Introduction

Back pain is one of the most common reasons people visit their doctor. According to the National Institute of Arthritis and Musculoskeletal and Skin Diseases, 8 out of 10 people have some type of backache.

Back pain can be acute or chronic. Acute pain develops suddenly and goes away within 6 weeks. Chronic pain can come on fast or slow, but it lasts longer than 3 months. Back pain can occur in any area of the back, but it is more common in the lower part, which supports most of the body’s weight.

The main nerve traveling down the leg is the sciatic nerve. Pain associated with the sciatic nerve usually originates when nerve roots in the spinal cord become compressed or damaged. Symptoms can include tingling, numbness, or pain that radiates to the buttocks, legs, and feet.

The Spine

The back is highly complex, and pain may result from damage or injury to any of various bones, nerves, muscles, ligaments, and other structures. Still, despite sophisticated techniques that provide detailed anatomical images of the spine and other tissues, the cause of most cases of back pain remain elusive.

Vertebrae. The spine is a column of small bones, or vertebrae, that support the entire upper body. The column is grouped into three sections.

• The cervical (C) vertebrae are the seven spinal bones that support the neck.
• The thoracic (T) vertebrae are the twelve spinal bones that connect to the rib cage.
• The lumbar (L) vertebrae are the five lowest and largest bones of the spinal column. Most of the body's weight and stress falls on the lumbar vertebrae.

Click the icon to see an image of the spine.

Below the lumbar region is the sacrum, a shield-shaped bony structure that connects with the pelvis at the sacroiliac joints.

At the end of the sacrum are two to four tiny, partially fused vertebrae known as the coccyx or "tail bone."

Click the icon to see an image of the sacrum.

Each vertebra is designated by using a letter and number, which allows the doctor to determine where it is in the spine.

• The letter reflects the spinal region where the vertebra is located: C=cervical (neck region), T=thoracic (chest, or middle back, region), and L=lumbar (lower back).
• The number signifies the vertebra's place within that spinal region. The numbers start with 1 at the top of a region and count up as the vertebrae descend within the region. For example, C4 is the fourth bone down in the cervical region and T8 is the eighth thoracic vertebrae.

The Disks. Vertebrae in the spinal column are separated from each other by small cushions of cartilage known as intervertebral disks. Each disk is 80% water and contains two structures.

• Inside each disk is a jelly-like substance called the nucleus pulposus.
• The nucleus pulposus is surrounded by a tough, fibrous ring called the annulus.

This structure plus its heavily fluid-content makes the disk both elastic and strong. The disks have no blood supply of their own, however, but need to rely on nearby blood vessels to keep them nourished.

Click the icon to see an image of an intervertebral disk.

Processes. Each vertebra in the spine has a number of bony projections called processes. The spinal and transverse processes attach to the muscles in the back and act like little levers, allowing the spine to twist or bend. The particular processes form the joints between the vertebrae themselves, meeting together and interlocking at the zygapophysial joints (more commonly known as facet or z joints).

Spinal Canal. Each vertebra and its processes surround and protect an arch-shaped central opening. These arches, aligned to run down the spine, form the spinal canal, which encloses the spinal cord.

Click the icon to see an image of the vertebrae and spinal cord.

Spinal Cord. The spinal cord is the central trunk of nerves that connects the brain with the rest of the body. Each nerve root passes from the spinal column to other parts of the body through small openings bounded on one side by the disk and the other by the facets. When the spinal cord reaches the lumbar region, it splits into four bundled strands of nerve roots called the cauda equina (meaning horsetail in Latin).

Click the icon to see an image of the cauda equina.

Low Back Pain

Low back pain is usually defined as either acute or chronic.

• Acute low back pain lasts less than a month and is not caused by serious medical conditions. Most cases clear up in a few days without medical attention, although recurrence after a first attack is common.
• Chronic low back pain persists beyond 6 months. It constitutes only 1 - 5% of all low back pain cases.

The source of low back pain can result from many of the following problems.

• Injuries and small fractures in the spine
• Muscle spasm
• Rupture in the weakened disk (herniated disk)
• The facets (z-joints) can become misaligned or deteriorate
• The spinal canal itself can become narrowed (spinal stenosis)
• Scar tissue in the lower spine
• Tears in muscles and ligaments that support the back

Click the icon to see an image of the nucleus pulposus.

Sciatica

At some time, up to 40% of people experience pain called sciatica. This condition occurs when the sciatic nerve is trapped or inflamed.

The Sciatic Nerve. The sciatic nerve has an extensive pathway.

• It first branches from the nerve roots that descend off the lowest part of the spinal cord (in the lumbar and sacral areas). Each of the two branches of the sciatic nerve is about as wide as a thumb.
• Each branch of the nerve threads through the pelvis and deep into either side of the buttocks.
• The nerve branches then pass down each hip and along the back of each thigh to the foot.

Causes of Sciatica. A herniated disk pressing on the sciatic nerve is the most common cause of sciatica, although spinal stenosis or other vertebral abnormalities that press on the sciatic nerve can also cause pain.

Symptoms of Sciatica

Pain due to sciatica can vary widely. It may feel like a mild tingling, dull ache, or a burning sensation. In some cases, the pain is severe enough to cause immobility.

The pain most often occurs on one side. Some people experience sharp pain in one part of the leg or hip and numbness in other parts. The affected leg may feel weak.

The pain often starts slowly. Sciatica pain may get worse:

• At night
• After standing or sitting for long periods of time
• When sneezing, coughing, or laughing
• After bending backwards or walking more than 50 to 100 yards (particularly if it is caused by spinal stenosis)

Sciatica pain usually goes away within 6 weeks, unless there are serious underlying conditions. Pain that lasts longer than 30 days, or gets worse with sitting, coughing, sneezing, or straining may indicated a longer recovery.

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Review Date: 4/26/2006
Reviewed By: Harvey Simon, MD, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital