Torsional behavior of nickel-titanium rotary instruments: what’s new?

As demonstrated by several studies, the two main causes of intracanal separation are undoubtedly the cyclic fatigue and the torsional stress to which the endodontic rotary instruments are subjected during the shaping of root canal system. The first one occurs when the instrument rotates in a canal curvature, undergoing to repetitive tensile and compressive stresses accumulated at the point of maximum curvature. Instead, the second one is a direct consequence of the blocking of a part of the instrument in the dentinal walls whilst the coronal part continues to rotate. This leads to a ductile fracture of the instrument due to an exceeding of its elastic limit.