Changes in electric activity of masseter and temporal muscles after mandibular sagittal split osteotomy.

The functioning of the masseter and anterior temporal muscles was assessed by electromyography in 18 patients before orthognathic surgery and 6 weeks, 3 months, 1 year, and 2 years afterwards. Electromyogram (EMG) recordings were made during maximal bite in intercuspal position and chewing. The mean electric activity in the masseter and anterior temporal muscles decreased markedly 6 weeks after surgery but increased clearly for 1 year in both functions. Only a slight further increase was observed at 2-year follow-up during chewing, but not during maximal bite in intercuspidation. The mean changes in electric activity increased more clearly in patients with hyperplastic mandible than in patients with hypoplastic mandible and in those with long shift of the mandible. The same finding was also more visible in men and older patients than in women and younger patients.