ORIGINAL RESEARCH Cyclic fatigue resistance of new and used Mtwo rotary nickel-titanium instruments in two different radii of curvature Ugur Inan, DDS, PhD 1 ; Cumhur Aydin, DDS, PhD 2 ; and Kadriye Demirkaya, DDS, PhD 2 1 Department of Restorative Dentistry and Endodontics, Faculty of Dentistry, Ondokuz Mayis University, Samsun, Turkey 2 Department of Endodontics and Conservative Treatment, Center for Dental Sciences, Gulhane Military Medical Academy, Ankara, Turkey Keywords cyclic fatigue, Mtwo instruments. Correspondence Dr Ugur Inan, Department of Restorative Dentistry and Endodontics, Faculty of Dentistry, 19 Mayis University, 55139 Kurupelit, Samsun, Turkey. Email: ugurinan29@hotmail.com doi:10.1111/j.1747-4477.2010.00236.x Abstract The fracture of nickel-titanium rotary instruments is a serious problem that can jeopardise the outcome of root canal treatment. The aim of this study was to compare the cyclic fatigue resistance of new and used Mtwo rotary nickel- titanium instruments in artificial canals with two different radii of curvature. Four hundred and twenty Mtwo instruments were used in the present study. Thirty new and 30 used instruments of each size were tested for cyclic fatigue in artificial canals with radii of curvature of 5 or 10 mm. Each instrument was rotated until fracture occurred and the number of cycles to fracture was calculated by multiplying time to fracture and rotational speed. Data were analysed using a t-test. According to the results of the study, radius of curvature was an important factor on cyclic fatigue resistance and clinical use signifi- cantly reduced the instrument lifespan. Introduction In the recent years, rotary nickel-titanium (NiTi) instru- ments have an important place in the endodontic practice (1). The super-elasticity of the NiTi allows the file to return its original shape after significant deformation unlike stainless steel that retains permanent shape change. They also reduce operator fatigue, procedural errors such as ledge formation and zipping and prepara- tion time (2). Because of these properties, rotary NiTi instruments are extensively used by general dentists and endodontists to facilitate the cleaning and shaping proce- dures in root canals. Intracanal file separation may be inevitable even with the most experienced endodontist (3). The fracture of rotary NiTi instruments may occur as a result of torsional, cyclic fatigue or a combination of the both (4). Torsional fracture occurs when an instrument tip is locked in a canal while the shank continues to rotate (5). This type of fracture can be seen when the elastic limit of the NiTi alloy is exceeded after binding of the instrument in the root canal (6). On the other hand, fracture because of cyclic fatigue is caused by work hardening and metal fatigue (7). Repeated tensile-compressive stresses occur at the curve by the rotation of the instrument, which results in fracture of the instrument (8). In order to avoid this type of fracture, manufacturers recommend discarding the NiTi instruments after a certain times of usage. Mtwo (VDW, Munich, Germany) rotary NiTi instru- ments have core width designed for maximum flexibility and have minimal radial contact with a non-cutting tip. In this system, all instruments are used at working length. Mtwo is the only system with #10.04 and #15.05 files and has no orifice shaper. Previous studies reported that cyclic fatigue resistance of a rotary file was dependent on the instrument size, the angle of canal curvature and the radius of the canal cur- vature and of these, radius of the canal curvature was the dominant factor causing cyclic fatigue (9). Therefore, the aim of this study was to evaluate the cyclic fatigue resis- tance of Mtwo rotary NiTi instruments in artificial canals with two different radii of curvature. Materials and methods Total 420 Mtwo instruments were used in this study. Thirty new and 30 used instruments of each size (10.04, 15.05, 20.06, 25.06, 30.05, 35.04 and 40.04) were tested Aust Endod J 2011; 37: 105–108 105 © 2010 The Authors Australian Endodontic Journal © 2010 Australian Society of Endodontology