Dynamic and cyclic fatigue of engine-driven rotary nickel-titanium endodontic instruments

The absence of adequate testing standards for engine-driven nickel-titanium (NiTi) instruments necessitates further study of these instruments in all areas. This study examined three groups of engine-driven rotary NiTi endodo ntic instruments (Profile, Hero, and Quantec) and assessed the times for dy namic fracture in relation to the radius of curvature to which the instrume nts were subjected during preparation, with the instrument diameter determi ned by size and taper and the mode by which the fracture occurred. Ten inst ruments were randomly selected representing each size and taper for each gr oup and for each radius of curvature: 600 in total. The instruments were ro tated at 350 rpm and introduced into a tempered steel curve that simulated a canal. Two radii of curvature of canals were used: 5 and 10 mm. Time at f racture was noted for all files, and the fracture faces of each file were a nalyzed with scanning electron microscopy. Radius of curvature was found to be the most significant factor in determining the fatigue resistance of th e files. As radius of curvature decreased, fracture time decreased. Taper o f files was found to be significant in determining fracture time. As diamet er increased, fracture time decreased. In all cases, fracture was found to be of a ductile nature, thus implicating cyclic fatigue as a major cause of failure and necessitating further analyses and setting of standards in thi s area.