MUSCLE-ACTIVITY DURING MANDIBULAR MOVEMENTS IN NORMAL AND MANDIBULAR RETROGNATHIC SUBJECTS

Purpose: The masticatory muscles function as a unit during precise man dibular positioning movements that occur during such activities as spe ech, singing, or playing musical instruments. This investigation was d esigned to assess jaw muscle recruitment patterns during controlled ma ndibular movement in normal subjects and in patients with mandibular r etrognathism. Patients and Methods: A computer-integrated electromyogr aphy (EMG) and movement monitoring (Selspot) system was used to collec t data over 7 seconds of a sagittal border movement (Posselt envelope) of the mandible and 4 seconds each of rest position, light tooth cont act, and maximum clench. Fine wire bipolar electrodes were placed into the inferior belly of the lateral pterygoid muscles bilaterally and s urface electrodes were placed bilaterally over the anterior belly of t he temporalis muscles and the masseter muscles, Ten subjects with Clas s I occlusion, normal cephalometric values, and an absence of temporom andibular joint (TMJ) dysfunction were compared with 12 patients with mandibular retrognathism, Class II malocclusion, and an absence of cli nical signs of TMJ internal derangement before and after a bilateral s agittal split and advancement of the mandible. Results: There was a wi de variation in standard deviations of EMG activity for the lateral pt erygoid muscles in the retrognathic patients compared with normal cont rols before surgery (P <.05). In light tooth contact, temporalis muscl e activity increased after surgery with respect to both control and th e presurgical levels (P <.05, P <.005, respectively). In maximum clenc h, activity in all muscle groups in the retrognathic patients, both be fore and after surgery, were below that of control subjects (P <.005). The lateral pterygoid muscles showed late recruitment, with low EMG a ctivity levels during the forward movement phase of the envelope, befo re surgery compared with controls (P <.001). After surgery, the latera l pterygoid muscle showed early recruitment in the forward movement si milar to control levels. Conclusion: The masticatory muscles function as a unit during mandibular positioning movements, Patients with mandi bular retrognathism have different muscle recruitment patterns from th ose 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.