REVIEW New thermomechanically treated NiTi alloys – a review J. Zupanc 1 , N. Vahdat-Pajouh 1 & E. Sch€ afer 2 1 Department of Periodontology and Operative Dentistry, Westphalian Wilhelms-University, M€ unster; and 2 Central Interdisciplinary Ambulance in the School of Dentistry, M€ unster, Germany Abstract Zupanc J, Vahdat-Pajouh N, Sch€ afer E. New thermomechanically treated NiTi alloys – a review. International Endodontic Journal, 51, 1088–1103, 2018. In the past 10 years, several proprietary processing procedures for nickel titanium (NiTi) alloy were devel- oped to improve the mechanical properties of NiTi endodontic instruments. Beside specific thermal and mechanical treatments, manufacturers introduced several machining procedures (e.g. twisting, electrical discharge machining), as well as techniques for final surface finishing. NiTi alloys used for endodontic instruments can be subdivided into instruments that mainly contain the austenite phase (austenitic: con- ventional NiTi, M-Wire, R-Phase) and those mainly containing the martensite phase (martensitic: CM Wire, Gold and Blue heat-treated NiTi). Thermome- chanically treated NiTi alloys have been reported to be more flexible with improved cyclic fatigue resistance and greater angle of deflection at failure when compared to conventional NiTi. These enhanced properties may be attributed to a modified phase composition containing varying amounts of R- phase and martensite. Endodontic instruments made of austenitic alloys possess superelastic properties because of stress-induced martensite transformation and consequently tend to spring-back to their original form after deformation. In contrast, the martensitic instruments can easily be deformed due to the reori- entation of the martensite variants and show a shape memory effect when heated. The use of martensitic alloy results in more flexible instruments, with an increased cyclic fatigue resistance compared with aus- tenitic alloy. Keywords: austenitic NiTi alloy, controlled memory, martensitic NiTi alloy, metallurgical proper- ties, thermomechanical treatment. Received 21 December 2017; accepted 16 March 2018 Introduction Nickel titanium (NiTi) endodontic instruments have been reported to be more flexible with an increased torsional fracture resistance when compared to stain- less steel instruments (Walia et al. 1988). These enhanced characteristics allowed a substantial improvement in engine-driven endodontic instru- ments (Fife et al. 2004). Though, fracture of rotary NiTi instruments remains an inadvertent incident during clinical use (Pruett et al. 1997, Gutmann & Gao 2012). Beside variations in the design of NiTi instruments, manufacturers have introduced several proprietary manufacturing procedures including ther- mal, mechanical and surface treatment to improve the mechanical properties of NiTi alloys, in order to produce instruments with enhanced resistance to fracture and increased flexibility. Preparation errors during canal instrumentation (e.g. zips, ledges, perforations or canal transporta- tions) can occur. Both, stainless steel and NiTi endodontic instruments tend to straighten within a curved root canal resulting in a lateral force upon the root canal wall. To minimize this lateral force and to ensure a centred preparation, great flexibility is a Correspondence: Edgar Sch€ afer, Central Interdisciplinary Ambulance, Waldeyerstr. 30, D-48149 M€ unster, Germany (fax: + 251/834 3749; e-mail: [email protected]). © 2018 International Endodontic Journal. Published by John Wiley & Sons Ltd International Endodontic Journal, 51, 1088–1103, 2018 doi:10.1111/iej.12924 1088
New thermomechanically treated NiTi alloys – a review
Jun 29, 2023
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