Anatomy, Back, Intervertebral Discs

Excerpt

Adjacent vertebrae articulate through zygapophyseal joints between the respective superior and inferior facets of the vertebral articular processes as well as through the joints of the vertebral bodies. While the former serves to limit the spine’s range of motion, the latter increases it and provides the majority of the spine’s weight-bearing capacity. The inferior surface of the superior vertebral body articulates with the superior surface of the inferior vertebral body through intervertebral (IV) discs. These 25 discs (7 cervical, 12 thoracic, 5 lumbar, and 1 sacral) account for about 25% to 33% of the length of the spine. They allow the spine to be flexible without sacrificing a great deal of strength. They also provide a shock-absorbing effect within the spine and prevent the vertebrae from grinding together. They consist of three major components: the inner or nucleus pulposus (NP), the outer or annulus fibrosus (AF), and the cartilaginous endplates that anchor the discs to adjacent vertebrae.

The NP is a gel-like structure that sits at the center of the intervertebral disc and accounts for much of the strength and flexibility of the spine. It is made of 66% to 86% water, with the remainder consisting primarily of type II collagen (it may also contain type VI, IX, and XI) and proteoglycans. The proteoglycans include the larger aggrecan and versican that bind to hyaluronic acid and several small leucine-rich proteoglycans. Aggrecan is largely responsible for retaining water within the NP. This structure also contains a low density of cells. While sparse, these cells produce the extracellular matrix (ECM) products (aggrecan, type II collagen, etc.) and maintain the integrity of the NP.

The AF is a ring-shaped disc of fibrous connective tissue that surrounds the NP. This structure is highly organized, consisting of 15 to 25 stacked sheets, or “lamellae,” of predominantly collagen, with interspersed proteoglycans, glycoproteins, elastic fibers, and the connective tissue cells that secrete these ECM products. Each lamella contains collagen uniformly oriented in a plane that differs in orientation to the adjacent lamella by about 60 degrees. This alignment leads to the parallel orientation of alternate lamella. This “radial-ply” formation provides exceptional strength compared to an entirely longitudinal setup and has been mimicked in the construction of products such as car tires. The lamellae are interconnected through translamellar bridges. The number of translamellar bridges per unit area is set to achieve a balance between strength and flexibility. A greater number of bridges would provide greater resistance to compressive forces but would limit flexibility and vice-versa.

The AF contains an inner and an outer portion. They differ primarily in their collagen composition. While both are primarily collagen, the outer annulus contains mostly type I collagen, while the inner has predominantly type II. The inner annulus also contains more proteoglycans than the inner. The ratio of type I to type II changes gradually; as the distance from the NP increases, the amount of type II collagen decreases, while the amount of type I increases. Another major difference between the two segments is the morphology of the connective tissue cells that secrete ECM. The cells of the inner annulus are described as round, while the outer annulus has a more oblong, fibroblast-like appearance.