| Nickel-titanium (NiTi) instruments are in common use, especially rotary NiTi instruments are increasingly used by dentists. However, the 'unexpected' separation during use remains problematic, as deployed in curved narrow canals. Several recent studies have evaluated the wear of rotary NiTi files under simulated clinical and laboratory conditions. Tommpson & Dummer stated that the incidence of NiTi instrument separation was related to instrument design and mechanical properties.Some published reports indicated the inability to remove or bypass a broken instrument as deleterious to the prognosis of endodontically treated teeth. Technical equipment, skill and experience of the operator as well as anatomical factors (root canal curvature, root canal diameter) may be considered the factors influencing success or failure of removal procedures. Today, in combination, the microscope, ultrasonic instrumentation and iRS have dramatically improved the potential and safety when removing broken instruments.The purpose of this study was to analyze the clinical fracture of nickel-titanium instruments and its causes, to evaluate the influence of several factors on the success or failure of attempts to remove the NiTi fractured instruments from the root canals, to observe the types of the deflects of the discarded ProTaper rotary NiTi files reaching the pre-determined number in clinic and to draw conclusions regarding the reasons for failure, to assess the success of the combination of piezoelectric ultrasonic and iRS technique at removing fractured ProTaper instrument from different levels in narrow, curved root canals, and to describe the problems encountered and the strategy of retreatment of broken instrument after resinifying therapy.PART I Clinical analysis of the failure of nickel-titanium instruments duringroot canal therapyMethods The fracture of ProFile and NiTi K-file instruments for root canal preparation during 2001.5-2003.5, and the relation between the fracture and the teeth were analyzed. Results The majority of the clinical fracture of nickel-titanium instruments are flexural fatigue which frequently occurred in molar, the midpoint of the curvature, canal with curves >30?and very complex canals. The amount of the broken nickel-titanium instruments in the first year is twice that in second year. Conclusions The fracture of nickel-titanium instruments is closely related to the state of the root canal, the operator's experience and canal preparation technique and so on.PART II Factors associated with removing the NiTi fractured instruments from root canalsStudy design In 72 teeth with a separated NiTi instrument, removal attempts were undertaken using a variety of techniques and armamentarium. Factors including type of tooth, degree of root canal curvature, location of fragment in relation to the root canal curvature, and radiographic length of fragment were analyzed. Success of treatment was defined as removal or complete bypassing of the fragments. Results The overall success rate was 53%. The success rate for ProFile fragment was 41% and for a NiTi K-file 60%. The success rate in maxillary teeth was obviously higher than that in mandibular teeth. Of 52 instruments in molars, 28 were successfully removed or bypassed. Of the 12 fragments in premolars, only 2 were removed. All 8 cases in anterior teeth were retrieved completely. When the fragment was localized before the curvature, complete removal was achieved. When the fragments were located at and beyond the curvature, the success rates were 60% and 27%, respectively. In canals with a slight, moderate and severe curvature, the success rates were 100%, 83% and 43%, respectively. In general, the longer the fragment is, the greater the chance for successful removal or bypass. Conclusions Favorable factors for removal of separated NiTi fragmentsare straight canals, anterior teeth, localization before the curvature, length of fragment more than 5 mm and hand NiTi K-file.PART III Deflects in ProTaper rotary nickel-titanium instrument after c...
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