Intervertebral disc prolapse

The intervertebral disc bursts and fragments of the outer annulus fibrosus, together with some of the inner gelantinous nucleus pulposus, press on ligaments and nerves running close to the disc and produce pain.Once this has happened, that disc will never be normal again. It's resistance to further injury is considerably reduced and anyone who has had a burst intervertebral disc is always at risk of further episodes of acute pain.
Pathophysiology
Although any disc in the entire spine can prolapse or burst, the most common ones to which this happens are the lowest two, that is between the fourth and fifth lumbar vertebrae and between the fifth lumbar and the top of the sacrum. The most likely explanation is that the stresses experienced by the spine are the greatest at these levels. Also the sacrum does not stand vertically but is tilted backwards, so producing a sharp curve in this region and giving a wedge shape to the discs. This wedge shape may concentrate the stresses in the back of the disc so making these lower lumbar discs particularly liable to damage. Briefly in a prolapse, the disc ruptures or bursts so that debris from the disc protrudes and can damage the surrounding structures. Most commonly these ruptures occur at the back of the disc but to one side or the other. These are known technically as the postero-lateral edges of the disc. Sometimes, however, the burst may occur directly backwards in the midline and this is known as a central posterior prolapse. Prolapsed disc bursting backwards and to one side due to the presence of the posterior longitudinal ligament in the spinal canal.
Stages of Spinal Disc Herniation
Disc herniation can occur in any disc in the spine, but the two most common forms are lumbar disc herniation and cervical disc herniation. The former is the most common, causing lower back pain (lumbago) and often leg pain as well, in which case it is commonly referred to as sciatica.
Lumbar disc herniation occurs 15 times more often than cervical (neck) disc herniation, and it is one of the most common causes of lower back pain. The cervical discs are affected 8% of the time and the upper-to-mid-back (thoracic) discs only 1 - 2% of the time.
The following locations have no discs and are therefore exempt from the risk of disc herniation: the upper two cervical intervertebral spaces, the sacrum, and the coccyx.
Cervical disc herniation
Cervical disc herniations occur in the neck, most often between the fith & sixth (C5/6) and the sixth and seventh (C6/7) cervical vertebral bodies. Symptoms can affect the back of the skull, the neck, shoulder girdle, scapula,shoulder, arm, and hand. The nerves of the cervical plexus and brachial plexus can be affected.
Thoracic disc herniation
Thoracic discs are very stable and herniations in this region are quite rare. Herniation of the uppermost thoracic discs can mimic cervical disc herniations, while herniation of the other discs can mimic lumbar herniations.
Lumbar disc herniation
Lumbar disc herniations occur in the lower back, most often between the fourth and fifth lumbar vertebral bodies or between the fifth and the sacrum. Symptoms can affect the lower back, buttocks, thigh, and may radiate into the foot and/or toe. The sciatic nerve is the most commonly affected nerve, causing symptoms of sciatica. The femoral nerve can also be affected.[9] Can cause the patient to experience a numb, tingling feeling throughout one or both legs and even feet or even a burning feeling in the hips and legs.
Risk Factors
-Disc herniations can occur from general wear and tear, such as jobs that require constant sitting, but especially jobs that require lifting.
-Traumatic (quick) injury to lumbar discs commonly occurs from lifting while bent at the waist, rather than lifting while using the legs with a straightened back.
- Minor back pain and chronic back tiredness is an indicator of general wear and tear that makes one susceptible to herniation on the occurrence of a traumatic event from bending to pick up a pencil or a traumatic injury from a fall.
-Smoking is a major risk factor as the chemicals within smoke cause diminished nutrition and oxygenation of the discs leading to dehydration & degeneration which can then proceed to herniation.
-Mutation in genes coding for proteins involved in the regulation of the extracellular matrix, such as MMP2 and THBS2, has been demonstrated to contribute to lumbar disc herniation.

reference
www.nlm.nih.gov/medlineplus/ency/article/000442.htm