To date, more than 160 rotary NiTi systems have been used in clinical practice. 3 Then, in the 1990s, commercial rotary NiTi files became available, and John McSpadden and Johnson, known as the fathers founders of rotary NiTi files, developed a file with a taper of 0.02 in 1992 and the ProFile Orifice Shaper (Dentsply Sirona, USA) with tapers of 0.04 and 0.06, respectively, in 1994. 2 In 1988, the first NiTi hand file (#15) was introduced with orthodontic wire. In 1975, Civjan proposed the concept of fabrication of endodontic instruments using NiTi. 1) and is also known as shape memory alloy (SMA). Due to its superelasticity (SE), shape-memory effect (SME), and biocompatibility, NiTi alloy was used to fabricate orthodontic wires in 1971 (ref. space program at the Naval Ordnance Laboratory in 1963 and was called nitinol (NiTi alloy). Nickel-titanium alloy was first successfully developed by the U.S. Introduction and crystal phase composition of NiTi alloy Research on the mechanical properties of NiTi instruments has been increasing since 2007. The number of Chinese papers increased rapidly during the 5 years from 2002 to 2007, including 265 clinical studies investigating quality of obturation, clinic emergency, and efficacy of cleaning. But the English studies about clinical performance of NiTi files is relatively low, with a total of 35 papers. In the past 5 years from 2015 to 2021, the studies on efficacy of cleaning in English has increased slightly. The number of English studies on shaping ability ranks second. Mechanical properties of NiTi files cover the torsion, stiffness, and fatigue resistance of files and surface structure. From 2005 to 2021, more than 50% of the published studies, a total of 398 papers with an annual average of 27 papers, focused on the mechanical properties of NiTi files and ranking first among the topics. With the popularization of engine-driven rotary NiTi instruments in clinical practice, researchers continue to find and solve problems, and the technology, design, and manufacturing of NiTi instruments are constantly changing, thus promoting the rapid increase in research studies (Figs. From 1995 to 2005, the number of studies had slowly increase. These studies included 4 research directions: cleaning efficacy of instrumentation, shaping ability of root canal preparation, mechanical properties of NiTi files, and clinical outcome of root canal therapy.
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