[The "automatic activation" of labial arch wires during orthodontic treatment with expansion plates]

In laboratory experiments, progressive transversal jaw expansion with screw plates was simulated and the resulting increase in labial archwire force and deflection (autoactivation) was analysed. The results showed that progressive activation of a screw plate caused an initial increase in labial archwire force, which was subsequently followed by a minor reduction, and which stabilised after 90 seconds. Archwire force increases with the diameter of the wire, spring-hard wires developing higher forces than hard wires, and deflection decrease progressively with increasing size of the single activation step, as well as with increasing total activation. These results may have the following practical implications: on adjustment of the screw of the plate, the labial archwire is automatically activated to a reasonable degree for retroclination of the front teeth in screw plate treatment. This is particularly true when an 0.8 mm spring-hard wire is used. More constant archwire force is obtained by applying frequent small activations of the screw rather than a few large ones. To maintain the labial archwire force at a useful level, throughout treatment, fabrication of a new plate is recommended before the full range of screw adjustment is exhausted.