Comparative Buckling Strength and Metallurgical Analysis of Five Classic NiTi Endodontic Rotary Files

Nickel-titanium (NiTi) rotary instruments have revolutionized endodontic practice through continuous advancements in metallurgy and design. Despite these improvements, mechanical failure remains a clinical concern. This study aimed to evaluate and compare the design features, metallurgical properties, and buckling resistance of five widely used NiTi rotary endodontic systems.

A total of 250 new NiTi rotary instruments from five systems (ProTaper Next, Mtwo, ProFile, EndoSequence, and GT Series X) were analyzed. Design features were assessed using dental microscopy and scanning electron microscopy. Metallurgical properties were evaluated through energy-dispersive X-ray spectroscopy and differential scanning calorimetry. Buckling resistance was measured using a universal testing machine equipped with a 1 kN load cell, applying a compressive load at 1 mm/min until 1 mm of lateral displacement was achieved. Statistical analysis was performed using the Shapiro-Wilk test to assess normality, followed by the non-parametric Kruskal-Wallis test to compare groups. A significance level of p<0.05 was adopted.

ProFile instruments exhibited the highest number of spirals (19) and spiral density (1.19 spirals/mm), while GT Series X featured the shortest cutting blade length (≤12 mm). All systems demonstrated near-equiatomic nickel-titanium ratios. ProTaper Next and GT Series X showed higher R-phase and austenitic transformation temperatures. Buckling resistance was significantly greater in the ProFile (0.04 and 0.06 taper) and EndoSequence 35/.06 and 40/.06 instruments (p<0.05). In contrast, EndoSequence 0.04 files, Mtwo, and ProTaper Next exhibited lower resistance.

Design features, taper, and metallurgical composition significantly influence the buckling resistance of NiTi rotary endodontic instruments.