Defects in protaper S1 after clinical use: longitudinal examination

Objectives: To evaluate the defects in ProTaper shaping instrument S1 after a defined schedule of clinical use. Methods: Among all ProTaper files discarded from an endodontic clinic at a stomatological school in China over a period of 17 months, 122 S1 instruments were collected. They were ultrasonically cleaned, autoclaved and then examined in the laboratory. Any instrument separation was noted; the average length involved was determined. The 0.5 mm region on either side of this length of discarded but intact instruments was examined circumferentially by SEM. The region adjacent to the broken end of the fractured instruments was also examined in the same way. Results: One specimen was lost during processing. Of the remaining 121 instruments, 27 were separated with a mean fracture length of 3.67 mm from the tip. Of these, 2 files showed macroscopically torsional fracture and the others showed signs of flexural fatigue. Only one file that had not fractured showed visible unwinding defects. Examination of the 3.1~4.1 mm region of other unfractured instruments revealed the presence of microcracks, surface debris, pitting and/or wear of their cutting edges. Some debris particles seemed to have been trapped in crack-like structures. Conclusion: Multiple use of ProTaper S1 predisposed the instrument to microcracks formation and wear of its cutting edges. There was a low prevalence of torsional failure and most ProTaper S1 instruments failed without discernible sign of unwinding of the flutes. Further studies should address the mode of failure and the role of debris particles in the fracture mechanism.