22 November 2022

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Resistance to torsion, static and dynamic cyclic fatigue of Nickel Titanium instruments

Lara Figini

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Rotary nickel titanium (NiTi) instruments are widely used for root canal preparation due to their many benefits in reducing procedural errors and working time, but they are also susceptible to breaking, especially in narrow and curved canals. 

The intracanal separation of the NiTi instrument can emerge due to torsional or cyclic fatigue. Torsional fracture occurs when the tip of the instrument gets stuck in the canal while the shaft continues to rotate. Cyclic fatigue failure occurs more frequently in curved canals, especially at the maximum point of curvature due to the alternating and repeated compressive and tensile stresses generated during file rotation. 

Among the NiTi systems, considered in the study, One RECI (OR; Micro-Mega, Besançon, France) is an alternative single-row system characterized by a heat treatment (C-wire) and variable decentralized cross-section, starting from a triple helix with progressive change in an S shape towards the stem. The tool is designed to work in reciprocal motion at specific angles (170 counter clockwise [CCW] and 60 clockwise [CW]) generating a counter clockwise cutting action. 

WaveOne Gold (WOG; Dentsply Maillefer, Ballaigues, Switzerland) is also a single-row system consisting of a “gold” alloy with a parallelogram section and an off-center design with two cutting edges. One Curve (OC, Micro-Mega) instruments are similar to OR, except that they work in continuous rotation motion with a clockwise cutting action. 

ProTaper Next2 (PTN X2; Dentsply Sirona, Charlotte, NC) is a root canal instrument developed using M-Wire heat treatment technology and designed with an off-center rectangular section.

Materials and methods

In an in vitro study, published in November 2022 in the Journal of Endodontics, the authors evaluated the torsional, dynamic and static cyclic fatigue resistance of several reciprocating and continuous rotating NiTi nickel titanium rotary files: One RECI (OR ; Micromega, Besançon, France), WaveOne Gold (WOG; Dentsply Maillefer, Ballaigues, Switzerland), One Curve (OC, MicroMega) and ProTaper Next X2 (PTN X2; Dentsply Sirona, Charlotte, NC). 

Study authors analyzed a total of 120 OR (n = 30), WOG (n = 30), OC (n = 30) and PTN X2 (n = 30). The authors measured the torque and angle of rotation up to failure under static conditions and the torsional load using ISO 3630-1. 

The team investigated static and dynamic resistance and fatigue time (TTF) in a stainless-steel artificial canal with an angle of 60 ° and a radius of curvature of 5 mm at intracanal temperature. The results were analyzed with 1-way analysis of variance and the post hoc Tukey test. The alpha-type error was set at 5%. Fracture instruments from torsion and fatigue tests were examined with a scanning electron microscope.

Results

OR showed higher static fatigue strength and greater fatigue rotation angle than WOG, OC and PTN X2 (P, .05). WOG showed greater torsional strength than the others (P, .05). The dynamic cyclic fatigue tests showed a TTF mode higher than the static one for the PTN X2 and WOG groups (P, .05). In dynamic tests, OR and WOG showed a higher TTF than OC and PTN X2 (P, .05).

Conclusions

Under these experimental conditions, One RECI exhibited suitable mechanical properties with the highest cyclic fatigue resistance and angle of rotation among the tested instruments.

Eugenio Pedullà, Naji Kharouf, Simone Caruso, Giusy Rita Maria La Rosa, Hamdi Jmal, Youssef Haikel, Davide Mancino. "Torsional, Static, and Dynamic Cyclic Fatigue Resistance of Reciprocating and Continuous Rotating Nickel-Titanium Instruments." Journal of Endodontics. 2022 Nov;48(11):1421-1427. DOI: 10.1016/j.joen.2022.08.005