Impact of metallurgical features and cross-sectional design on the cyclic and torsional properties of NiTi rotary instruments

ABSTRACT This study evaluated six NiTi rotary instruments' mechanical and metallurgical properties with varying thermal treatments and cross-sections. A total of 240 instruments (sizes 25.06 and 35.04) were tested: VDW Rotate (VDW), Plex V (PV), Pro-Flexi Files (PFF), MG3 Gold (MG3), RCS Rainbow One (RRO), and AF F One (AFR). Cyclic fatigue resistance was assessed in an artificial canal (60° curvature, 5 mm radius) (n=20), and torsional strength was measured by maximum torque and angular deflection at the instrument tip (n=20). Metallurgical properties were analyzed using Differential Scanning Calorimetry (DSC) and X-ray Diffraction (X-RD). ANOVA and Tukey tests were applied (α=5%). RRO 25.06 showed the highest NCF (Number of Cycles to Fracture) (p<0.05), while RRO 25.06 and PFF 25.06 had the lowest torsional strength (p<0.05). For size 35.04, RRO and PFF exhibited the highest NCF (p<0.05), whereas MG3 had the lowest torsional strength (p<0.05). DSC analysis revealed that VDW instruments had the lowest austenite start and finish temperatures. X-RD showed the highest austenite content in VDW 25.06 and AFR 35.04. Mechanical resistance varied significantly among instruments, with DSC and X-RD confirming distinct metallurgical features strongly influencing performance. Phase quantification and its correlation with laboratory data improve understanding new thermally treated files.