The effect of temperature on the mechanical behavior of nickel-titanium orthodontic initial archwires

Objectives: To investigate and compare the characteristics of commonly used types of traditional and heat-activated initial archwires at different temperatures by plotting their load/deflection graphs and quantifying three parameters describing the discharge plateau phase. Materials and Methods: Forty-eight archwires of cross-sectional diameters ranging from 0.010 inches to 0.016 inches were obtained from seven different manufacturers. A modified three-point wire-bending test was performed on three analogous samples of each type of archwire at 55 degrees C and 5 degrees C, simulating an inserted archwire that is subjected to cold or hot drinks during a meal. For each resulting load/deflection curve the plateau section was isolated and the mean value of each parameter for each type of wire was obtained. Results: Permanent strain was exhibited by all wires tested at 55 degrees C. Statistically significant differences were found between almost all wires for the three considered parameters when tested at 55 degrees C and 5 degrees C. Loads were greater at 55 degrees C than at 5 degrees C. Differences were also found between traditional and heat-activated archwires, the latter of which generated longer plateaus at 55 degrees C, shorter plateaus at 5 degrees C, and lighter mean forces at both temperatures. The increase in average force seen with increasing diameter tended to be rather stable at both temperatures. Conclusions: All nickel-titanium wires tested showed a significant change related to temperature in terms of behavior and force for both traditional and heat-activated wires. Stress under high temperatures can induce permanent strain, whereas the residual strain detected at low temperatures can be recovered from as temperature increases. (Angle Orthod. 2013;83:298-305.)