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Instrumentation employed an automated endodontic device with controlled downward loading and torque/force sensing, comparing 1, 2, and 3 N and continuous rotation (CR) versus optimum torque reverse (OTR). All groups completed without fracture or ledge formation. OTR-3N reduced upward screw-in force versus OTR-2N, while CR-1N outperformed OTR-1N in time. At 1 mm from the apex, CR-2N showed a larger canal-centering ratio deviation than OTR-2N. Downward load 2–3 N in OTR matched CR shaping efficiency, with reduced screw-in force and improved apical canal-centering, supporting OTR as a promising alternative for curved 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Heat-Treated Nickel–Titanium Rotary Instrument  \nRisako Yamamoto 1, Keiichiro Maki 1, *, Shunsuke Kimura 1, Satoshi Omori 1, Keiko Hirano 1, Arata Ebihara 1, Yoshio Yahata 1 and Takashi Okiji 1,2  \nAcademic Editor: Bongju Kim  \nReceived: 18 November 2025  \nRevised: 20 December 2025  \nAccepted: 23 December 2025  \nPublished: 28 December 2025  \nCopyright: © 2025 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license.  \n1 Department of Pulp Biology and Endodontics, Division of Oral Health Sciences, Graduate School of Medical and Dental Sciences, Institute of Science Tokyo, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan; [r.yamamoto.endo@tmd.ac.jp](r.yamamoto.endo@tmd.ac.jp) (R.Y.); [s.kimura.endo@tmd.ac.jp](s.kimura.endo@tmd.ac.jp) (S.K.); [s.omori.endo@tmd.ac.jp](s.omori.endo@tmd.ac.jp) (S.O.); [k.hirano.endo@tmd.ac.jp](k.hirano.endo@tmd.ac.jp) (K.H.); [a.ebihara.endo@tmd.ac.jp](a.ebihara.endo@tmd.ac.jp) (A.E.); [yahata.yoshio@tmd.ac.jp](yahata.yoshio@tmd.ac.jp) (Y.Y.); [t.okiji@tky.ndu.ac.jp](t.okiji@tky.ndu.ac.jp) (T.O.)  \n2 Department of Endodontics, The Nippon Dental University School of Life Dentistry at Tokyo, 1-9-20 Fujimi, Tokyo 102-8159, Japan  \n* [Correspondence: makendo@tmd.ac.jp](Correspondence: makendo@tmd.ac.jp); Tel.: +81-3-5803-5494  \nAbstract  \nThis study analyzed how different downward loads and rotational kinematics influence NiTi rotary instrumentation outcomes. Heat-treated NiTi instruments were used to prepare extracted human single-rooted premolars with a moderate canal curvature. Instrumentation was performed using an automated endodontic instrumentation device with controlled downward loading and torque/force sensing, under different downward load settings (1, 2, and 3 N), employing either continuous rotation (CR) or optimum torque reverse (OTR) motion, which is a torque-sensitive reciprocation. Instrumentation was completed without instrument fracture or ledge formation in all six groups. OTR-3N specimens displayed a significantly lower upward force (i.e., screw-in force) than OTR-2N specimens (p \u003C 0.05) . OTR-1N specimens required a significantly longer instrumentation time than CR-1N specimens and the other OTR specimens (p \u003C 0.05) . At 1 mm from the apex, CR-2N specimens showed a significantly larger canal-centering ratio (i.e., larger deviation) than OTR-2N specimens (p \u003C 0.05) . Overall, applying a downward load of 2–3 N in OTR mode provided shaping efficiency similar to CR, but with a reduced screw-in force and enhanced canal-centering in the apical region, supporting the use of OTR as a promising alternative to CR for curved canal preparation using heat-treated NiTi instruments.  \nKeywords: nickel–titanium rotary instrument; root canal treatment; optimum torque reverse mode; canal-centering ratio; torque; vertical force; downward load; instrumentation time  \n1. Introduction  \nApical periodontitis develops as an inflammatory response of the periradicular tissues to bacterial infections originating from the root canal system. Root canal instrumentation plays a critical role in mechanically eliminating infected material, shaping the root canal to facilitate efficient irrigation, and preparing it for proper obturation, thereby reducing the microbial load and promoting healing of the periapical tissues [1–3] . Nickel–titanium (NiTi) rotary instruments have become widely accepted for root canal treatments owing to their superior flexibility [4], which helps preserve the original root canal curvature. Nevertheless, clinical challenges persist, such as unexpected instrument fracture [5] and  \ncanal transportation [6,7] . To overcome these issues, NiTi rotary instruments have undergone progressive advancements, such as heat treatments that enhance their flexibility [8] and improved","cbCaiiqmxPu6RFvg","https://ap.wps.com/l/cbCaiiqmxPu6RFvg","pdf",1535192,12,"English","# Introduction\n## Apical periodontitis and role of root canal instrumentation\n## Advancements in NiTi rotary instruments\n## Operational factors: vertical load and insertion speed\n## Rotation modes: continuous rotation and reciprocation\n## Optimum torque reverse mode","[{\"question\":\"What variables were tested in the study?\",\"answer\":\"The study compared downward loads of 1, 2, and 3 N and two rotational kinematic modes: continuous rotation (CR) and optimum torque reverse (OTR).\"},{\"question\":\"What were the main findings regarding instrument fracture and ledge formation?\",\"answer\":\"Instrumentation was completed without instrument fracture or ledge formation in all six experimental groups.\"},{\"question\":\"How did OTR performance compare with CR for curved canals?\",\"answer\":\"OTR with a 2–3 N downward load provided shaping efficiency similar to CR, with reduced screw-in force and enhanced canal-centering in the apical region.\"}]","Impact of Downward Load and Rotational Kinematics on Root Canal Instrumentation with a Heat-Treated Nickel–Titanium Rotary Instrument | PDF",1790771078]