Abstract : Purpose : The objective of this study was to clarify the effects of high-pressure steam sterilization (autoclave) on the structural properties of rotary files made of nickel-titanium alloy (NiTi file) exhibiting superelastic or shape-memory characteristics.

 Materials and Methods : Two types each of NiTi files with superelastic or shape-memory characteristics were prepared, with six files of each type. Half of the files from each group were autoclaved. The crystal structure, surface chemical state, and surface morphology of the as-received and the autoclaved NiTi files were examined using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM), respectively.

 Results and Discussion : XRD revealed that NiTi files with superelastic properties had peaks assigned to the austenite phase, while those with shape-memory properties showed the martensite phase. No changes were observed in the XRD patterns after autoclaving. In XPS analysis of as-received NiTi files, the autoclaved files showed significantly increased carbon content and decreased Ni and Ti contents. Additionally, autoclaved NiTi files were confirmed to be in the hydrated state. No changes were observed in the SEM images with or without autoclaving. Therefore, autoclaved as-received NiTi files could be considered to preserve their superelastic and shape-memory properties, as the autoclaving did not affect their crystal structure or surface morphology. Since the surface of autoclaved NiTi files becomes hydrated, this suggests the potential for degradation of the NiTi files due to hydrogen embrittlement.

 Conclusion : Within the limitations of this study, autoclaving did not affect the crystal structure or surface morphology of NiTi files with superelastic or shape-memory properties, but caused carbide formation and hydration on the surface.