Influence of heat treatment on torsional resistance and surface roughness of nickel‐titanium instruments.

Aim: To evaluate the influence of controlled memory (CM) thermal treatment on the torsional resistance and surface roughness of nickel‐titanium instruments with identical geometric and dimensional characteristics. The influence of environmental temperature on torsional resistance was also evaluated. Methodology: A sample of 25 conventional NiTi alloy and 25 CM thermal‐treated NiTi instruments, with an identical geometric design and a nominal size of 0.25 mm at D0 and a nominal taper of.06 mm mm−1 were selected. The torsional strength and the angle of rotation of the instruments were tested following ISO 3630‐1 at two temperatures: 21 and 35 °C. The fracture surfaces of all tested instruments were verified with a scanning electron microscope. The surface roughness of the blades of conventional NiTi alloy and CM wire files was evaluated by using a New View 7100 Profilometer. Statistical analysis was performed using Student's t‐test and paired t‐test to analyse the torsional resistance and surface roughness at a significance level of P < 0.05. Results: The torsional strength of thermally‐treated instruments was significantly lower than nontreated instruments (P < 0.05). However, thermally‐treated instruments had significantly greater angular rotation to fracture than nontreated instruments (P < 0.05). Temperature did not influence the torsional strength (P > 0.05) and the angular rotation (P > 0.05) of the instruments. Regarding the roughness measurements, groove depth was lower at the surface of thermally‐treated instruments when compared to nontreated instruments (P < 0.05). Conclusion: Thermal treatment influenced the torsional resistance and surface roughness of NiTi instruments with identical geometric and dimensional characteristics. Temperature did not affect torsional behaviour. [ABSTRACT FROM AUTHOR]
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