Cyclic fatigue of different nickel-titanium endodontic rotary instruments Teresa Roberta Tripi, MD, PhD, a Antonio Bonaccorso, MD, b and Guglielmo Guido Condorelli, MD, PhD, c Catania, Italy UNIVERSITY OF CATANIA Objective. A comparative study of the fatigue resistance of rotary nickel-titanium endodontic instruments was performed with the aim of assessing the influence of both instrument design and surface treatment on flexural fracture. Study design. To evaluate fatigue resistance of different rotary instruments, a total of 120 instruments were tested; these came from different sources: ProFile, RaCe, K3, Hero, and Mtwo. To compare the effect of electro-polishing procedures on fatigue resistance, a group of RaCe instruments (which are normally electro-polished) without surface treatment was used. A scanning electron microscope (SEM) study for each instrument was performed before and after fatigue study to determine the mode of fracture and the aspect of tips and cross-sectional surface areas. Results. ProFile instruments gave the best values for fatigue resistance. It was seen that for RaCe instruments the surface treatment reduces the presence of micro-cracks, surface debris, and machining damage. Conclusions. This study shows that the instrument design often proves to be an important factor in the fatigue resistance of NiTi rotary instruments. In RaCe instruments the electro-polishing surface treatment increases the fracture- related fatigue resistance. (Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2006;102:e106-e114) An increasing number of new nickel-titanium (NiTi) rotary file systems are currently available. Recently developed NiTi files possess unique design properties in terms of cross-sectional shape, taper, surface treat- ment (electro-polishing), the number and angle of flutes, and so forth. However, little is known about the fatigue resistance of new continuously rotating nickel- titanium instruments. Moreover, no study has been published about the effect of surface treatment on fa- tigue resistance. FKG, the manufacturers of RaCe, states that one of the benefits of electro-polishing pro- cedures is to reduce instrument fatigue. Stated simply, electropolishing is the controlled electrochemical re- moval of surface metal, resulting in a brilliant appear- ance and improved properties. Sometimes described as “reverse plating,” or “Super Passivation,” the process has a leveling effect, which produces smoothness and increased reflectivity. More importantly, the deformed, amorphous outer layer of the metal is removed, leaving a passive surface free of imbedded contaminants and work-induced residual stresses. 1 In 1988, Walia et al. 2 introduced a new material for the manufacturing of endodontic instruments: nickel- titanium. NiTi instruments exhibit more elastic flexi- bility in bending and torsion, as well as superior resis- tance to corrosion compared with stainless-steel files manufactured by the same processes. 3,4 A number of studies have shown that these NiTi rotary systems are able to prepare root canals with excellent taper, less canal transportation, greater conservation of tooth structure, and at a much faster rate than hand files. 5,6 However, despite the evident advantages of the new technique, NiTi rotary instruments may experience fail- ure by fatigue or torsion. 7,8 Different studies have sug- gested that cyclic fatigue (flexural and torsional) caused by repeated tensile-compressive stress plays an impor- tant role in the fracture of NiTi instruments. 9-11 Tension is the force that pulls the structure of the material apart; compression is the force that pushes the structure of the material together. 7 In cyclic fatigue, fracture can occur even though the load amplitude may be much less than the yield strength of the material. 12 Precedent studies with scanning electron microscope (SEM) analysis showed the presence of micro-fissures and defects on the surface of both new and used NiTi instruments. These could be the prelude to further de- terioration of the instrument and could explain the reports of increased breakage of these instruments in root canals and in operative conditions that are appar- ently simple and without risk of breakage. 13 For Kuhn et al., 14 in the breakage process, the crack nucleation stage is facilitated by a high density of surface defects and the successive fatigue failure is largely due to a crack propagation process. For Cheung et al., 15 the a Professor, Faculty of Dentistry, University of Catania. b Staff Specialist, Faculty of Dentistry, University of Catania. c Professor, Department of Chemical Science, University of Catania. Received for publication Oct 31, 2005; returned for revision Dec 13, 2005; accepted for publication Dec 14, 2005. 1079-2104/$ - see front matter © 2006 Mosby, Inc. All rights reserved. doi:10.1016/j.tripleo.2005.12.012 e106