The original authors of this video (unknown) did an amazing job of illustrating normal TMJ rotation and translation.
1) Jaw Resting Position.
Although the round condyle looks like a perfect fit in the articular fossa, the resting position (dictated by the disc and the length of the ligaments) has the jaw sitting in line with the peak of the articular eminence, meaning the jaw always slides forwards/backwards, rather than ascending/descending out of the fossa.
Look at the middle of the condyle as the jaw opens. It only moves in the horizontal plane, there is no vertical plane of movement. Compare this to the resting position in the Anteriorly displaced disc with reduction video.
2) Compartmentalised Movement
The initial movement is pure rotation of the condyle of the jaw on the disc, or movement in the inferior compartment of the joint. As the condyle moves to its limit of range, the disc and condyle lock and slide forward together creating translation. The translation moment occurs between the disc and the skull, or the superior compartment. Either of these compartments can cause an movement derangement.
3) Retrodiscal Tissue
This is the nerve supply and blood supply to an otherwise aneural and avascular joint. The retrodiscal tissue is clear of compression when the disc is sitting in the correct position. As the jaw opens, you can see it stretch but it always remains free of the condyle.
4) Alveolar Bone Density
When excessive force is applied to a sphere, the force is transferred to the centre of the sphere and it will break down from the inside out. Look at the quality, colour and density of the alveolar bone in the head of the condyle and the articular eminence and compare it to the Anteriorly displaced disc with reduction video.
On X-ray, a jaw joint under sustained compression will not necessarily show up as degeneration as you don’t see changes to the joint surface until the alveolar bone has collapsed.
5) Disc Position
The articular disc sits on top of the condyle like a beanie and divides the jaw into 2 compartments. It is almost hour-glass or butterfly shaped and throughout the movement the condyle stays in the middle of the bulk of the tissue.
Author: Nigel Smith