Temporal muscle activity during the first year of Class II, division 1 malocclusion treatment with an activator.

The activity of the anterior and posterior temporal muscles in response to treatment with a splint type of activator was studied in children with distal occlusion. The effect on muscle activity was compared with that in a similar group of children being treated with a headgear and with that in a control group receiving orthodontic treatment for Class I malocclusion. Electromyographic recordings were made with the mandible in the rest position and, during maximal bite, in the intercuspal position. The recordings were made before the start of the treatment and on three later occasions at 4-month intervals. The activity in the rest position was constant during the 1-year period of observation. During maximal bite the activity of the posterior temporal muscle decreased significantly in the group with headgear and the control group and in a subgroup of children with large protrusions in the construction bite who had been treated with activators. This decrease was considered to be an effect of occlusal instability brought about by the treatment. There was no evidence of a decrease in the postural (rest) activity of the posterior temporal muscle, although such a decrease has been described as a sign of forward displacement of the mandible during treatment with a functional appliance.