Study on the Fracture Mechanisms of Nickel Titanium Rotary Instruments

• Main Authors: LI, UEI-MING, 李偉明
• Other Authors: LAN, WAN-HONG
• Format: Others
• Language: zh-TW
• Published: 2001
• Online Access: http://ndltd.ncl.edu.tw/handle/75790509060647015650

碩士 === 國立臺灣大學 === 臨床牙醫學研究所 === 89 === Over the years, the revolutionary development of incorporating nickel-titanium（NiTi）into endodontic files has greatly transformed the methods of root canal instrumentation. They help minimize the undesirable complications often encountered during instrumentation in fine and curved canals. Files made from this alloy are biologically acceptable, highly flexible and considerably stronger in fatigue resistance than stainless steel (SS) files. Despite its increased strength and flexibility, separation is still a concern with NiTi instruments, and they have been reported to undergo unexpected fractures. Endodontic instruments upon rotation are subjected to both tensile and compressive stress in curved canals. This stress is localized at the point of curvature. This is the most destructive mode of cyclic loading. But little scientific data have been published about the life span of Ni-Ti rotary instruments while they were located at the mode of pecking motion or static condition, and activated at different rotational speeds. The purpose of this study was to investigate the possible fracture patterns and mechanisms of nickel titanium rotary instruments, and lead to recommendations for preventing instrument separation. The specific aims of this study were as follows: the first aim is fractographic study of nickel titanium rotary instruments. One hundred and twelve fractured instrument fragments had been collected, recording their size, taper, and the length of fracture site from the tip, and were examined under scanning electron microscope（SEM）. The second aim is to evaluate the cyclic fatigue of 0.04 ProFile® nickel titanium rotary instruments operating at different rotational speeds and varied moving distances of pecking motion in the metal blocks that simulated curved canals. A total of 150 ProFile® instruments were made to rotate freely in a 75° sloped metal block at speeds of 200, 300, or 400 rpm by a contra-angle handpiece mounted on an Instron machine. The electric motor and Instron machine were activated until the instruments were broken in two different modes, static and dynamic pecking-motion. The fractured surfaces of separated instruments were examined under a scanning electron microscope. All data obtained were analyzed by a stepwise multiple regression method using a 95% confidence interval. The results revealed that the first, no matter what instrument size or taper, all the mean length of fracture site from the tip locate between 3 mm and 5 mm. In addition, almost 70﹪ instrument separation is owing to torsional overload after exceeding their elastic limit, and the other reveals ductile fracture as the fatigue failure mode. The second, the time to failure significantly decreased as the angles of curvature or the rotational speeds increased. As the moving distances of pecking motion increased, the time as well as the numbers of cycles to fracture increased. Microscopic evaluation indicated that ductile fracture was the major cyclic failure mode. In order to prevent breakage of a NiTi rotary instrument appropriate rotational speeds and continuous pecking motion in the root canals are recommended.