An evaluation of the transition temperature range of super-elastic orthodontic NiTi springs using differential scanning calorimetry

Differential scanning calorimetry (DSC) was used to determine the transitio n temperature ranges (TTR) of four types of super-elastic orthodontic nicke l-titanium coil springs (Sentalloy). A knowledge of the TTR provides inform ation on the temperature at which a NiTi wire or spring can assume superela stic properties and when this quality disappears. The spring types in this study can be distinguished from each other by their characteristic TTR duri ng cooling and heating. For each tested spring type a characteristic TTR du ring heating (austenite transformation) and cooling (martensite transformat ion) was evaluated. The hysteresis of the transition temperature, found bet ween cooling and heating, was 3.4-5.2 K. Depending on the spring type the a ustenite transformation started (As) at 9.7-17.1 degrees C and finished (Af ) at 29.2-37 degrees C. The martensite transformation starting temperature (Ms) was evaluated at 32.6-25.4 degrees C, while Mf (martensite transformat ion finishing temperature) was 12.7-6.5 degrees C. The results show that the springs become super-elastic when the temperature increases and As is reached. They undergo a loss of super-elastic properti es and a rapid decrease in force delivery when they are cooled to Mf. For t he tested springs, Mf and As were found to be below room temperature. Thus, at room temperature and some degrees lower, all the tested springs exert s uper-elastic properties. For orthodontic treatment this means the maintenan ce of super-elastic behaviour, even when mouth temperature decreases to abo ut room temperature as can occur, for example, during meals.