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Intervertebral discs (or intervertebral fibrocartilage) lie between adjacent vertebrae in the spine. Each disc forms a fibrocartilaginous joint to allow slight movement of the vertebrae, and acts as a ligament to hold the vertebrae together. Their role as shock absorbers is crucial.
Discs consist of an outer fibrous ring, the anulus fibrosus, which surrounds an inner gel-like centre, the nucleus pulposus. The anulus fibrosus consists of several layers of fibrocartilage. The layers of fibrocartilage contain the nucleus pulposus and this helps to distribute pressure evenly across the disc. The nucleus pulposus contains loose fibers suspended in a mucoprotein gel. The nucleus of the disc acts as a shock absorber, absorbing the impact of the body's activities and keeping the two vertebrae separated. It is the remnant of the notochord.
There is one disc between each pair of vertebrae, except for the first cervical segment, the atlas. The atlas is a ring around the roughly cone-shaped extension of the axis (second cervical segment). The axis acts as a post around which the atlas can rotate, allowing the neck to swivel. There are 23 discs in the human spine: 6 in the neck (cervical region), 12 in the middle back (thoracic region), and 5 in the lower back (lumbar region). For example, the disc between the fifth and sixth cervical vertabrae is designated "C5-6".
The intervertebral disc functions to separate the vertebrae from each other and provides the surface for the shock-absorbing gel of the nucleus pulposus. The nucleus pulposus of the disc, functions to distribute hydraulic pressure in all directions within each intervertebral disc under compressive loads. The nucleus pulposus consists of large vacuolated notochord cells, small chondrocyte-like cells, collagen fibrils, and proteoglycan aggrecans that aggregate through hyaluronic chains. Attached to each aggrecan molecule are the glycosaminoglycan (GAG) chains of chondroitin sulfate and keratan sulfate. Aggrecan is negatively charged, allowing the nucleus pulposus to attract water molecules. The amount of water and glycosaminoglycans decreases with age and degeneration.
During development and at birth, vertebral discs have some vascular supply to the cartilage end plates and the anulus fibrosus. These quickly deteriorate leaving almost no direct blood supply in healthy adults.
A prolapsed disc can happen when the gel-like material of the nucleus pulposus is forced out of the surrounding anulus fibrosus which can put pressure on the nerve located near the disc. This can give the symptoms of sciatica if it impinges on the roots of the sciatic nerve.
Before age 40 approximately 25% of people show evidence of disc degeneration at one or more levels. Beyond age 40, more than 60% of people show evidence of disc degeneration at one or more levels on a MRI.
One effect of aging and disc degeneration is the nucleus pulposus begins to dehydrate and the concentration of proteoglycans in the matrix decreases, thus limiting the ability of the disc to absorb shock. This general shrinking of disc size is partially responsible for the common decrease in height as humans age. The anulus fibrosus also becomes weaker with age and has an increased risk of tearing. In addition, the cartilage end plates begin thinning, fissures begin to form, and there is sclerosis of the subchondral bone.
While this may not cause pain in some people, in others one or both of these may cause chronic pain. Other spinal disorders can affect the morphology of intervertebral discs. For example, patients with scoliosis commonly have calcium deposits (ectopic calcification) in the cartilage end plate and sometimes in the disc itself. Herniate discs are also found to have a higher degree of cellular senescence than non-herniated discs.
The intervertebral disc space is typically defined on an X-ray photograph as the space between adjacent vertebrae. In healthy patients, this corresponds to the size of the intervertebral disc. The size of the space can be altered in pathological conditions such as discitis (infection of the intervertebral disc).
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