Mechanical behavior of thermo-responsive orthodontic archwires.
OBJECTIVE
This investigation was conducted to describe the mechanical behavior of thermo-responsive nitinol archwires in flexure at 5 degrees and 37 degrees C.


METHODS
Four same-sized (but different force level) rectangular archwires were examined using a three-point bend test. Samples were tested at 5 degrees and 37 degrees 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 degrees but not at 5 degrees C. Permanent deformation was greater at 5 degrees than 37 degrees. The initial slope of the load-deflection data averaged 1230 g/mm at 37 degrees, which was significantly different from the average at 5 degrees (500 g/mm). Loads at the apparent yield point and the loads at 1, 2, and 3 mm of deflection were greater at 37 degrees C than at 5 degrees. 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 degrees and then unloaded at 37 degrees, the mechanical hysteresis of the wires tested at 37 degrees and the permanent deformation of the wires tested at 5 degrees were reduced for all wires.


CONCLUSIONS
Nitinol wires are available with a variety of mechanical properties. The different mechanical properties of thermo-responsive wires at 5 degrees and 37 degrees C result in clinically useful shape-memory behavior. Utilizing the superelastic and shape-memory features of thermo-responsive wires has clinical advantages.
Mullins
W S
WS
MET Electrical Testing Co., Columbia, Maryland, USA.
Bagby
M D
MD
Norman
T L
TL
D000497:Alloys / Q000737:chemistry*; D000704:Analysis of Variance; D004548:Elasticity; D016014:Linear Models; D008422:Materials Testing; D019563:Mechanics; D009969:Orthodontic Wires; D013816:Thermodynamics
D000497:Alloys; C013616:nitinol