Superelastic nickel titanium alloy retraction springs--an experimental investigation of force systems

The purpose of the present investigation was to study the mechanical characteristics of canine retraction springs made of superelastic nickel titanium (NiTi) alloys. A modified Burstone T-loop was used to construct an experimental canine retraction spring 10 mm in height and 10 mm in length. Twenty-five NiTi T-segments were hand made from the superelastic orthodontic alloys Ormco NiTi and Soar Sentalloy (dimensions 0.016 x 0.022"). The T-segments were equipped with arms made of rectangular standard steel wire (0.017 x 0.025"). The following geometrical and mechanical parameters of the retraction springs were analysed: radius and bending angles of the T-segments, distalizing force and M/F ratio during activation and the force/deflection rate of the springs. The error in the geometric parameters was in the range of 5-10 per cent, irrespective of the alloy used to produce the T-segments. On the other hand, the force systems of the springs were strongly influenced by the alloy and the batch under investigation. There were differences in the distalizing force of up to 100 per cent, i.e. at the beginning of the unloading plateau the distalizing force varied from 0.4 to 2.5 N. The force/deflection rate varied between a value of 0.06 and 0.15 N/mm, whereas the moment/force ratio reached values of 6.5-7.0 mm. Within a single batch, a reproducibility of these mechanical properties of approximately 5 per cent could be obtained. These results confirm that each orthodontic device made of superelastic NiTi alloys has to be calibrated individually. The manufacturers should pay more attention to keeping the material properties of their NiTi alloys constant.