Mechanical behavior of thermo-responsive orthodontic archwires

Objectives. This investigation was conducted to describe the mechanical behavior of thermo-responsive nitinol archwires in flexure at 5° and 37°C. Methods. Four same-sized (but different force level) rectangular archwires were examined using a three-point bend test. Samples were tested at 5° and 37°C. Linear regressions were fit to different segments of the load-deflection plots. Regression parameters of the segments and other properties were statistically analyzed. Results. Superelastic behavior was exhibited by all wires tested at 37° but not at 5°C. Permanent deformation was greater at 5° than 37°. The initial slope of the load-deflection data averaged 1230 g/mm at 37°, which was significantly different from the average at 5° (500 g/mm). Loads at the apparent yield point and the loads at 1, 2, and 3 mm of deflection were greater at 37°C than at 5°. While the slope and length of the superelastic region were not judged to be clinically significantly different, the average load of the superelastic region was significantly different: F300 (340 g) > F200 (250 g) > F100 (180 g) and Bioforce (180 g). When loaded at 5° and then unloaded at 37°, the mechanical hysteresis of the wires tested at 37° and the permanent deformation of the wires tested at 5° were reduced for all wires. Significance. Nitinol wires are available with a variety of mechanical properties. The different mechanical properties of thermo-responsive wires at 5° and 37°C result in clinically useful shape-memory behavior. Utilizing the superelastic and shape-memory features of thermo-responsive wires has clinical advantages.