Assessment of longevity of nickel-titanium files by AC impedance measurement

Objectives: Nickel-titanium rotary files exhibit more elastic flexibility in bending and torsion, as well as superior resistance to corrosion compared to stainless steel files. Despite these advantages, separation of the files can occur during clinical use without any visible signs of permanent deformation. The manufacturers state that the only predictable way to prevent the failure is to discard the files regularly after a certain number of uses. However, there is little consensus on the number of uses which discard the file. The objective of this study is to evaluate the effect of sodium hypochlorite (NaClO) on the fatigue lifetime of a nickel-titanium file and to clarify the relationship between AC impedance and fatigue lifetime. 

Methods: A brand of nickel-titanium files (K³, SybronEndo), size 25, 0.06 taper, 25 mm long, was used. Eighty new files with a same lot number were randomly divided into four groups. In two control groups, the number of rotations until failure in rotational bending test was measured in silicon oil and in saline. In third group, 6 % NaClO solution was used as a corrosive environment. In the last group, the fatigue testing in 6 % NaClO solution was interrupted periodically and AC impedance of the file was measured.

Results: The mean fatigue lifetime (3466 cycles) in the NaClO solution significantly (p<0.05) decreased compared with those in two control groups, 3834 cycles in silicon oil and 3754 cycles in saline, respectively. The AC impedance at 2250 cycles, which corresponds to about 60 % of mean fatigue lifetime, had decreased significantly (p<0.05) compared to that before fatigue test. 

Conclusions: These results indicate that a decrease in AC impedance is associated with the progress of fatigue, and the measurements of AC impedance in the clinic can be used to predict the number of uses which discard the files.