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 D-0 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 degrees 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 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.