Cyclic fatigue resistance of rotary nickel-titanium instruments submitted to nitrogen ion implantation

The aim of this study was to assess cyclic fatigue resistance in rotary nickel-titanium instruments submitted to nitrogen ion implantation by using a custom-made cyclic fatigue testing apparatus.

Thirty K3 files, size #25, taper 0.04, were divided into 3 experimental groups as follows: group A, 12 files exposed to nitrogen ion implantation at a dose of 2.5 x 10(17) ions/cm(2), accelerating voltage of 200 kV, currents of 1 microA/cm(2), 130 degrees C temperature, and vacuum conditions of 10 x 10(-6) torr for 6 hours; group B, 12 nonimplanted files; and group C, 6 files submitted to thermal annealing for 6 hours at 130 degrees C. One extra file was used for process control. All files were submitted to a cyclic fatigue test that was performed with an apparatus that allowed the instruments to rotate freely, simulating rotary instrumentation of a curved canal (40-degree, 5-mm radius curve). An electric motor handpiece was used with a contra-angle of 16:1 at an operating speed of 300 rpm and a torque of 2 N-cm. Time to failure was recorded with a stopwatch in seconds and subsequently converted to number of cycles to fracture. Data were analyzed with the Student t test (P < .05).

Ion-implanted instruments reached significantly higher cycle numbers before fracture (mean, 510 cycles) when compared with annealed (mean, 428 cycles) and nonimplanted files (mean, 381 cycles).

Our results showed that nitrogen ion implantation improves cyclic fatigue resistance in rotary nickel-titanium instruments. Industrial implementation of this surface modification technique would produce rotary nickel-titanium instruments with a longer working life.