Crystallographic Metallurgy of Nickel Titanium Instruments in Endodon-tics: A Comprehensive Review
Endodontic instrument, metallurgy, NiTi alloy, thermomechanically heat treatment
AuthorsAbstract:A solid grasp of the mechanical and physical properties of nickel titanium (NiTi) endodontic files, as well as their link to metallurgical characteristics is required for clini-cians to appreciate the role and proper use of NiTi instruments during root canal treatment. Various thermo mechanical treatment techniques have been created to increase the clinical performance and improve the fracture resistance of NiTi instruments.Knowledge will also assist clinicians in selecting the most appropriate instruments forroot canal treatment. During heat treatment, NiTi alloy undergoes phase change to increase mechanical characteristics. This narrative analysis evaluate how various thermo mechanical heat treatment technique affect the metallurgical properties of NiTialloys, as well as discussing newly introduced thermomechanical process altered NiTiendodontic instruments.
Introduction:Since the time of Walia et al., nickel titanium (NiTi) alloys have been utilized in the last 3decades in endodontics and they have brought about a major breakthrough in root canaltreatment (16). NiTi endodontic instruments were more flexible than stainlesssteel (SS)instruments due to enhanced resistance against torsional fracture (46). These enhancedqualities resulted in a significant improvement in engine or machine driven of endodontic instruments (2). In contrast to SS instruments, the use of NiTi instruments in engine or machine driven endodontic handpieces reduces the likelihood of procedural errors (3). However, despite the improved metallurgical properties, NiTi instruments are still vulnerable to be brittle during the biomechanical preparation of root canals (3). During mechanical instrumentation within the root canal, two forms of fracture of NiTi devices can occur: flexural fracture and torsional fracture (4). NiTi instruments may suffer a flexural fracture during biomechanical instrumentation of curved root canals due to an increase in their cyclic fatigue (4). Torsional fractures develop as a result of constant phase transformation inside the material produced by the repeated loading and unloading of the instrument during function (4,5). Applying stress to the instrument causes in microstructural changes, which produce phase transformation. Several thermomechanical modifications of the alloy can have an impact on these phenomenon (6). Numerous patented thermal processing techniques for NiTi alloys have been designed to enhance their mechanical properties (9). In addition to thermal and mechanical treatment techniques, manufacturers have also implemented some machining procedures such as the twisted method and the electrical discharge machining method and chemoelectrical surface finishing procedures (10). NiTi alloys used for endodontic devices from are classified into two types by major phases: those with a high concentration of austenite phase (traditional / conventional NiTi instruments, R- phase, M- wire) and the others with a high concentration of the martensite phase (Gold and Blue heat treated NiTi, controlled memory wire) (9). Whenthe proprietary thermo mechanically processed NiTi alloys were compared with theconventional NiTi, the thermo mechanically treated NiTi alloys demonstrated increasedflexibility, superior cycle fatigue resistance, and a larger distortion angle before torsionalfailure (7).These improved properties might be ascribed to a modified phase including differentproportions of martensitic alloy as well as Rphase alloy (11). Endodontic instruments composed of austenitic alloys have superelastic (SE) capabilities ••••••••••••••••••••••••••••••••
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