Instrument fracture: mechanisms, removal of fragments, and clinical outcomes

Abstract

Instrument fracture can be an unpleasant mishap during non‐surgical root canal treatment. While most stainless‐steel instruments appear to fail by excessive amounts of torque, the combined action of torsional stress and cyclic loading (i.e. fatigue, as a result of rotational bending, or repeated torsion) is responsible for the breakage of NiTi rotary files in use. Factors affecting the incidence and mode of failures include the instrumentation technique, use of torque‐controlled motor, the dimension and surface condition of the instrument, rotation rate, radius (and, to a certain extent, angle) of canal curvature, and the presence of a straight‐line access and a ‘glide path’ to the apical portion of the canal. If breakage should occur clinically, the patient should be informed of the incident and consideration should be given as to whether or not to remove the fragment. Managed properly, the presence of a broken fragment per se may not adversely affect the outcome of root canal treatment. This review examines the various aspects (including mechanisms, contributing factors, prognosis and management) of instrument fracture. Emphasis is placed on the separation of nickel–titanium rotary files and ways to prevent this in the clinical situation.

Micro-CT assessment of two different endodontic preparation systems

“…13 Fractures caused by fatigue or high levels of torsion may cause failure of the endodontic treatment. 14 In the present study, a comparison was made between a rotary and an oscillating system used for instrumenting root canals. The study data indicated that the EEA system prepared a larger root canal surface area in comparison to the PTU system.…”

Current Challenges and Concepts of the Thermomechanical Treatment of Nickel-Titanium Instruments

“…Two mechanisms that may lead to NiTi rotary fracture, namely cyclic fatigue and torsional overloading, have been described (2,13,(49)(50)(51). Cyclic fatigue occurs as a result of rotation around a curve by repeated extension and compression of the metal and, finally, work hardening followed by fracture (15,49).…”

“…Cyclic fatigue occurs as a result of rotation around a curve by repeated extension and compression of the metal and, finally, work hardening followed by fracture (15,49). Torsional fracture occurs when the tip of the rotating instrument binds in the canal while the motor continues to rotate (2,13). Torsional fracture of the material occurs when the ultimate strength of the material is exceeded.…”

“…Four methods (13) have been used to investigate in vitro fatigue fracture resistance of NiTi rotary endodontic instruments: (1) curved metal tube (49), (2) grooved block-and-rod assembly (54-60), (3) rotation against an inclined plane (61, 62), and (4) 3-point bend of a rotating instrument (32,63,64). However, in an effort to understand the nature of the metal alterations and their impact on enhancing resistance to fatigue in particular, an international standard for cyclic fatigue testing of NiTi rotary instruments should be developed in order to ensure the uniformity of methodologies so that the resulting data can be compared (15).…”

“…Despite their undeniably favorable qualities, there is a potential risk of ''unexpected'' fracture with NiTi instruments (1)(2)(3)(4)(5)(6)(7)(8)(9)(10). Even though there have been considerable improvements in the file design and manufacturing methods on endodontic rotary instruments made of NiTi alloys, rotary instrument intracanal separation caused by cyclic fatigue remains a concern, especially in root canals with severe curvatures (11)(12)(13)(14)(15)(16). The mechanical behavior of NiTi alloy is determined by the relative proportions and characteristics of the microstructural phases.…”