Forces Released by Nickel-titanium Springs Due to Temperature Changes

Objectives: Evaluate the forces delivered by super-elastic springs, after the sudden application of high (50°C) and low (5°C) temperatures.
Methods: 18 springs from three commercial houses were used (GAC®, American Orthodontics® and Ormco®)
9mm coil spring discharge loading curves were established when elongated to 12mm. In unloading the universal testing machine was stopped 6mm inside the super-elasticity zone for 4min. After that the hot or cold water was applied at 210, 270 and 330 seconds marks. Water temperature remained constant in Julavo thermal baths.

T-Student tests, Variance Analysis (Anova) and the Bonferroni t test were performed. The significance was established at p<0.05.
Results: The data showed homocedasticity and normality. There were significant differences in the force delivered depending on the temperature in all the springs and between commercial houses, cold and hot. The change in strength was punctual and short-lived. After that there was a tendency to return to the baseline. There were differences in the percentages of recovery between cold and heat applications. In cases of application to heat, there was no complete return to the baseline. In cold applications, the force after recovery exceeded the value of the baseline. With each application of cold or heat the force delivered by the spring changed and their differences were significant.
Conclusions: Super-elastic springs are highly sensitive to temperature within the common mouth ranges. For this reason it is not possible to deliver constant forces. In spite of that the changes are of short duration. The biological and mechanical impact of this should be a reason for future research.