[New wires and their force delivery--consequences for orthodontic therapy].

Levelling arches made of stainless-steel wires were compared with nickel-titanium wires of the new generation which are distinguished by their shape memory, superelasticity and their excellent spring-back. The data received from a bending test have been analysed statistically and revealed a linear unloading curve of the twisted stainless-steel wires. The nickel-titanium wires, however, showed a clear deviation form the linearity with a curviform course. Besides the unusual unloading curve with a constant force mechanism in the middle range of deactivation the wires give off the same amount of force irrespective of the degree of activation (that is to say, regardless whether the level difference between the brackets amounts to 1 or to 4 mm). The diagram shows that the greater the activity between the brackets, the longer the period of superelasticity. Therefore the nickel-titanium wires are suited for large level differences or deflections. At a low activation rate the characteristic line takes a very steep and almost linear deactivation curve. In these cases twisted stainless steal wires should be preferred because of their lower load deflection rate and flexibility.