![]() ![]() ![]() Near-equiatomic NiTi alloys have three microstructural phases (austenite, martensite, and R-phase), and their properties and their respective proportions influence the mechanical properties of the metal ( 15). Similar to other metallic systems, NiTi alloys can come in a variety of crystallographic forms. The NiTi alloys of the endodontic instruments are made of ~56% nickel and 44% titanium by weight, or a 1:1 atomic ratio (equiatomic) ( 14). Properties of Each Phase (Austenitic, Martensitic, and R-Phase) In recent years, many new NiTi instruments have been introduced, and understanding the nature of NiTi alloy depending on their phases and their effects on instrument performance is important for clinicians for attaining favorable clinical results. New NiTi instrument systems with a titanium oxide surface layer are made from NiTi alloy heat-treated in a special way. Novel NiTi instruments produced by using thermomechanical techniques, such as M-wire, R-phase, and controlled memory (CM) files, have been launched in recent years and shown to have enhanced flexibility and cyclic fatigue resistance when compared to conventional superelastic NiTi files ( 11– 13). Nickel–titanium alloy has found a unique commercial application in the endodontic industry, because of its shape memory effect and corrosion resistance results from phase transformation. Heat treatment (thermal processing) has been reported to influence the fatigue resistance of NiTi instruments and is one of the most common methods for adjusting NiTi alloy transition temperatures ( 7– 10). The relative proportions and properties of the microstructural phases govern the mechanical behavior of NiTi alloy. Despite significant advancements in file design and manufacturing procedures for NiTi rotary instruments during the last two decades, fracture of rotary instruments induced by torsional or cyclic fatigue remains a concern for clinicians, particularly in calcified or severely curved root canals ( 4– 6). ![]() The first NiTi rotary instruments were marketed in the 1990s ( 3). Since Walia and colleagues first introduced nickel–titanium (NiTi) instruments in the late 1980s, NiTi instruments have revolutionized the root canal instrumentation by reducing the majority of iatrogenic instrumentation issues commonly associated with stainless steel files such as zipping, ledges, transportation, and perforation ( 1, 2). In this study, the impact that such developments have on the properties of endodontic NiTi files is discussed. This review focuses on the metallurgical properties of endodontic NiTi files, with a special emphasis on recent developments and improvements in metallurgy and the effects of heat treatment and surface treatment. Understanding the behavior of the NiTi files guides the clinicians in choosing the correct instruments for different clinical and anatomical situations. Knowledge and thorough understanding of the characteristics of endodontic nickel–titanium (NiTi) files is paramount for dentists performing root canal treatments to patients. 3Department of Stomatology, Affiliated Hospital of Jining Medical University, Jining, China.2Faculty of Dentistry, Division of Endodontics, Department of Oral Biological and Medical Sciences, University of British Columbia, Vancouver, BC, Canada.1Department of Conservative Dentistry, School of Dentistry, Dental and Life Science Institute, Dental Research Institute, Pusan National University, Yangsan, South Korea.Sang Won Kwak 1,2 Ya Shen 2 He Liu 2,3 Zhejun Wang 2 Hyeon-Cheol Kim 1 Markus Haapasalo 2 * ![]()
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