Thermomechanical properties of nickel-titanium closed-coil springs and their implications for clinical practice

• Springs with large hysteresis do not maintain a constant force over time.
• The effects of preactivation are cancelled by the intake of warm food.
• Springs with minor hysteresis deliver more consistent, predictable forces.

IntroductionThe aim was to study nickel-titanium closed-coil springs in a clinically relevant test setting with respect to the accuracy of the “preactivation” for nickel-titanium closed-coil springs application and whether it is possible to keep activation forces constant during the whole time of treatment.MethodsWe tested 10 types of springs from 5 manufacturers under clinically relevant conditions, allowing us to study the interactions between load and temperature over time. Hystereses were compared using t tests.ResultsSprings with a large mechanical hysteresis also showed a large thermal hysteresis. After heating shock, these springs showed intensive force spikes and persistent high loads. Some springs showed negligible thermal and mechanical hysteresis. Such springs never showed any clinically significant persistent high loads.ConclusionsSprings with a large hysteresis were unable to keep activation forces constant during the whole time of treatment even after any preactivation, and they might cause persistently high loads and possibly overloading. Only springs with minor hysteresis, low temperature dependence of force, and a clinically useful plateau have the following clinical advantages: reduced chair time, optimal rates of tooth movement, reproducible clinical results, and conservation of anchorage.