Although a herniated disc sometimes follows a single traumatic injury, it is more often the result of gradual, aging-related degeneration of the disc. By the age of 30, intervertebral discs (discs that serve as cushions between the vertebrae) begin to flatten, deteriorate, lose water content, thin out and become brittle.

Eventually, this deterioration, known as degenerative disc disease, may cause the nucleus pulposus in the center of the disc to swell or bulge. The pressure of this bulging may eventually create tiny tears in the tough, fibrous outer covering of the disc. If the nucleus pulposus in the center of the disc pokes through these tears, it is known as a herniation or rupture. In some cases, the nucleus pulposus may break through far enough to cause pressure on and irritate a nerve and result in a pinched nerve. Pain in the back, neck or a limb may result.

Sometimes, a herniated disc will cause material to compress the bundle of lumbar and sacral nerve roots at the end of the spinal cord known as the cauda equina. This condition is called cauda equina syndrome, and it may cause permanent neurological damage if left untreated.

Herniated discs occur most commonly in people in their 30s to 50s. They are most likely to affect middle-aged men engaged in strenuous activity. Having a congenital (present at birth) or acquired condition that affects the size of the spinal canal, such as spinal stenosis, raises the risk for developing nerve compression from a herniated disc.

Other conditions that can promote disc herniation include:

• Spondylosis. Spinal osteoarthritis, usually in the cervical or lumbar spine, often involving osteophytes (bone spurs).
  
  
• Osteoporosis. A common condition in which the bones lose density. Osteoporosis can lead to vertebral and other fractures and contribute to disc degeneration.
  
  

Herniated discs cannot always be prevented. However, good posture and ergonomics, such as proper lifting techniques, and other preventive measures can lower the risk of a herniated disc. In addition, not all herniated discs cause pain.