Investigation of Transformations in Used and Heat-Treated Nickel-Titanium Endodontic Instruments

Nickel-titanium rotary instruments may exhibit reversible transformation from the parent austenitic structure (A) to the martensitic structure (M) under stresses during preparation of curved root canals. Such transformations might have fundamental consequences for clinical performance of these instruments. Temperature-modulated differential scanning calorimetry (TMDSC) provides an ability to differentiate overlapping transitions that is not possible with conventional DSC. Objective: Investigate transformation temperature ranges (TTR) in NiTi rotary instruments after clinical use, mechanical bending, and heat treatment. Methods: ProFile (Dentsply), ProTaper (Dentsply) and Liberator (Miltex) instruments were selected. ProFile and ProTaper instruments subjected to controlled clinical use and discarded instruments were compared to as-received instruments. All three groups of instruments were also subjected to 90° bending, and heat-treated for 15 minutes in a nitrogen atmosphere at 400°, 500° and 600°C. As-received, used, and heat-treated instruments were cut into 3 or 4 segments. TMDSC analyses (DSC Q100, TA Instruments) were conducted between –80° and 100°C, using a linear heating and cooling rate of 2°C per minute and an oscillation amplitude of 0.318°C with a period of 60 seconds. Nitrogen was used as the purge gas. Results: Clinically used instruments and bent instruments showed minimal change in TTR and enthalpy for overall transformation between the (A) and (M) structures. The presence of the intermediate R–phase was minimally evident with as-received and bent test specimens, and may be more apparent after heat treatment. Low-temperature martensitic transformations, resolved on the nonreversing heat flow curves, were not present on the reversing heat flow curves and were more apparent for heat-treated specimens than bent specimens. Conclusions: Heat treatment results in loss of clinical shape memory in the NiTi instruments, and allows for pre-curving. Further research is needed to compare cutting efficiency of heat-treated and as-received NiTi rotary instruments. Dentsply Tulsa Dental donated instruments for this study.