Cyclic Fatigue Resistance and Fractured Fragment Length of Three NiTi Rotary Systems: An In Vitro Comparison

Background: The aim of this in vitro study was to compare the cyclic fatigue resistance and fractured fragment length of three nickel-titanium (NiTi) rotary systems of identical tip size and taper (25/.06). NiTi rotary instruments fracture unpredictably in curved canals, and no cyclic fatigue data have been published for either the Endo Magic All in One or the Denco Pro-Flexi system; this study provides the first independent fracture data for both, benchmarked against the established One Curve Mini. Methods: A total of 60 instruments (n= 20 per group) of One Curve Mini 25/.06, Denco Pro-Flexi 25/.06, and Endo Magic All in One 25/.06 were tested. Cyclic fatigue testing was performed in an artificial canal within a stainless-steel block (60° curvature angle, 5 mm radius of curvature, 1.5 mm internal diameter) in distilled water at 35 ± 1 °C. Instruments were rotated at 300 rpm until fracture. The number of cycles to failure (NCF) was calculated, fragment lengths were measured with a digital caliper, and cross-sectional geometry was evaluated by scanning electron microscopy (SEM). Data were analysed using the Kruskal–Wallis and Dunn’s tests (α = 0.05). Results: Significant differences were found among the groups for both NCF and fragment length (p < 0.001). The highest NCF was recorded for Endo Magic All in One (9165.50 ± 2049.58), followed by One Curve Mini (3007.50 ± 470.79) and Denco Pro-Flexi (2023.00 ± 1134.43); all pairwise comparisons were significant. Fragments in the Denco Pro-Flexi group were significantly longer (7.51 ± 1.05 mm). SEM revealed circular, triple-helix, and square cross-sections for Endo Magic All in One, One Curve Mini, and Denco Pro-Flexi, respectively. Conclusions: Instruments identical in tip size and taper differed up to 4.5-fold in cyclic fatigue resistance, so comparable fracture risk cannot be assumed from these specifications alone; fragment length data further indicate that the least resistant system also separates at a level that complicates retrieval. The systems differed in cross-sectional geometry and in the alloy processing reported by their manufacturers; determining the relative contribution of these factors will require metallurgical characterisation.