INVESTIGATION OF THE SIMULATED ROOT CANAL CUTTING LIMIT OF NICKEL-TITANIUM ROTARY FILES WITH REPEATED PHASE-TRANSFORMATION HEAT TREATMENT

抄録

Objective: As a reference for the safe use of nickel-titanium rotary files (Ni-Ti files), the limit number of root canals for consecutive cutting with the Hyflex CM (Coltene Whaledent, Altstätten, Switzerland) and the JIZAI (MANI, Tochigi, Japan) R-phase Ni-Ti files with repeated heat treatments for phase transformation was evaluated, and their courses of transition to fracture occurrence were compared. Materials and methods: A simulated root canal (Endo Training Block J-shaped simulated root canal) was subjected to root canal enlarging and shaping with Hyflex CM (25 mm, #25, .06 taper) or JIZAI (25 mm, #25, .06 taper). As a low-speed engine, the Tri Auto ZX2 (Morita, Kyoto, Japan) was used in the OTR mode. After root canal enlargement, the file was submerged for 5 s in heated water at 60-70℃ to induce phase transformation, and file deformation was observed microscopically before and after heating. Fracture surfaces were also observed by a scanning electron microscope (SEM). Results: The mean number of root canals to file fracture was 16.4 (±5.4) with Hyflex CM and 90.2 (±12.0) with JIZAI, significantly higher with JIZAI. The mean number of root canals to onset of deformation was also found to be markedly higher with JIZAI, at 15.5, than with Hyflex CM, at 1.8. In terms of permanent deformation before and after heating, moreover, permanent deformation was found for Hyflex CM after a mean of 9 root canals, but JIZAI, in contrast, was found to return to its initial file form after a mean of ≥20. In both files, the point of fracture was located approximately 3 mm from the tip, and observation of the fracture surfaces by SEM showed a dimpled structure in both files, with many dimples located in the center of the Hyflex CM cross-section, but near the radial land in the JIZAI cross-section. Conclusion: It was found that, in root canal shaping with an R-phase Ni-Ti file, elimination of file deformation by heat treatment can effectively increase the number of times the file can be used for root canal enlarging and shaping. If recovery from deformation fails to occur, however, the probability of file fracture increases, and replacement of the file is therefore recommended.