Causes

In about 85% of back pain cases, the origin of the pain is unknown and even imaging studies usually fail to determine the cause. Disk herniation and disk degeneration due to aging are the most common causes of low back pain. Other problems can also cause this pain, however.

Lumbar Degenerative Disk Disease

Over the years, the disk can degenerate and produce low-grade inflammation and irritation. This age-related condition is a major source of chronic low back pain.

Herniated Disk and the Inflammatory Response

A herniated disk, sometimes, but incorrectly, called a slipped disk, is widely held to be the most common cause of severe back pain and sciatica. A disk in the lumbar area becomes herniated when it ruptures or thins out and degenerates to the point that the gel within the disk (nucleus pulposus) pushes outward. This event can take many forms.

  • A bulge -- The gel has been pushed out slightly from the disk and is evenly distributed around the circumference.
  • Protrusion -- The gel has pushed out slightly and asymmetrically in different places.
  • Extrusion -- The gel balloons extensively into the area outside the vertebrae or breaks off from the disk.

There is some debate, however, about how pain develops from a herniated disk and how frequently it causes low back pain. Many people have disks that bulge or protrude and do not suffer back pain. Extrusion (which is less common than the other two conditions) is highly associated with back pain, since the gel is likely to extend out far enough to press against the nerve root, most often the sciatic nerve. Extrusion is very uncommon, however, and sciatic and low-back pain is very common suggesting that there are other, more prominent causes of this pain.

Ordinarily, at the time of any injury, the immune system triggers key factors that are designed to promote healing. Evidence is now pointing to an abnormal and persistent immune response in the cells of the nucleus pulposus that may be responsible for nerve injury and pain in the lower back. In such cases, the nucleus pulposus in the herniated disk overproduces certain factors known as cytokines -- notably tumor necrosis factor (TNF) -- that, in high levels, cause inflammation and cell damage. Evidence now suggests that such cytokines cause a biochemical reaction in the regions surrounding the bulging or protruded nucleus pulposus, which results in pain.

Abnormalities in the Annular Ring. Research has also focused on tears in the annular ring--the fibrous band that surrounds and protects the disk. The annular ring contains a dense nerve network and high levels of peptides that heighten perception of pain. Tears in the annular ring are a frequent finding in patients with degenerative disk disease. Some cases of chronic low back pain may be caused by inward growth of nerve fibers into the annular ring, which triggers pain within the intervertebral disk.

Muscle and Ligament Injuries

Other than age-related degenerative disk disorders, injuries in the muscles and ligaments supporting the back are the major causes of low back pain. Of note, is the iliac crest pain syndrome (iliolumbar syndrome), in which there are tears in the ligaments that help support the pelvic bone.

Spinal Stenosis

Spinal stenosis is the narrowing of the spinal canal. This typically develops as a person ages and the disks become drier and start to shrink. At some point in this process, any disruption, such as a minor injury that results in disk inflammation, can cause impingement on the nerve root and trigger pain. Pain from spinal stenosis can occur in both legs, or can present as sciatica. Spinal stenosis occurs mostly in the elderly with degenerative osteoarthritis, but it can sometimes be caused by other problems, including infection and birth defects.

Spondylosis and Spondylolisthesis

Spondylosis is a condition in which the fourth or fifth lumbar vertebrae degenerate or develop small fractures. This condition affects 4% to 6% of the general population, and the rates may be higher in certain populations. As it progresses, the spine can become unstable and lead to spondylolisthesis, in which one vertebra slips forward over the other and causes sciatica. The condition most often occurs in older individuals with women having a higher risk than men. It is also a common cause of back pain from stress fractures in young athletes and can also be due to inherited problems, injury, or bone disease.

Piriformis Syndrome

Some cases of sciatica pain may occur when a muscle located deep in the buttocks pinches the sciatic nerve. This muscle is called the piriformis. The resulting condition is called piriformis syndrome. Piriformis syndrome usually develops after an injury. In rare cases leg swelling, deep-vein blood clots, or both may occur. Piriformis syndrome is sometimes difficult to diagnose.

Ankylosing Spondylitis

Ankylosing spondylitis is a chronic inflammation of the spine that may gradually result in a fusion of vertebrae. Symptoms include a slow development of back discomfort, with pain lasting for more than three months. The back is usually stiff in the morning; pain improves with exercise. In severe cases, the patient must continually stoop over. It can be quite mild, however, and it rarely affects a person's ability to work. It occurs mostly in young Caucasians in their mid-twenties. The disease is more common in men, but about 30% of the cases are in women. Researchers believe that in most cases it is hereditary. About 20% of people with inflammatory bowel disease and about 20% of people with psoriasis develop a form of ankylosing spondylitis. There are few effective treatments for this potentially disabling disease, although etanercept (Enbrel) and infliximab (Remicade), anti-inflammatory agents known as TNF-blockers, are proving to be beneficial.

Miscellaneous Abnormalities

Any abnormality in joints, vertebrae, or nerve roots can cause back pain:

  • The facet (z-joints) joints can wear down. In such cases, pain occurs on arching the back or when walking.
  • In some cases a segment (consisting of two vertebrae and their common joint and disk) becomes unstable when its parts wear down.
  • Injury to nerve roots, notably deep root ganglia (nerve cells in the spine whose fibers extend from skin to muscle tissue), may be important in some cases. Some patients may have scar tissue that traps the nerve roots in the lower spine and causes sciatica.
Review Date: 4/26/2006
Reviewed By: Harvey Simon, MD, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital