Cyclic Fatigue and Wear Analysis of OneShape and OneCurve Instruments

Objectives: OneShape and OneCurve instruments have been introduced to obtain a single file system to shape root canal in clockwise rotation. This in vitro study compared cyclic fatigue resistance and wear analysis of these two instruments.
Methods: Cyclic fatigue tests were performed on 40 new OneCurve-25.06 (n=20) and OneShape-25.06 (n=20) (Micromega, Besançon, France) using an artificial canal with 5.0mm radius of curvature and 90° curvature angle. Time to failure, number of cycles to fracture (NCF) and length of fractured instruments were measured.
OneCurve (n=2) and OneShape (n=2), were analysed by scanning electron microscope (SEM) (JSM-5200, JEOL, Tokyo, Japan) in as received conditions and then subjected to in-vitro wear tests, performed using straight canals selected from a pool of extracted teeth and evaluated radiographically. Instruments were used in 5 canals with a 16:1 reduction handpiece X-Smart (Dentsply Maillefer, Baillagues, Switzerland) following the manufacturer’s recommendations (350rpm;2.5N/cm²). After use, autoclaved samples were observed using SEM to identify the wear features, unwinding, microcracks, blade disruption and tip deformation. 2-way ANOVA (p<0.05) statistical analysis was performed.
Results: OneCurve files exhibited a significantly higher cyclic fatigue resistance compared to OneShape (p<0.01) (Table). No statistically significant differences were found between the groups for length of fractured instruments. Superficial analysis conducted by SEM on as-received files revealed the presence of a smooth regular texture with milling grooves noticeably flattened by the electropolishing process. No instrument fractured during ex-vivo use. Spiral distortions and microcracks were absent in both kind of instruments. Dentinal debris were found on the surface of used instruments. The cutting edge of both types of instruments is well-defined and rounded, without signs of wear.
Conclusions: OneCurve instruments presented a significant improvement of the previous version. The high NCF and longer Time to Failure suggested a safe use in difficult clinical conditions and/or severely curved canals.