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Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

What Is It?

The disks in your spine, called intervertebral disks, are thin, oblong structures that serve as cushions between the bones of your back (vertebrae). Each disk is made of a soft gel core surrounded by a tough, fibrous outer shell. This structure allows the disk to be firm enough to maintain the space between the vertebrae, but soft enough to compress when the spine flexes during bending, leaning and turning sideways.

In some people, mostly middle-aged adults, a disk's tough outer shell develops an area of weakness or a small tear. When this happens, part of the disk's soft inner core can bulge out of its normal position (herniate), producing a condition called a herniated disk. If the herniated disk presses on nerves in the nearby spinal canal, this can cause variety of nerve-related symptoms, including pain, numbness and muscle weakness. In the most severe cases, a herniated disk can compress nerves that control the bowel and bladder, causing urinary incontinence and loss of bowel control.

Scientists do not fully understand why disks herniate. Most theories attribute this condition to a combination of the following factors:

  • Disk aging Herniated disks are rare in young people, but common among people aged 35 to 55. Of all the factors responsible for herniated disks, aging is probably the most important. With age, the disk's outer shell appears to degenerate slowly, possibly because of decades of upright posture and back flexion.

  • Genetic factors In some families, several close relatives suffer from herniated disks, whereas other families are not affected at all. If the condition runs in a family, it may have an unusually early onset, even striking people younger than 21. Studies are beginning to identify specific genes linked to inherited forms of disk disease.

  • Individual risk factors You may be at increased risk of a herniated disk if you work at a job or participate in a sport that involves heavy lifting or excessive twisting or bending.

There are three distinct areas of the vertebral column where a herniated disk may occur:

  • The cervical area between the vertebrae in the neck

  • The thoracic area between the vertebrae in the upper back, near the ribs

  • The lumbar region between the vertebrae in the lower back, above the pelvis

Herniated disks are most common in the lumbar region. Herniated disks are relatively rare in the thoracic region, where they account for only 1 in every 200 to 400 disk herniations.

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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