The intervertebral disc
Though it is not strictly part of the usual diarthrodial joints, the structure of the intervertebral discs is shortly presented as its pathology is very common problem.
Two main parts of the intervertebral discs are the outer ring (annulus fibrosus) and the inner gelatinous material. (nucleus pulposus). The outer ring consists of fibrocartilage (type I collagen), while the inner material shows many similarity to the articular hyaline cartilage both in its biochemical (type II collagen) and histological appearance. The function of the outer ring would be to keep the inner substance within the disc and resist the rotational and loading forces. The inner material functions as shock absorber for axial loads. Intervertebral discs loose their water content with time and become narrower (the explanation behind the loss of height in elderly) as a normal aging process.