Influence of Previous Angular Deformation on Flexural Fatigue Resistance of K3 Nickel–Titanium Rotary Instruments Fabiola Ormiga Galvão Barbosa, MSc,* José Antônio da Cunha Ponciano Gomes, MSc,* and Marcos Cesar Pimenta de Araújo, DSc † Abstract Several studies have evaluated the influence of torsion and bending loads on the mechanical resistance of nickel-titanium instruments. The aim of the present study was to analyze the influence of previous angular deformation on K3 files resistance to flexural fatigue. New files and files previously submitted to different angular deformation were tested for flexural fatigue, and the number of cycles to fracture was compared. The results indicated that as the angular deformation increases, the number of cycles attained under flexural fatigue condition decreases. A reduction of fatigue resistance was registered even when a significant plas- tic deformation was not imposed. From the obtained results, it may be suggested that the sequential load- ing, including torsion and flexural fatigue, can be re- sponsible for instrument separation during clinical prac- tice. Successive torsion overloads, occurring together with flexural fatigue, reduce the mechanical resistance of nickel-titanium files. (J Endod 2007;33:1477–1480) Key Words Endodontic instrument, flexural fatigue, fracture, nick- el-titanium, torsion N ickel-titanium (NiTi) rotary files have been largely used in endodontic clinical practice. The main limitation to the use of these instruments is the risk of fracture. Two main factors may cause the fracture of these instruments: flexural and torsional tensions (1). Ankrum et al. (2) investigated the incidence of file breakage and distor- tion when the K3 system (SDS Kerr, Glendora, CA) was used to instrument canals in the severely curved roots of extracted molars. The proportion of files distorted was 8.3%, and the percentage of broken files was 2.1%. Alapati et al. (3) hypothesized that the clinical fracture of NiTi rotary files is largely caused by a single overload incident during instrumentation. They stated that the number of cycles imposed during clinical use is not sufficient to cause fatigue fracture. It is worth noting that the authors consider only the bending fatigue, even if different types of tension are never applied to the files in isolation. The isolated analysis of different kinds of solicitation has led to the suggestion of file-usage limits different from those observed during the canal instrumentation. Several studies have evaluated the influence of the torsion and bending loads on the same instrument (4–9). Yared et al. (4, 5) and Gambarini (6) examined the influence of previous clinical usage of ProFile instruments (Dentsply, Maillefer, Bal- laigues, Switzerland) on the results of flexural fatigue testing. Yared et al related that previous simulated clinical use or clinical practice did not cause a significant reduction of the number of cycles to fracture. Gambarini’s results, however, showed that the group of previously used files resisted a lower number of cycles than the new files. They observed that the use of lower torque limits during previous clinical usage resulted in a higher resistance to bending fatigue. Yared et al. (7) also examined the influence of previous clinical usage on torsional resistance of K3 files. The results showed that the previously used files presented a lower torque at fracture when compared with the new files. Therefore, they concluded that the torque and the rotation angle at the moment of fracture are significantly influenced by the usage of the files in simulated curved canals. Booth et al. (8) performed load testing on ProFile instruments under different curvatures (50° with a 3-mm radius and 50° with a 5-mm radius). The authors observed a significant difference between the acute and the gradual curves, obtaining for the latter a higher torque to fracture. Ullmann et al. (9) investigated the influence of previous flexural fatigue on the torsional resistance of ProTaper files (Dentsply, Maillefer, Ballaigues, Switzerland). The files were previ- ously submitted to flexural fatigue tests at 30%, 60%, and 90% of the number of cycles to fracture, with a 90° curvature and a 5-mm radius. The results showed that flexural fatigue decreased the torque to fracture. The present study aimed to evaluate the mechanical resistance of K3 files under a mixed mechanical solicitation mode. The influence of previous torsional angular deformation on the number of cycles to fracture during flexural fatigue tests was studied. Materials and Methods Seventy 25.06 NiTi K3 rotary files (SDS Kerr, Glendora, CA) were used on flexural and torsional mechanical testing. Scanning electron microscopy (SEM) was used to characterize the material under study. From the Departments of *Metallurgy and Materials and † Edodontics, Federal University of Rio de Janeiro, Rio de Janeiro, Brazil. Supported by FAPERJ and CNPq. Address requests for reprints to Dr José Antônio da Cunha Ponciano Gomes, Av Ipiranga, 910/02, Petrópolis, RJ, CEP 25610-150, Brazil. E-mail address: ponciano@metalmat. ufrj.br. 0099-2399/$0 - see front matter Copyright © 2007 by the American Association of Endodontists. doi:10.1016/j.joen.2007.08.014 Basic Research—Technology JOE — Volume 33, Number 12, December 2007 Angular Deformation and Flexural Fatigue of K3 Instruments 1477