Current Strategies in Metallurgical Advances of Rotary NiTi Instruments: A Review

NiTi alloy has been used as the raw material for making endodontic files. The interest in NiTi instrument research has not waned with time. In fact, there is a renewed interest in this field, notably through the application of modern research tools and techniques from other fields, making it a truly cross-disciplinary area of research. In 1988, Walia and colleagues introduced nickeltitanium (NiTi) files to endodontics [1]. Civjan et al. [2] first suggested the use of NiTi alloy for fabrication of hand and rotary instruments. Rotary NiTi instruments have become popular as they can clean and shape root canals with fewer procedural errors and more predictability than stainless steel hand files [3]. However, these instruments are prone to separation without warning. Two modes of fractures have been identified for NiTi files: flexural fracture and torsional fracture. Flexural fracture occurs due to the cyclic fatigue experienced by the files within a curved canal. Repeated loading and unloading of NiTi files during instrumentation causes repetitive phase transformation resulting finally in torsional fracture of the instrument when it goes beyond the unrecoverable plastic deformation state [4]. New materials and methods of manufacturing NiTi rotary instruments are in demand for better performance. The realization that the mechanical properties of near equiatomic NiTi alloys are strongly influenced by the stressinduced phase transformation taking place in this alloy and that the microstructural changes introduced by thermomechanical treatments can control this phase transformation constitutes the modern trend toward developing new rotary endodontic instruments with improved mechanical properties. This review will address the new developments in nickel-titanium metallurgy and their impact on rotary NiTi file systems.