More than 80 percent of Americans have back pain at some point in their lives.

For some, the pain is caused by inflammation of the sacroiliac joint that attaches the sacrum (the lowest part of the spine) to the pelvis. Often this condition is missed or inappropriately treated.

The disease is called sacroiliitis, and in people under age 60 an underlying medical condition is often present. These include inflammatory bowel disease (ulcerative colitis and Crohn's disease), psoriasis and a condition called ankylosing spondylitis.

Sacroiliitis can occur as a complication of infections in the heart, skin, joints or muscle. It can also follow an injury to the back and is more common in those who use illicit drugs. This form of back pain is a frequent complication of pregnancy as the pelvis stretches to accommodate childbirth. In older people the major cause is osteoarthritis and is similar to what occurs in many other joints.

In a typical patient, the symptoms can develop very suddenly with pain that is usually localized to one side of the back. The pain can be excruciating to the touch, and sometimes the patient will not allow close examination of the affected area. Here, any form of movement of the spine becomes very uncomfortable and painful.

Sometimes the pain is felt in the thigh and in the buttock. Pain in the joint leads to spasm of the muscles surrounding the joint that then, in turn, can cause compression of the sciatic nerve leading to pain that radiates down into the leg and on occasion numbness in the foot or leg. This can lead to limping.

The most severe forms of sacroiliitis are associated with the medical conditions listed above. There can be fever, eye inflammation, evidence of psoriasis (with an obvious skin rash) or inflammatory bowel disease that manifests with abdominal pain and diarrhea containing blood and mucus.

In the older patient, the pain can be quite severe, but is more often chronic, presenting with back pain that could have been present for weeks, months and even years. A unique feature of the pain is that it is made worse by sitting or lying down and is made better by standing up.

Quite frequently sacroiliac pain is mistaken for another cause of low back pain, such as a ruptured disk, collapsed vertebra, spinal stenosis or osteoarthritis of the joints in the spinal cord. Unless your doctor closely examines your back he may totally overlook the diagnosis. In those of us over the age of 60, the next step in order to make a diagnosis is an X-ray or MRI of the lower back.

At this age, abnormalities of the spine are very common, and therefore it is not surprising that the pain is ascribed to the wrong cause. Even if the sacroiliac joint is visualized, the X-ray is often normal. On occasion, surgery is done that, of course, will not be successful.

Because this condition is so frequently misdiagnosed, an empowered patient should always be aware of the sacroiliac joint as a cause of pain. A good way to pinpoint sacroiliac pain is to sit down and if the pain is on the right side of your back, place the ankle of your right leg on the thigh of your left leg. Then push your right knee downward. If this elicits pain the likely cause is the sacroiliac joint. Your doctor should also closely examine your back and press on the affected joint, which of course will lead to pain, and then, an accurate diagnosis.

Once sacroiliitis has been diagnosed, tests are needed to exclude illnesses associated with this problem, and if suspected, fluid can be removed from the joint to assure that it is not infected. If the cause is osteoarthritis, initial treatment should be with acetaminophen (Tylenol) and nonsteroidal anti-inflammatory drugs such as Aleve and Motrin.

If the pain is very severe, bed rest, muscle relaxants and a short course of steroids can be prescribed. Local injections with steroids or an anesthetic are often quite helpful, as is physical therapy, stretching and exercise.

Because failure to diagnose this condition is so common, it is important for every patient with back pain to be aware of sacroiliac pain and discuss it as a possibility with a doctor.

Dr. David Lipschitz is the author of the book "Breaking the Rules of Aging." To find out more about Dr. David Lipschitz and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate Web page at www.creators.com. More information is available at www.drdavidhealth.com.