Muscle activity during mandibular movements in normal and mandibular retrognathic subjects

The masticatory muscles function as a unit during precise mandibular positioning movements that occur during such activities as speech, singing, or playing musical instruments. This investigation was designed to assess jaw muscle recruitment patterns during controlled mandibular movement in normal subjects and in patients with mandibular retrognathism.

A computer-integrated electromyography (EMG) and movement monitoring (Selspot) system was used to collect data over 7 seconds of a sagittal border movement (Posselt envelope) of the mandible and 4 seconds each of rest position, light tooth contact, and maximum clench. Fine wire bipolar electrodes were placed into the inferior belly of the lateral pterygoid muscles bilaterally and surface electrodes were placed bilaterally over the anterior belly of the temporalis muscles and the masseter muscles. Ten subjects with Class I occlusion, normal cephalometric values, and an absence of temporomandibular joint (TMJ) dysfunction were compared with 12 patients with mandibular retrognathism, Class II malocclusion, and an absence of clinical signs of TMJ internal derangement before and after a bilateral sagittal split and advancement of the mandible.

There was a wide variation in standard deviations of EMG activity for the lateral pterygoid muscles in the retrognathic patients compared with normal controls before surgery (P < .05). In light tooth contact, temporalis muscle activity increased after surgery with respect to both control and the presurgical levels (P < .05, P < .005, respectively). In maximum clench, activity in all muscle groups in the retrognathic patients, both before and after surgery, were below that of control subjects (P < .005). The lateral pterygoid muscles showed late recruitment, with low EMG activity levels during the forward movement phase of the envelope, before surgery compared with controls (P < .001). After surgery, the lateral pterygoid muscle showed early recruitment in the forward movement similar to control levels.

The masticatory muscles function as a unit during mandibular positioning movements. Patients with mandibular retrognathism have different muscle recruitment patterns from those of normal subjects with the mandible at rest and during mandibular movement. After orthognathic surgery, adaptation occurs in the phasic timing of jaw muscle activity.