Chemo-Mechanical Fatigue of Endodontic NiTi Rotary Files

Objective: Endodontic Nickel Titanium (NiTi) rotary files are susceptible to fracture due to fatigue exacerbated by corrosion. The objective of this study is to elucidate the corrosive effect on fatigue of endodontic NiTi files in vitro.

Method: 60 NiTi rotary files, size S1 ProTaper®, were randomly divided into two groups. The corrosion resistance of endodontic NiTi files to NaOCl solution and autoclave sterilization was investigated by immersing the files (group 1, 30 files) in NaOCl solution of different concentrations (1.0% or 5.25%) or water (control) and/or subjecting to standard autoclave sterilization. The chemo-mechanical fatigue behavior of endodontic NiTi files (group 2, 30 files) was elucidated by drilling the hand-file pretreated root canal models with a contra-angle handpiece at 16:1 reduction and 300 RPM for 30 seconds, equivalent to 150 cycles, for each specimen, and additional specimens were used until fracture occurred under irrigation solutions of NaOCl (1.0% or 5.25%) or water (control) combined with autoclave sterilization. Specimens were examined using optical and electron microscopes.

Result: For the corrosion study, the 5.25% NaOCl concentration (three cumulative 1 min immersions) caused significant pit corrosion in NiTi files than autoclave sterilization (three cumulative cycles), while 1% NaOCl, autoclave, and water (control) caused minimal corrosion in these files (n=5 for each test). For the chemo-mechanical study, the number of total cycles to failure were calculated for each file: 830±12 (mean±SD, n=10) for 5.25% NaOCl significantly lower than 1116±12 (n=10) for 1% NaOCl and 1287±8 (n=10) for water (control).  t-test showed P<0.001 between all test groups.

Conclusion: 5.25% NaOCl solution significantly reduces the fatigue resistance of NiTi files. 1% NaOCl solution, however, can effectively disinfect the root canal and remove the compromised tissues, while having minimal effect on the fatigue behavior of NiTi files.  Supported by NIH/NIDCR-R01DE017925 and NSF/CMMI-0758530.