Force relaxation in orthodontic arch wires.

Wire selection for use in an orthodontic appliance is usually based upon clinical impressions. This is unsatisfactory. A far better basis for selection would be measured values of the forces exerted by the arch wire, both initially on insertion of the appliance and their variation with time as a result of tooth movement and of stress relaxation in the wire. An apparatus has been designed to measure force relaxation in orthodontic arch wires shaped into a representative arch form. Measurements of the force relaxation have been made in stainless steel, Elgiloy, and nitinol wires at 21 degrees C and 37 degrees C. All the wires were found to relax at 21 degrees C, but no relaxation was observed in nitinol at 37 degrees C. The rates of relaxation were found to vary widely; stainless steel wires produced by Wilcock in Australia relaxed significantly less than Elgiloy wires. A heat treatment performed on green Elgiloy eliminated the relaxation altogether and had a significant but lesser effect on blue Elgiloy.