Is the reciprocating movement per se able to improve the cyclic fatigue resistance of instruments?

Abstract

Aim: To compare cyclic fatigue resistance of two geometrically similar nickel-titanium instruments, used in conditions similar to clinical use in reciprocating and continuous rotary motion.

Methodology: Four groups of eighteen instruments each, Reciproc(®) files sizes ISO 25 and 40 (R25 and R40) and Mtwo(®) files sizes ISO 25 and 40 (M25 and M40), were tested in reciprocating and continuous rotary motion, employing a novel experiment device. An artificial root canal (diameter, 1.4 mm; angle of curvature, 60 °; and curvature radius, 5 mm) was milled into a stainless steel block. To simulate clinical conditions, instead of rotating the file in static position, the set-up was designed to produce a continuous up-and-down pecking motion along the vertical axis of the instrument. Time to fracture (TTF) and push-pull cycles (PPC) were recorded, the number of cycles to fracture (NCF) was determined, and fractured instrument surfaces were examined under a scanning electron microscope (SEM).

Results: Mean time to fracture was 34.44 ± 8.58 min for R25 in reciprocation motion, 35.77 ± 4.82 min for R40 in reciprocation motion, 12.15 ± 1.74 min for M25 in continuous rotary motion and 13.27 ± 2.02 min for M40 in continuous rotary motion, whereas 28.52 ± 3.27 min for R25 in continuous rotary motion, 23.87 ± 1.52 min for R40 in continuous rotary motion, 31.07 ± 1.79 min for M25 in reciprocation motion and 31.08 ± 3.26 min for M40 in reciprocation motion. There was a significant difference (P < 0.0001) for the cyclic fatigue resistance between the reciprocation motion and the continuous rotary motion groups. Reciproc(®) files in reciprocating movement had a significantly higher NCF than Mtwo(®) files, when used in continuous rotation. The highest resistance to failure was shown by Reciproc(®) files in reciprocation movement, followed by Mtwo(®) files in reciprocation and Reciproc(®) files in continuous motion. Mtwo(®) files in continuous rotary movement had the least resistance. SEM analysis of the fracture surface confirmed typical features of cyclic fatigue failure.

Conclusion: Reciprocating movement increased the cyclic fatigue resistance of NiTi instruments.

Keywords: continuous rotary motion; cyclic fatigue; fracture resistance; nickel-titanium rotary file; reciprocating motion.