Characterization of new rotary endodontic instruments fabricated from special thermomechanically processed Nickel titanium wire

Although NiTi rotary instruments are very popular for endodontic treatment, instrument separation is still a challenge in clinic. A new NiTi rotary instrument (GTRTM Series X(TM), Dentsply Tulsa Dental Specialties) has recently been marketed that is machined from a wire (termed M-Wire) that has been subjected to a proprietary novel thermomechanical processing procedure. The manufacturer has claimed that this new M-Wire instrument has considerably improved flexibility and resistance to cyclic fatigue, compared to conventional rotary instruments that are machined from superelastic (SE) austenitic NiTi wire. Clinical use has confirmed that these new GT RTM Series X(TM) rotary instruments have outstanding clinical fatigue resistance (private communication from Dr. John Nusstein, Division of Endodontics, College of Dentistry, The Ohio State University). However, the mechanism for the improved clinical performance of these instruments is unknown.;The objective of this study was to employ a variety of metallurgical laboratory techniques to determine the origin of these improved mechanical properties for the new rotary instruments. Specimens from as-received M-Wire instruments, clinically used M-Wire instruments, and conventional instruments made from SE wire were prepared for evaluation. The temperature range for phase transformation was examined by differential scanning calorimetry (DSC). Vickers hardness measurements were made since hardness variations for the same type of alloy has been found to correlate with variations in mechanical properties. The microstructures of the NiTi alloys were revealed by acid etching and examined with an optical microscope and a scanning electron microscope that was also capable of X-ray energy-dispersive spectrometric analyses (SEM/EDS). Wear resistance of clinically used M-Wire instruments was investigated by examining their surfaces with an SEM. In a complementary study, bright-field images of M-Wire blanks were obtained by scanning transmission electron microscopy (STEM). STEM images from that study are included in this dissertation to provide further insight into the mechanism for the mechanical properties of the M-Wire instruments.;DSC study showed that M-Wire instruments have much higher Af (austenite-finish) temperatures (over 40°C) than conventional superelastic rotary instruments (below room temperature), and are a mixture of martensite, R-phase and austenite at room temperature. The Vickers hardness of M-Wire instruments is significantly higher than that of conventional rotary NiTi instruments. Mean Vickers hardness number for the tip, intermediate region and shank region of size 30/.04 taper M-Wire instruments were about 374, 380 and 392 each, whereas values of Vickers hardness number for the corresponding regions of conventional instruments were typically less than about 320, 350 and 380.;Better wear resistance was observed with the SEM on clinically used M-Wire instruments, which presented less microcracks and evidence of permanent deformation on the surface compared with surfaces of clinically used conventional NiTi instruments. This improved wear resistance is attributed to increased hardness for surface region of the M-Wire instrument. Acid-etched M-Wire instruments (surfaces and cross-sections) presented a classical lenticular martensite structure when observed with the optical microscope and SEM. EDS analyses of the microstructures of the M-Wire instruments revealed titanium-rich precipitates. The complementary STEM examinations of M-Wire blanks revealed much coarser grains, twinning, and a high density of dislocations, which were not observed in starting superelastic NiTi wire blanks for conventional instruments.;In summary, increased hardness, which is indicative of higher strength and improved wear resistance, was found for M-Wire instruments, compared with conventional superelastic ProFileRTM instruments, which served as a control for this study. The STEM observations show that the improved mechanical properties of the starting M-Wire (and the rotary instruments manufactured from this special NiTi wire) arise from strengthening mechanisms in the martensitic structure, which were induced by extensive thermomechanical processing.