Effect of Intermittent Use on the Dynamic Cyclic Fatigue Resistance of Heat-Treated Nickel-Titanium Endodontic Files

This study aimed to investigate the effect of in vitro intermittent file use on the dynamic cyclic fatigue resistance of heat-treated nickel-titanium (NiTi) rotary files.

iRaCe and RaCse EVO rotary files (n = 30 each) were divided into two groups, wherein the files operated in a dynamic motion using either continuous or intermittent use (n = 15), following the recommended speeds by the manufacturer (600 rpm for iRaCe and 800 rpm for RaCe EVO files). All files were tested inside an artificial metallic canal with a 60° angle of curvature, a 5 mm radius of curvature, and the maximum point of curvature located 5 mm from the canal terminus, under a 35 ± 1 °C water bath. In the first group (continuous use), the file executed repeated vertical strokes spanning 3 mm in amplitude, moving both apically and coronally to the full canal length until it fractured. In the second group (intermittent use), the same strokes were performed but for 10 seconds. Then, the file was completely pulled out from the canal and then reinserted to perform another 10-second cycle until the file fractured. The number of insertions was recorded. Video recording was made for the tested files to identify the time to fracture and calculate the number of cycles to fracture (NCF). Statistical analyses were performed using SPSS at a 5% significance level.

The NCF of the RaCe EVO was higher than iRaCe, especially when operated intermittently (P < 0.01). Within each file system, the intermittent use exhibited better cyclic fatigue resistance (P < 0.01). The number of insertions before fracture was greater with the RaCe EVO (P < 0.01).

The intermittent use extended the lifespan of files tested under cyclic fatigue. Further NiTi file failure studies should be conducted intermittently to better mimic clinical practice.