Influence of cyclic torsional loading on the fatigue resistance of K3 instruments M. G. A. Bahia 1 , M. C. C. Melo 1 & V. T. L. Buono 2 1 Department of Restoration Dentistry, Faculty of Dentistry, Federal University of Minas Gerais, Belo Horizonte, MG; and 2 Department of Metallurgical and Materials Engineering, School of Engineering, Federal University of Minas Gerais, Belo Horizonte, MG, Brazil Abstract Bahia MGA, Melo MCC, Buono VTL. Influence of cyclic torsional loading on the fatigue resistance of K3 instruments. International Endodontic Journal, 41, 883–891, 2008. Aim To evaluate the influence of cyclic torsional loading on the flexural fatigue resistance and torsional properties of rotary NiTi instruments. Methodology Twelve sets of new K3 instruments, sizes 20, 25 and 30 with an 0.04 taper, and sizes 20 and 25 with an 0.06 taper, were torsion tested until rupture, to establish their mean values of maximum torque and angular deflection. Twelve new K3 instru- ments of each of the following dimensions, size 30, 0.04 taper and sizes 20 and 25 with 0.06 taper, were tested to failure by rotation bending in a fatigue test device. Cyclic torsional loading was performed in 20 cycles from zero angular deflection to 180° and then return to zero applied torque. After cyclic loading, the same number of instruments were tested until rupture in torsion and flexural fatigue. Data obtained were subjected to a one way analysis of variance (anova) at 95% confidence level. Results Cyclic torsional loading caused no significant differences in maximum torque or in maximum angular deflection of the instruments analysed, but comparative statistical analysis between measured NCF values of new and previously cycled K3 instruments showed signifi- cant differences for all tested instrument. Longitudinal cracks, that is, cracks apparently parallel to the long axis of the instruments cycled in torsion was observed. Conclusions Cyclic torsional loading experiments in new K3 rotary endodontic instruments showed that torsional fatigue decreased the resistance of these instruments to flexural fatigue, although it did not affect their torsional resistance. Keywords: cyclic loading, endodontic instruments, fatigue resistance, longitudinal cracks, nickel–titanium. Received 15 February 2008; accepted 16 May 2008 Introduction The peculiar properties of shape memory alloys are related to a reversible solid-to-solid phase transforma- tion, the martensitic transformation, which can be thermally or stress-induced (Otsuka & Wayman 1998). One of these properties is superelasticity, which has allowed for the development of NiTi endodontic instru- ments. Facilitating the preparation of curved and narrow root canals whilst maintaining the original anatomy is the major characteristics of these instru- ments (Scha ¨fer & Florek 2003). During root canal shaping, rotary NiTi instruments undergo flexural and torsional cyclic loads simulta- neously. These time-varying stresses can lead to fatigue and failure (Pruett et al. 1997). Cyclic loading is one of the generic characteristic features of many of the present applications of NiTi shape memory alloys. It is well accepted that the behaviour of different engineer- ing materials under cyclic loading depends on material strength, microstructure, surface quality and fatigue loading type (Eggeler et al. 2004). Cyclic loading is associated with structural and functional fatigue, both limiting the service life of moving components. Correspondence: Prof. Vicente T. L. Buono, Department of Metallurgical and Materials Engineering, Federal University of Minas Gerais, Rua Espı ´rito Santo 35 room 206, 30160-030 Belo Horizonte, MG, Brazil (Tel.: +55 31 3238 1859; fax: +55 31 3238 1815; e-mail: vbuono@demet.ufmg.br). doi:10.1111/j.1365-2591.2008.01449.x ª 2008 International Endodontic Journal International Endodontic Journal, 41, 883–891, 2008 883