Decrease in the fatigue resistance of nickel-titanium rotary instruments after clinical use in curved root canals Maria Guiomar A. Bahia, DDS, MS, PhD, a and Vicente Tadeu Lopes Buono, BS, MS, PhD, b Belo Horizonte, Minas Gerais, Brazil FEDERAL UNIVERSITY OF MINAS GERAIS Objective. The changes in fatigue resistance of nickel-titanium rotary ProFile instruments after clinical use for shaping 10 curved molar root canals were evaluated in this study. Study design. Twenty-five sets of files #20, #25, and #30 and tapers .04 and .06 were divided into 2 groups, one with 10 sets of new files that were tested in a fatigue test bench device as a control. The other, experimental group, with 15 sets of clinically used files, was tested in the same device. The Student t test was employed to compare mean values of the measured parameters. Results. A statistically significant decrease in the number of cycles to failure was determined for the clinically used files, as compared with the new ones. The fracture point was the same for all files tested. Conclusions. The clinical use of ProFile instruments for shaping curved canals reduces their fatigue resistance. (Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2005;100:249-55) Root curvature is a frequent occurrence in the human dentition, especially in molars, being one of the various anatomical complexities characterizing the root canal system, and introduces factors that, if not properly controlled during canal preparation, may lead to undesirable technical results. The parameters radius and angle of curvature, as defined by Pruett et al, 1 are generally employed to describe the geometrical char- acteristics of curved root canals. The radius of curvature describes, for canals with the same angle of curvature, how abrupt is the canal deviation from a straight line: the smaller the curvature radius the more abrupt the canal deviation. Abrupt curvatures, especially those localized in the apical one-third of a canal, present the greatest difficulty because they do not allow much alteration of the curvature radius via canal coronal enlargement, since their influence is expressed through a very short lever arm. 2 For this reason, the high incidence of secondary curvatures in human mandibular molars, 30%, and the fact that they are usually localized in the apical one-third of the canal, at a mean distance of 2.2 mm from the foramen, 3 make root canal instrumen- tation even more difficult. In the continuous search for improvement of endodontic instruments, the greatest innovation in- troduced in the last few years was the development of nickel-titanium (NiTi) rotary files, which made the safe instrumentation of curved canals easier and reduced canal transportation. NiTi instruments are superelastic and flex far more than stainless steel files before exceeding their elastic limit. 4 Despite the many advantages of NiTi instrumentation, unexpected file separation occurs during clinical use, and the removal of the broken fragment may not be feasible, compromising the outcome of the root canal treatment. 5 When using NiTi rotary instruments to clean and shape curved root canals, the continuous tensile and compressive stress cycles in the region of maximum curvature of the canal lead to mechanical fatigue. Thus, the knowledge of how these files behave under cyclic loading is relevant, because fatigue resistance is the parameter that de- termines, in most cases, the applicability of the device. 6 Once the risk of occurrence of fatigue failure in NiTi rotary endodontic files during shaping curved root canals has been established, the need to evaluate the remaining fatigue life of these files after routine clinical use becomes clear. Therefore, the purpose of this study was to evaluate how the clinical use of ProFile NiTi endodontic instruments in shaping curved molar root canals influences the deterioration of their fatigue life. MATERIAL AND METHODS Twenty-five sets of .04 and .06 ProFile instruments (Dentsply Maillefer, Ballaigues, Switzerland), in the sizes 20, 25, and 30, totaling 150 files, were divided into This work was partially supported by Fundac ¸ao de Amparo a Pesquisa do Estado de Minas Gerais (FAPEMIG), Belo Horizonte, MG, Brazil, and Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq), Brası ´lia, DF, Brazil. a Associate Professor, Department of Restorative Dentistry, Faculty of Dentistry, Federal University of Minas Gerais, Belo Horizonte, MG, Brazil. b Associate Professor, Department of Metallurgical and Materials Engineering, School of Engineering, Federal University of Minas Gerais, Belo Horizonte, MG, Brazil. Received for publication Sep 23, 2004; returned for revision Oct 12, 2004; accepted for publication Oct 13, 2004. Available online 25 January 2005. 1079-2104/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.tripleo.2004.10.013 249