating tension/compression cycles at the point of maximum flexure until the fracture occurs. This repeated tension-compression cycle, caused by rotation within curved canals, increases cyclic fatigue of the instrument over time and may be an impor- tant factor in instrument fracture (6, 12). Efforts are ongoing to identify factors that impact NiTi rotary fracture resistance (eg, file sequence, torque-limiting motors, cross-sectional file design, and file surface treatments as electropolishing and magneto- electropolishing processes) (13, 14). One additional factor potentially limiting the resistance to fatigue and torsional fracture is corrosion that may occur in the presence of NaOCl solution (15–17). Sodium hypochlorite (NaOCl) is the most common irrigant used in root canal treatment. It has a very unpleasant odor; it tends to discolor (18) and corrode surgical instruments (19). NiTi instruments come into contact with NaOCl during disinfection (20) or when the solution is present in the pulp chamber and root canal during instrumentation (21), and for this reason the time course and extent of corrosive action of NaOCl on NiTi surfaces are currently unclear (22). The corrosion patterns, involving selective removal of nickel from the surface, can create micropitting (23). It is supposed that these microstructural defects can lead to areas of stress collection and crack formation, weakening the structure of the instrument (24). The corrosive effect of NaOCl on endodontic NiTi instruments has been studied previously. Haikel et al (25) used hand instruments totally immersed in a 2.5% solution for 24 to 48 hours and detected no obvious corrosion. Busslinger et al (26) used 5% NaOCl for 30 or 60 minutes and Lightspeed rotary instruments and found corrosion patterns, even if the authors were not sure of the clinical implications (Discus Dental, Culver City, CA). However, these studies provide a contact of endodontic instru- ments with hypochlorite solution only in static mode and for different times from what occurs in clinical conditions. In fact, the working part of endodontic instruments contacts dynamically the NaOCl solution only for a few minutes during endodontic From the *Department of Surgery, University of Catania, Catania, Italy; and † Department of Endodontics, Sapienza University of Rome, Rome, Italy. Address requests for reprints to Dr Eugenio Pedull a, Via Cervignano, 29, 95129, Catania, Sicily, Italy. E-mail address: eugeniopedulla@gmail.com 0099-2399/$ - see front matter Copyright ª 2011 American Association of Endodontists. doi:10.1016/j.joen.2011.04.008 Basic Research—Technology JOE — Volume 37, Number 8, August 2011 Cyclic Fatigue of Rotary Instruments after NaOCl Immersion 1139