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

INTRODUCTION: The 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. METHODS: We 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. RESULTS: Springs 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. CONCLUSIONS: Springs 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.

Thermo-mechanical properties of NiTi closed coil springs--force degradation and force regeneration over time, viscous properties.

INTRODUCTION: The aim of this study was to find out the impact of degradation and regeneration of force over time at NiTi springs on the value and course of the final acting force and to verify the possibility of using these phenomena for a directed transition to the reverse plateau and its maintaining. METHODS: Static and cyclic mechanical loadings were performed. At first unused springs were tested. Afterwards the springs were mechanically stabilized by stress cycling and finally tested again. The difference in shape of the working curves was assessed. For simulation and description of the force degradation the modified Voight model was used. RESULTS: New springs, mainly those with large hysteresis, showed a significant stress-strain curve movement and shape changes during the cycling. The effect of the stress-strain curve course change disappeared fully in the stabilized springs. Multiple loading led to an overall decrease of force value during the measurement. The effect of force degradation and regeneration over time by simple static loading varies in the range of percentage of the nominal force in the plateau area. The transition between stress-strain curve phases caused by the degradation or regeneration of the force wasn't observed in case of mechanically stabilized springs. CONCLUSIONS: Springs should be mechanically stabilized before their application. The degree of force degradation over time is insignificant for mechanically stabilized springs. Degradation or regeneration of force over time, mechanical stabilization or micromovements in the mouth don't cause any transition between individual stress-strain curve phases.