The intervertebral discs may be thought of as soft tough pads that separate the bones (vertebrae) of the spine from one another. Their basic functions are:

• they act as a ligament by holding the vertebrae of the spine together,
• they act as a shock absorber which carries the downward weight of the body (axial load) while in an up right position,
• they act as pivot point, which allows the spine to bend and twist.

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). The disc is made up of three basic structures: the gelatinous nucleus pulposus in the center, the fibrous rings around the nucleus called the anulus fibrosus, and the vertebral end-plates. Although their composition percentage differs, the three structures are made of three basic components: proteoglycan (protein), collagen (cartilage), and water.

The Nucleus Pulposus is the water rich gelatinous center of the disc which is under very high pressure when the human is up-right. It has two main functions, to bear or carry the downward weight (aka: axial load) of the body, and to act as a 'pivot point' from which all movement of the lower trunk occurs. It also acts as a ligament and binds the vertebrae together. The Anulus Fibrosus is a much more fibrous structure that the nucleus pulposus. It has a higher collagen content and lower water content. Its job is to 'corral' the pressurized nucleus and keep that it from exploding outward. It is made of 15 to 25 concentric sheets of collagen, (a cartilage like substance) called the Lamellae . The lamellae are arranged in a special configuration which makes them extremely strong and easily able to contain that pressurized nucleus pulposus.

In the annulus, collagen fibers that make up the lamellae have a wavy, planar crimped pattern. This crimping plays a role in disc biomechanical function by allowing collagen fibers to stretch during compression.

Collagen in the intervertebral disc, tendons, and ligaments display a wavy collagen pattern termed "crimping". This crimp morphology contributes an important component to the mechanical responsiveness of these tissues. When these tissues experience loading, the collagen crimp architecture is gradually straightened; this makes possible the slight elongation of collagen at the loaded site.

Illustration above is of the organization of the intervertebral disc with attention to lamellar structure in the annulus and crimping in the collagen fibers.

The biomechanical behavior of the disc is closely linked to this specialized crimp morphology. The collagen fibers which form the annulus possess the same planar crimped geometry as is seen in the tendon. In the disc, however, crimp parameters vary according to the radial distance through the annulus. The lamellar structure of the annulus consists of continuous layers of collagen fibers that encircle the nucleus pulposus. In successive lamellar layers, the fiber orientation changes with respect to the spinal column axis. From the outer annulus inward, the interlamellar angle decreases linearly. Within each lamellar bundle, the collagen fibers are aligned in parallel arrays, and the fibers display a planar crimped waveform with all fibers in register. A gradient of crimp angle also is present in the disc, with crimp angle increasing and the crimp period decreasing from the periphery of the disc inward.

This complex and sophisticated architectural organization of lamellar disc structure serves to meet the unique biomechanical needs of the healthy disc. As you age, alteration in the crimping behavior of collagen may contributes to the underlying tissue changes which culminate in fissures and annual tears in the degenerating disc. These tears allow the nucleus to herniate towards the outer anulus layers. The stages are a bulging of the nucleus, a herniation, and the last last stage, the extrusion.

> What are the signs and symptoms of a herniated disc?
People with common herniated lumbar discs experience some combination of back and leg pain such as numbness, tingling, and/ or weakness along the compressed nerve. At first, most people experience severe back pain in conjunction with muscle spasm.

After a few days, the back pain usually subsides, and pain in one leg takes over. This pain is often described as an "electric shock", or burning sensation, that follows the nerve from the buttock, down the back of the thigh and into the calf or foot. This type of pain is called sciatica for the sciatic nerve that runs in the same area.

• There may be numbness, tingling, or a "pins and needles" feeling as well as other abnormal sensations along the pinched nerve.
• The muscles controlled by the nerve may weaken because they are not getting normal signals from the brain and spinal cord. Eventually, these muscles may atrophy (become smaller) because they are not being used.
• Although these are the most common symptoms, there may be any combination of back and leg pain in varying degrees of severity, from a very mild ache to unbearable pain requiring a visit to a hospital emergency room.

The most dangerous ruptured disc compresses all the nerves passing through the lower back, including those that control bladder and bowel function. These nerves are well protected so this situation is rare. However, when it does happen, it is a true emergency that requires immediate surgery. Symptoms include:

• an inability to control urine or stool functions
• an inability to urinate at all
• numbness around the rectum, buttocks and genital area
  

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