The Lateral Pterygoid Muscle: Function and Dysfunction

The human lateral pterygoid muscle plays an important role in the control of jaw movements. This review provides an update on some aspects of the normal function of the lateral pterygoid muscle and its response to alterations, including mandibular advancement, occlusal changes, and experimental orofacial pain. Both the upper or superior head (SHLP) and the lower or inferior head (IHLP) of the lateral pterygoid insert predominantly into the neck of the condyle. A number of recent studies have carried out recordings of jaw movement and electromyographic (EMG) activity from the lateral pterygoid muscle, where verification of electrode location has been achieved through computer tomography imaging. From these studies, there is no evidence of background EMG activity within IHLP or SHLP when the jaw is in the clinically determined postural jaw position. There is little evidence for a reciprocal relation in activity between SHLP and IHLP, and both SHLP and IHLP play an important role in contralateral, protrusive, and jaw-opening movements and force generation. There is evidence for independent activation of subcompartments within the lateral pterygoid muscle to allow a range of force vectors to be delivered to the condyle. In terms of the role of the lateral pterygoid muscle in mandibular advancement and occlusal changes, the lack of studies of these issues, where verified recordings have been made from the lateral pterygoid muscle, means that there is no definitive evidence in humans for the “lateral pterygoid hypothesis,” and there is also little reliable information as to the effect of occlusal variables on the activity of the lateral pterygoid muscle. There is also little information on the effect of pain on lateral pterygoid muscle activity, although recent studies demonstrate that the pattern of pain-induced changes in lateral pterygoid muscle EMG activity is not clear-cut but can vary with the task performed and jaw displacement magnitude. The lateral pterygoid muscle is a difficult muscle to access, and only verified recordings from the muscle will yield definitive conclusions.