Hidden Effect of Repetitive Use on the Fracture Resistance of Rotary and Reciprocating Nickel-Titanium Files

This study aimed to investigate the effects of repeated use on the fracture resistance of thermally treated nickel-titanium (NiTi) files with identical S-shaped cross-sections but different kinematics.

Reciproc Blue (RB; reciprocating, single-file technique) and VDW.Rotate (VR; continuous rotation, multiple-file sequence) were divided into subgroups based on 0, 2, or 4 simulated uses. Each file prepared standardised J-shaped resin canals, with preparation protocols following each manufacturer's recommended clinical sequence. After each usage cycle, files were tested for cyclic fatigue resistance using a custom device simulating each motion at body temperature, and for torsional resistance according to ISO 3630-1. Fractured fragments were examined with a scanning electron microscope. Data were statistically analysed using one way analysis of variance (ANOVA) and Duncan's post hoc tests at a significance level of 95%.

Both systems showed increased toughness and distortion angle after 2 uses (P < .05), likely due to stress-induced martensitic (SIM) transformation. RB exhibited higher cyclic fatigue resistance than VR. After 4 uses, mechanical performance declined in both systems (P < .05), while maximum torque remained unchanged (P > .05). Scanning electron microscope (SEM) revealed typical features of cyclic fatigue and torsional fractures, with machining grooves present even in new files.

Limited repetitive uses of NiTi files can transiently improve torsional and cyclic fatigue resistance. Continuous repetitive use, however, decreases mechanical performance due to fatigue accumulation and microstructural defects. These findings underscore the importance of restricting clinical reuse to preserve the mechanical integrity and safe performance of NiTi rotary instruments.