An investigation into the mechanism of "locking" in the myofascial pain dysfunction syndrome (MPD).

Locking of the temporomandibular joint is often attributed to uncoordinated muscle activity and muscle spasm. A simple experiment has been devised to test the hypothesis that locking occurs when uncoordinated elevator muscle tension (temporalis, masseter and medial pterygoid) precedes and interferes with the opening action of the lateral pterygoid muscle. By asking the patients with manifest locking to protrude prior to maximal opening (protrude-open maneuver, POM) a highly significant increase of mouth opening was found over that produced by simple maximal opening (MO). No significant difference was found in a group of patients with myofascial pain as their only symptom, or in a group of healthy controls, suggesting that pain does not contribute to the mechanism of locking.