Long-term time effect on load–deflection characteristics of orthodontic wires

The purpose of this study was to examine the effect of time on the load–deflection characteristics of orthodontic wires in three-point bending when they were unloaded after having been held for 21 days before unloading.Using a modified three-point bending system, three types of round wires (stainless steel, NiTi and β-Ti) of 0.016 in. diameter (0.41 mm) were tested in the following two ways: one in the conventional manner with no delay before unloading, and the other allowing a 21-day hold. Comparison was made both of change of load during the 21-day hold and of load–deflection characteristics.During the 21-day hold at the turn, all three wires showed load decrease: NiTi by 18%, β-Ti by 14% and stainless steel by 6%. The effects of the 21-day delay in unloading on the load–deflection characteristics were as follows: stainless steel and β-Ti underwent 7.2% and 5.3% permanent deformation, respectively, with β-Ti also losing pseudoelasticity, while for NiTi, the range of the mechanical hysteresis increased by 14% with the lower plateau moving lower than without the delay.It is suggested that the load values clinically exhibited by these orthodontic wires might be smaller than those experienced in past experimental research that allowed immediate unloading.