Defects in nickel-titanium instruments after clinical use. Part 5: single use from endodontic specialty practices.

INTRODUCTION Single use of endodontic nickel-titanium (NiTi) instruments has been recommended to reduce instrument fatigue and the possibility of cross-contamination. The purpose of this study was to evaluate the defects of three NiTi rotary systems (ProFile series 29 [Dentsply Tulsa Dental Products, Tulsa, OK], ProFile [Dentsply Tulsa Dental Products], and ProTaper [Dentsply Maillefer, Ballaigues, Switzerland]) that were discarded after single use by two endodontic clinics. METHODS A total of 1,071 ProFile .04, 432 ProFile series 29 .04, and 1,895 ProTaper rotary instruments were collected over 12 months and analyzed. These discarded files were ultrasonically cleaned and autoclaved. The lateral view of the deformed instruments and fracture surface of the separated instruments were examined by scanning electron microscopy. Qualitative X-ray energy-dispersive spectrophotometric (EDS) spot analyses were performed on particles adherent to the surface and those apparently wedged in surface microcracks. RESULTS There were no fractures or deformations in the ProFile Series 29. The overall prevalence of deformation was 2.9% in ProTaper and 0.75% in ProFile. The incidence of instrument separation was 0.26% in ProTaper, whereas no fractures occurred in ProFile instruments. The majority of instrument defects occurred in size 25 (6/8) for ProFile and in Sx for ProTaper (22/60). The separated ProTaper instruments failed mostly because of shear stress. Some surface deposits and microcracks were found in single-use NiTi instruments. EDS indicated that surface deposits may be dentin. CONCLUSION The risk of NiTi rotary instrument fracture in the canal is low when a new instrument is used by experienced endodontists. The most common cause of failure, albeit rare, was shear failure.