The ABC's of Intervertebral Disks
The Intervertebral Disk
The intervertebral disk does not get the credit it deserves. It is, in fact, the first thing one complains
about when back pain sets in. This often means, however, that the disk's function is completely overlooked.
What does a disk look like?
As the name implies, an intervertebral disk has a disk-like form. The disk itself is composed of a gelatinous
core, the nucleus pulposus, surrounded by a fibrous ring, the anulus fibrosis. Water makes up roughly 90% of
the gelatinous core. All in all the disks account for about a fourth of the total length of the spinal column.
How are the intervertebral disks supplied with nutrients?
Blood vessels supply the disks until about the second year of life, after which the disks make do with a
much slower metabolism. Essential minerals and fluids required for regeneration enter the disks passively
during the night. Each disk loses a little bit of fluid each day, resulting in a loss of height at the
end of the day of approximately three centimeters.
What function does an intervertebral disk serve?
Intervertebral disks, vertebrae and ligaments are all passive elements of the musculoskeletal system. The movements of
the spinal column occur in segments. These segments consist of vertebrae, disks, ligaments and vertebral
joints. A single segment is itself capable of only very restricted movement, but all the segments together
allow for the great flexibility of the spinal column as a whole. The disks accommodate each movement as
best as possible, the give and take between nucleus pulposus and anulus fibrosis making smooth movement
possible. The nucleus pulposus acts as an incompressible pillow of water. The core of the healthy disk
is centered when at rest and moved outward during movement. The anulus fibrosis guides the gelatinous
core during such movement. Vertical pressure such as exerted by gravity is absorbed by the disks. The
disks are, in fact, the perfect shock absorbers, capable of snapping back into place and adjusting
to various kinds of movement.
What happens as a disk becomes older or diseased?
Disk degeneration is not only something that happens with age; the first changes can already manifest
themselves during one's 20's and 30's. The ability of the disk to replenish itself with fluid, the
mobility of the nucleus pulposus and the disk's shock absorbing capacity gradually decreases. As a
result the spacing between two separate vertebrae is diminished. This leads to increased load on
the vertebral disks and to eventual fusion of vertebral joints. With the loss of shock absorption
in the nucleus pulposus the anulus fibrosis becomes brittle and pain nerves begin growing into it.
The result is a disk which itself causes pain.
The weakened disk can protrude toward the back and press on a nerve root - the classic protruding disk.
A herniated disk occurs when a tear allows the nucleus pulposus to squeeze through the anulus fibrosis.
A sequestered disk is disk material separated from the main disk which can then settle in the spinal canal.
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the abc of intervertebral disks