Fracture Resistance of Heat-treated NiTi Instruments by the Load of Mechanical Preparation According to Tooth Type and Canal Number

Abstract

Introduction

This study aimed to evaluate the impact of mechanical loading, induced by preparing anatomically standardized premolar and molar canal models, on the cyclic fatigue and torsional fracture resistance of heat-treated nickel–titanium (NiTi) rotary instruments.

Methods

Sixty NiTi instruments (Race Evo) were randomly assigned to 3 groups (n = 20): unused controls (N0), instruments used to prepare 2 canals in premolar models (P2), and 4 canals in molar models (M4). Cyclic fatigue testing was performed in dynamic mode with a 4 mm axial pecking motion at 37 ± 1 °C. Time to fracture and the number of cycles to fracture were recorded; the lengths of fractured fragments were measured using a digital caliper. Torsional resistance was assessed by fixing the apical 3 mm and rotating the shaft until fracture. Maximum torsional load and angle of rotation were recorded. Fractured surfaces were examined using scanning electron microscopy, and phase transformation behavior was assessed via differential scanning calorimetry. Data were analyzed using one-way analysis of variance and Tukey's post hoc test at a 95% significance level.

Results

P2 and M4 groups showed significantly higher cyclic fatigue resistance than N0 (P < .001), with no difference between P2 and M4. Torsional resistance parameters were significantly reduced in P2 and M4 compared to N0 (P < .05). Scanning electron microscopy revealed distinct fracture morphologies; differential scanning calorimetry showed consistent phase transformation behavior.

Conclusion

Within the limitations of this study, standardized simulated procedural loading paradoxically increased the cyclic fatigue resistance of heat-treated NiTi instruments, while simultaneously producing a moderate reduction in torsional strength.