A comparison of nickel-titanium rotary instruments manufactured using different methods and cross-sectional areas: ability to resist cyclic fatigue So-Ram Oh, DDS, a Seok-Woo Chang, DDS, MSD, b Yoon Lee, DDS, MSD, c Yu Gu, MD, a Won-Jun Son, DDS, PhD, a Woocheol Lee, DDS, PhD, a Seung-Ho Baek, DDS, PhD, a Kwang-Shik Bae, DDS, PhD, a Gi-Woon Choi, DDS, PhD, d Sang-Min Lim, DDS, a and Kee-Yeon Kum, DDS, PhD, a Seoul and Wonju, Korea SEOUL NATIONAL UNIVERSITY, SUNGKYUNKWAN UNIVERSITY, YONSEI UNIVERSITY, AND KYUNGHEE UNIVERSITY Objective. This study examined the effect of the manufacturing methods (ground, electropolished, and twisted) and the cross-sectional area (CSA) of nickel-titanium (NiTi) rotary instruments on their cyclic fatigue resistance. Study design. A total of 80 NiTi rotary instruments (ISO 25/.06 taper) from 4 brands (K3, ProFile, RaCe, and TF) were rotated in a simulated root canal with pecking motion until fracture. The number of cycles to failure (NCF) was calculated. The CSA at 3 mm from the tip of new instruments of each brand was calculated. The correlation between the CSA and NCF was evaluated. All fractured surfaces were analyzed using a scanning electron microscope to determine the fracture mode. Results. The TF instruments were the most resistant to fatigue failure. The resistance to cyclic failure increased with decreasing CSA. All fractured surfaces showed the coexistence of ductile and brittle properties. Conclusion. The CSA had a significant effect on the fatigue resistance of NiTi rotary instruments. (Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2010;109:622-628) The fatigue resistance of root canal instruments affects the outcome of instrumentation in curved root canals. Many factors have been implicated in the fatigue failure of NiTi rotary instruments. Of these, the surface topog- raphy has received particular attention. Reported results have demonstrated that machining scratches on the instrument surface can act as sites for crack initia- tion. 1-3 The traditional grinding process across the grains of a NiTi wire limits the overall strength of the instrument owing to the formation of machining defects along the flute surface and residual stress within the internal struc- ture. 4-7 Therefore, an electropolishing procedure (EP) has been attempted during the manufacturing process to re- duce the number of machining defects and residual stress of ground NiTi rotary instruments. This procedure re- moves the outer layer of a metal, leaving the surface free of contaminants, microcracks, and work-induced residual stress. 8 However, the effect of EP on the cyclic fatigue resistance is still controversial. 9-13 Recently, a new file system, TF (for “Twisted File”), came onto market with a new manufacturing process, including twisting of a file blank, R-phase heat treat- ment, and specific surface treatment, to decrease the formation of machining defects during the grinding process. The manufacturer claims that this manufactur- Supported by the 2008 SNUDH research fund (4-2008-0023). The first two authors contributed equally to this work. a Department of Conservative Dentistry, Dental Research Institute, BK21 Program, School of Dentistry, Seoul National University. b Department of Conservative Dentistry, Institute of Oral Health Sci- ence, Samsung Medical Center, School of Medicine , Sungkyunkwan University. c Department of Dentistry, Wonju College of Medicine, Yonsei Uni- versity. d Department of Conservative Dentistry, School of Dentistry, Kyung- hee University. Received for publication Oct 21, 2009; returned for revision Nov 27, 2009; accepted for publication Dec 3, 2009. 1079-2104/$ - see front matter © 2010 Mosby, Inc. All rights reserved. doi:10.1016/j.tripleo.2009.12.025 622 Vol. 109 No. 4 April 2010 ENDODONTOLOGY Editor: Larz S.W. Spångberg