Normal movements of the jaw during function, such as chewing, are known as excursions. There are two lateral excursions ( left and right ) and the forward excursion, known as protrusion, the reversal of which is retrusion.
When the jaw is moved into protrusion, the lower incisors or front teeth are moved so that they first come edge to edge with the upper incisors, and then move past them, producing a temporary underbite. This is accomplished by sliding of the condyle down the articular eminance ( in the upper portion of the TMJ ) without any more than the slightest amount of rotation taking place ( in the lower portion of the TMJ ), other than that necessary to allow the lower incisors to come in front of the upper incisors without running into them.
During chewing, the jaw moves in a specific manner as delineated by the two TMJs. The side of the mandible that moves sideways 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 The latter terms, although a bit outdated, are actually more precise, as they define the sides by the movements of the respective condyles.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 jaw is moved into a lateral excursion, the working side condyle ( the condyle on the side of the jaw 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 jaw is moved primary by four muscles: the masseter, medial pterygoid, lateral pterygoid and the temporalis. These four muscles work in different groups to move the mandible in different directions. Contraction of the lateral pterygoid acts to pull the disc and condyle forward; thus, the action of this muscle serves to open the mouth. 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.
When the mouth opens, two distinct motions occur at the joint. The first motion is ROTATION around a horizontal axis through the condylar heads. The second motion is TRANSLATION. The condyle and meniscus move together interiorly beneath the articular eminence.
In the closed mouth position, the thick posterior band of the meniscus lies immediately above the condyle. As the condyle translates forward, the thinner intermediate zone of the meniscus becomes the articulating surface between the condyle and the articular eminence.When the mouth is fully open, the condyle may lie beneath the anterior band of the meniscus.
