Defects in nickel-titanium instruments after clinical use. Part 4: an electropolished instrument

This study aimed to examine the type and location of defects observed in one brand of electropolished nickel-titanium instruments after routine clinical use. A total of 414 RaCe instruments with structural deformations, discarded from one endodontic clinic during a period of 24 months, were analyzed. The lateral and fracture surfaces of those separated instruments were examined by scanning; the locations of the fractures were recorded. Of all defective instruments, 388 had an area of structural change, and 26 were fractured. The distance from the tip of the instrument to the end of the deformed area was significantly longer in .02 taper instruments than in other tapers (P < 0.01), whereas in .04 instruments the distorted area ended closer to the tip (P < 0.05). In 31% of all fractured instruments, the fragment was 7-8 mm long, more frequently in .04 taper than in others. Most of the fractures were found adjacent to the spiraled sector. Shear fracture was diagnosed for 22 (85%) of all those fractured, whereas fatigue occurred in only 4 instruments (15%) (P < 0.05). There was a marked increase in the amount of defects after the seventh (45.4%) and eighth (100%) use. The results of this study indicated that RaCe instruments commonly exhibited unwinding defects after repeated use, and the most common cause of failure was shear failure.