Functional roles of the superior and inferior heads of the human lateral pterygoid muscle in movements of the mandibular condyle and articular disk

Abstract

Electromyographic (EMG) activities of the superior (SUP) and inferior heads (INF) of the lateral pterygoid muscle (LPT) were recorded in humans during voluntary stepwise changes in biting force and jaw-position which were adopted to exclude the effects of acceleration and velocity of jaw movements on the muscle activity. The SUP behaved like a jaw closing muscle and showed characteristic activity in relation to the biting force. It showed considerable amount of background activity ( 5 -32% of the maximum) even in the intercuspal position without teeth-clenching, and reached a nearly maximum activity at relatively lower biting force levels than the jaw closing muscles during increment of the biting force. Stretch reflexes were found in the SUP, the function of which could be to stabilize the condyle against the biting force that pulls the condyle posteriorly. This notion was verified by examining the biomechanics on the temporomandibular joint. The complex movements of the mandibular condyle in a sagittal plane was decomposed into displacement in the antero-posterior direction (Ac) and angle of rotation (RAc) around a kinesiological specific point on the condyle. In relation to Ac, each head of the LPT showed quite a similar behavior to each other in all types of jaw movements across all subjects. Working ranges of the muscle activ-ities were almost constant (Ac < 3 mm for the SUP and Ac > 3 mm for the INF) . The amount of EMG activity of the SUP changed in inverse proportion to Ac showing a hyperbola-like relation, whereas that of the INF changed rather linearly. The EMG amplitude of the SUP showed a quasi-linear inverse relation with RAc in the hinge movement during which the condyle rotated with no movement in antero-posterior direction. This finding suggests that the SUP controls the angular relationship between the articular disk and the condyle. On the other hand, the position of the disk in relation to the maxilla, not to the condyle, is controlled indirectly by the INF, because the disk is attached to the condyle by tendinous ligaments.