Abstract: BACKGROUND: Heat-treated nickel-titanium files have gradually become the mainstream instruments in clinical applications due to their excellent flexibility and good root canal shaping ability. However, root canal preparation of S-shaped curved root canals is a challenging aspect of clinical root canal treatment, and instrument separation is prone to occur in complex root canals.
OBJECTIVE: To compare the cyclic fatigue resistance of five heat-treated nickel-titanium files in simulated S-shaped root canals at different temperatures. 
METHODS: Twenty samples each of Hyflex CM, M3, Plex, Hyflex EDM, and R-phase nickel-titanium files were selected. A double-curved metal simulated S-shaped root canal was used. The cyclic fatigue resistance of the five types of nickel-titanium files was tested at room temperature (24 °C) and at 65 °C using a heating device. Ten samples of each type of nickel-titanium file were tested at each temperature. The number of cyclic fatigue and the fragment length were recorded. The fragment surfaces of the nickel-titanium files were observed and analyzed in detail using a scanning electron microscope.
RESULTS AND CONCLUSION: (1) At both temperatures, the five types of nickel-titanium files showed similar fragment patterns in the S-shaped root canal: fragment first occurred in the apical curvature region, followed by fragment at the coronal curvature. The fragment surfaces of all five groups of nickel-titanium files showed typical cyclic fatigue characteristics under scanning electron microscopy. (2) Results at room temperature: The Hyflex EDM group showed the highest number of cyclic fatigue in the apical segment, while the Plex group showed the lowest. The Hyflex CM group showed the highest number of cyclic fatigue in the coronal segment, while the M3 group showed the lowest. The TF group had the longest apical fragment length, while the Hyflex EDM group had the shortest. The Plex group had the longest coronal fragment length, while the Hyflex CM group had the shortest. (3) Results at 65 °C: The TF group showed the highest number of cyclic fatigue in the apical segment, while the M3 group showed the lowest. The Hyflex EDM group showed the highest number of cyclic fatigue in the coronal segment, while the Plex group showed the lowest. The Plex group had the longest apical fragment length, while the M3 group had the shortest. The Hyflex CM group had the longest coronal fragment length, while the Plex group had the shortest. (4) When the temperature increased to 65 °C, except for the TF group, the cyclic fatigue values of the other four nickel-titanium file groups in both the apical and coronal segments were significantly lower than the values measured at room temperature. With increasing temperature, the apical fragment lengths of the Hyflex CM, Plex, and Hyflex EDM groups showed an increasing trend, and the coronal fragment lengths of the Hyflex CM and M3 groups also showed an increasing trend. The results indicate that, at room temperature and under heating, the Hyflex EDM nickel-titanium file and the R-phase nickel-titanium file TF exhibited superior cyclic fatigue resistance in S-shaped simulated root canals. Heating to 65 °C may reduce the cyclic fatigue resistance of Hyflex CM and Hyflex EDM nickel-titanium files.

Key words: nickel-titanium endodontic files, nickel-titanium files, S-shaped root canal, cyclic fatigue resistance, heat treatment, number of cyclic fatigue