Changes in activity of the temporal, masseter and lip muscles after surgical correction of mandibular prognathism

Abstract

The activity of the anterior and posterior portions of the temporal muscle, the masseter muscle and the upper lip was recorded electromyographically in 18 patients before and on three occasions after surgical correction of mandibular prognathism. The muscle activity was recorded in the postural position of the mandible, during maximal bite, during chewing and swallowing of peanuts and during swallowing of water. The postural activity before and 8 months after surgery did not differ much from that in subjects with normal occlusion but there was a tendency towards increased activity during the period of intermaxillary fixation. The activity during maximal bite before surgery was far below that in individuals with normal occlusion. It declined still further during intermaxillary fixation but increased to values approaching those in normal individuals at follow-up examination 8 months after surgery. The number of chewing cycles necessary to triturate the test food and the duration of the act of chewing before surgery were greater than in normal individuals but decreased as a result of treatment to values close to normal. The activity in the posterior temporal and in the masseter muscles during chewing did not change much following surgical correction, while the activity in the anterior muscle increased. Positional changes of the mandible during intermaxillary fixation were related to the muscle activity before surgery. A large backward displacement of the mandible was related to low masseter activity and a large superior displacement of the gonion to high activity of the anterior temporal muscle before surgery.