Impact of different cross.section designs of rotary instruments on smear layer removal, cyclic and torsional fatigue resistance: In vitro study

Introduction: In endodontics, mechanized nickel–titanium (NiTi) instruments are widely adopted for both initial treatments and retreatment because of their flexibility and minimal impact on root canal trajectory. This study aimed to assess the effect of four rotary NiTi systems on the quantity of smear layer remaining on dentin walls after instrumentation and to compare their resistance to cyclic and torsional fatigue. Materials and Methods: Forty mandibular incisors ( n = 10) were divided into four groups according to the file used for instrumentation: spin, S2, rotate, and platinum V.EU rotary files. Dentin wall evaluation for smear layer was performed through scanning electron microscopy. For mechanical tests, 80 instruments were used in total, with 40 instruments ( n = 10 per group) undergoing cyclic fatigue analysis using an artificial canal apparatus and another 40 instruments ( n = 10 per group) undergoing torsional fatigue analysis using a custom-designed torsion machine. All instruments had a 0.25 mm diameter and underwent cyclic and torsional fatigue analysis. Statistical analysis was conducted at a 5% significance level. Results: Intergroup comparisons showed a comparable percentage of smear layer on dentin walls ( P > 0.05). Cyclic fatigue tests indicated that spin and S2 instruments possessed the highest resistance ( P < 0.05). The rotate system demonstrated the greatest angular deflection and the lowest torque ( P < 0.05). Conclusion: All tested rotary systems yielded similar smear layer accumulation but exhibited varying cyclic and torsional properties.