Function

Each temporomandibular joint is classed as a "ginglymoarthrodial" joint since it is both a ginglymus (hinging joint) and an arthrodial (sliding) joint. The condyle of the mandible articulates with the temporal bone in the mandibular fossa. The mandibular fossa is a concave depression in the squamous portion of the temporal bone.

These two bones are actually separated by an articular disc, which divides the joint into two distinct compartments. The inferior compartment allows for rotation of the condylar head around an instantaneous axis of rotation, corresponding to the first 20mm or so of the opening of the mouth. After the mouth is open to this extent, the mouth can no longer open without the superior compartment of the temporomandibular joints becoming active.

At this point, if the mouth continues to open, not only are the condylar heads rotating within the lower compartment of the temporomandibular joints, but the entire apparatus (condylar head and articular disc) translates. Although this had traditionally been explained as a forward and downward sliding motion, on the anterior concave surface of the mandibular fossa and the posterior convex surface of the articular eminence, this translation actually amounts to a rotation around another axis. This effectively produces an evolute which can be termed the resultant axis of mandibular rotation, which lies in the vicinity of the mandibular foramen, allowing for a low-tension environment for the vasculature and innervation of the mandible.

The necessity of translation to produce further opening past that which can be accomplished with sole rotation of the condyle can be demonstrated by placing a resistant fist against the chin and trying to open the mouth more than 20 or so mm.

The resting position of the temporomandibular joint is not with the teeth biting together. Instead, the muscular balance and proprioceptive feedback allow a physiologic rest for the mandible, an interocclusal clearance or freeway space, which is 2 to 4 mm between the teeth.

Jaw movementedit

Normal full jaw opening is 40-50 millimeters as measured from edge of lower front teeth to edge of upper front teeth.

When measuring the vertical range of motion, the measurement must be adjusted for the overbite. For example, if the measurement from the edge of the lower front teeth to the edge of the upper front teeth is 40 millimeters and the overbite is 3 millimeters, then the jaw opening is 43 millimeters.

During jaw movements, only the mandible moves.

Normal movements of the mandible during function, such as mastication, or chewing, are known as excursions. There are two lateral excursions (left and right) and the forward excursion, known as protrusion. The reversal of protrusion is retrusion.

When the mandible is moved into protrusion, the mandibular incisors, or front teeth of the mandible, are moved so that they first come edge to edge with the maxillary (upper) incisors and then surpass them, producing a temporary underbite. This is accomplished by translation of the condyle down the articular eminence (in the upper portion of the joint) without any more than the slightest amount of rotation taking place (in the lower portion of the joint), other than that necessary to allow the mandibular incisors to come in front of the maxillary incisors without running into them. (This is all assuming an ideal Class I or Class II occlusion.)

During chewing, the mandible moves in a specific manner as delineated by the two temporomandibular joints. The side of the mandible that moves laterally is referred to as either the working or rotating side, while the other side is referred to as either the balancing or orbiting side. The latter terms, although a bit outdated, are actually more precise, as they define the sides by the movements of the respective condyles.

When the mandible is moved into a lateral excursion, the working side condyle (the condyle on the side of the mandible that moves outwards) only performs rotation (in the horizontal plane), while the balancing side condyle performs translation. During actual functional chewing, when the teeth are not only moved side to side, but also up and down when biting of the teeth is incorporated as well, rotation (in a vertical plane) also plays a part in both condyles.

The mandible is moved primarily by the four muscles of mastication: the masseter, medial pterygoid, lateral pterygoid and the temporalis. These four muscles, all innervated by V₃, or the mandibular division of the trigeminal nerve, work in different groups to move the mandible in different directions. Contraction of the lateral pterygoid acts to pull the disc and condyle forward within the glenoid fossa and down the articular eminence; thus, action of this muscle serves to protrude the jaw, it with assistance of gravity and the digastricus muscle also opens the jaw. The other three muscles close the mouth; the masseter and the medial pterygoid by pulling up the angle of the mandible and the temporalis by pulling up on the coronoid process of the mandible.