Experimental analysis of torque characteristics of orthodontic wires

A major requirement for orthodontic wires is superior elasticity. The question arises whether today's nickel titanium wires display a distinctively superelastic plateau also under torsional loading, and whether typical torsion angles are sufficient to produce adequate torquing moments with superelastic wires. The objective of this study was to compare various nickel titanium and steel wires in a pure torsion experiment.

To experimentally examine the torque characteristic of orthodontic wires, we used nickel titanium wires manufactured by Dentaurum (Tensic®) and ODS (Euro Arch NiTi) measuring 0.40 × 0.40 mm2, 0.40 × 0.56 mm2, 0.43 × 0.64 mm2, 0.46 × 0.64 mm2, 0.48 × 0.64 mm2, 0.51 × 0.51 mm2, and steel wires made by Dentaurum (remanium®), 3M Unitek (HI-T™ II Stainless Steel) measuring 0.40 × 0.40 mm2, 0.40 × 0.56 mm2, 0.43 × 0.56 mm2, 0.43 × 0.64 mm2, 0.46 × 0.64 mm2, 0.48 × 0.64 mm2, 0.51 × 0.51 mm2. Torque was measured at an ambient temperature of 37 ° C for the nickel titanium wires. The distance between the bearing points was 10 mm for all measurements. The steel wires' torquing moments were determined at a torsion angle of 20 °, the torquing moments of the nickel titanium wires were investigated on the plateau, and we compared the results of the means and the standard deviations of all wires. Starting and end points of the loading and unloading plateau were determined for all nickel titanium wires.

The torquing moment/torsion angle diagrams of the nickel titanium wires by different manufacturers displayed the curves typical of superelastic wires. The torquing moments of the wires with a smaller cross-section yielded values below 5 Nmm and thus appear obviously too small (e.g. Dentaurum Tensic®, 0.40 × 0.40 mm2: 1.3 Nmm). The mean values of the loading plateau's starting point yielded values of about 20 °.

As the torsional play in the bracket slot of the wires with a smaller cross-section is typically about 10 °, we doubt whether these wires reach the superelastic plateau. Moreover, we observed production-associated variations in the material properties of various nickel titanium wires. It is thus difficult for the practitioner to draw a correlation between a wire's crosssection and the actual torquing moment delivered.