Temperature- and deflection- dependences of orthodontic force with Ni-Ti wires

Orthodontic forces of Ni-Ti wires examined under the retrained condition on the dental arch model were evaluated with the changes in temperature and deflection. The tested specimens were a commercially available superelastic (W1) wire and two shape memory wires with their nominal A(f) points were 35 degrees C (W2) and 40 degrees C (W3), respectively. They showed typical superelastic hysteresis loops under the restraint condition at 40 degrees C. The force levels were significantly larger than those generally obtained by simple three-bending test. The recovery forces in the plateau region at 1.0 mm deflection were much larger than desired in the clinical guidelines around oral temperatures. In the shape memory wire W3, the recovery force rapidly decreased to zero by a small reduction of the deflection from its maximum. However, the wire again exerted the force with the remaining permanent deflection by temperature rising. It was small compared to the guidelines of desirable orthodontic force and seemed to be useful especially for the hypersensitive patients.