The temporomandibular joint is a ginglymo-arthrodial joint; the parts entering into its formation on either side are: the anterior part of the mandibular fossa of the temporal bone and the articular tubercle above; and the condyle of the mandible below.
Movements.—The movements permitted in this articulation are extensive. Thus, the mandible may be depressed or elevated, or carried forward or backward; a slight amount of side-to-side movement is also permitted. It must be borne in mind that there are two distinct joints in this articulation—one between the condyle and the articular disk, and another between the disk and the mandibular fossa. When the mouth is but slightly opened, as during ordinary conversation, the movement is confined to the lower of the two joints. On the other hand, when the mouth is opened more widely, both joints are concerned in the movement; in the lower joint the movement is of a hinge-like character, the condyle moving around a transverse axis on the disk, while in the upper joint the movement is of a gliding character, the disk, together with the condyle, gliding forward on to the articular tubercle, around an axis which passes through the mandibular foramina. These two movements take place simultaneously, the condyle and disk move forward on the eminence, and at the same time the condyle revolves on the disk. In shutting the mouth the reverse action takes place; the disk glides back, carrying the condyle with it, and this at the same time moves back to its former position. When the mandible is carried horizontally forward, as in protruding the lower incisor teeth in front of the upper, the movement takes place principally in the upper joint, the disk and the condyle gliding forward on the mandibular fossa and articular tubercle. The grinding or chewing movement is produced by one condyle, with its disk, gliding alternately forward and backward, while the other condyle moves simultaneously in the opposite direction; at the same time the condyle undergoes a vertical rotation on the disk. One condyle advances and rotates, the other condyle recedes and rotates, in alternate succession.
The mandible is depressed by its own weight, assisted by the Platysma, the Digastricus, the Mylohyoideus, and the Geniohyoideus. It is elevated by the Masseter, Pterygoideus internus, and the anterior part of the Temporalis. It is drawn forward by the simultaneous action of the Pterygoidei internus and externus, the superficial fibers of the Masseter and the anterior fibers of the Temporalis; and backward by the deep fibers of the Masseter and the posterior fibers of the Temporalis. The grinding movement is caused by the alternate action of the Pterygoidei of either side.