[Changes of masticatory muscle activity after surgical orthodontic treatment for skeletal mandibular protrusion]

The purpose of this study was to analyze the recovery process of masticatory muscle activity after mandibular set-back osteotomy. The subjects were 11 adult patients with skeletal mandibular protrusion. They received surgical orthodontic treatment and attained a stable occlusion. E. M. G. recordings of clenching and habitual jaw movement were taken from the temporalis anterior, posterior, and masseter muscles with surface electrodes before preoperative orthodontic treatment just before surgery, and 6 months, 1 year and over 5 years after surgery. The parameters of E. M. G. analysis were 70% of the maximum voltage, the buildup time from the onset of activity to 70% of the maximum voltage and the time difference of the onset of activity in reference muscles. The results were as follows: 1. The initial level of E. M. G. activity was related to the rate of recovery following surgery. 2. Differences in masticatory muscle activity were reflected in the activity levels of the respective muscles during the recovery period. 3. A fairly complete recovery of E. M. G. activity was attained about ten months after surgery. 4. The time from the onset of activity to attaining 70% of the maximum voltage became progressively shorter. 5. The time difference of the onset of activity in reference to muscles decreased. It was I concluded that masticatory muscle activity could well adapt to the newly surgically-established maxillomandibular skeletal relationship.