Influence of the surface roughness on the torsional resistance of nickel-titanium rotary endodontic instruments

Aim: The aim of the present study was to evaluate the influence of the surface roughness on the torsional resistance of polished and unpolished nickel-titanium BR4C endodontic instruments. Materials and Methods: Ten instruments of each type (electropolished and unpolished) were evaluated with a profilometer to quantify the surface roughness on the working part of the instruments and then subjected to torsional stress to evaluate the maximum torque and angular deflection at failure. Torsion was applied using a custom-made device attached to the crosshead of a universal testing machine (Emic DL 10.000, São José dos Pinhais, PR, Brazil). Data obtained from the maximum torque, angular deflection and from the roughness of the polished and unpolished instruments were statistically analysed by Student t-test. Results: The profilometer analysis showed that the surface grooves on the unpolished instruments were deeper compared with their electropolished counterparts. Significant differences in roughness (peak and valley) (P 0.05) were observed between the polished and unpolished specimens. In the torsion tests, the maximum torque and angular deflection were not significantly different (P > 0.05). Conclusion: This study showed that the depth of the surface grooves resulting from machining during the manufacturing process did not affect the maximum torque and angular deflection of the instruments tested. Keywords: roughness, surface finish, torsional fracture