Objectives: The aim of this study was to investigate the optimal extension with least affected heating cycle of 4 different brands of nickel-titanium closed coil spring by focusing on load deflection curve, chemical composition and phase transformation.
Methods: These springs were examined by using a universal testing machine, scanning electron microscope-energy dispersive X-ray microanalysis (SEM-EDS) and differential scanning calorimetry (DSC). Four specimens for each brand were tested with the tensile testing machine at constant temperature (37oC) and stepwise temperature changes (37-60oC and to 37oC). The effect of the heating cycle was assessed at selected points, 3, 5, 7 and 9 mm. extensions.
Results: Load-deflection curve revealed differences in force, hysteresis width and also curve characteristics. The unrecovered force after the heating cycle was analyzed in relation to the original force and reported in percent changes. The 3M (Unitek) spring demonstrated constant value with 1.04-1.71% changes from 5-9mm. extension while the Highland and Ormco springs demonstrated constant value from 3-9mm. extension with 3.13-4.56% and 4.23-6.05% respectively. However, the Tomy spring did not show constant value at any extension. These values can be used to predict the force changes after heating cycle.
Conclusion: The results showed that the optimum extension of the springs with least effect from the heating cycle was different among different brands. The chemical compositions and TTR of the spring from the 4 manufacturers were also different. This information is valuable in making predictions of the change of force in the clinic after the heating cycle.