Features February 2004 Issue

Knee Osteonecrosis: When Bone Death Knocks On Your Door

Early diagnosis can disarm this doomful-sounding joint disorder.

A diagnosis of knee osteonecrosis could understandably have you worried sick, for the term—literally meaning “bone death”—is ominous indeed. You may feel that you’ve been unfairly singled out to suffer a rare condition, that the disease is inevitably progressive, and that you are ultimately doomed.

A Brighter View
None of this is true. First, it is not a rare condition. It is estimated that thousands of Americans are diagnosed with osteonecrosis annually. Dr. Kenneth Marks—orthopaedic surgeon at The Cleveland Clinic and AA’s Editor-in-Chief—believes that the incidence of the disease is probably even higher than that, noting: “Many cases are not even recognized because they are totally without symptoms.”

Secondly, the bone death that occurs in knee osteonecrosis is most often confined to a specific area, usually small in size, and does not fan out to other joints or bones. In most cases, disability resulting from the condition can be remedied if therapy is begun early enough. And even advanced cases can respond favorably to surgical treatment.

What Goes Awry?
Knee osteonecrosis (also called avascular necrosis) occurs when a segment at the end of one of the large bones that meet in the joint—either the thigh bone (femur) or shin bone (tibia)—loses its blood supply. This decreased circulation causes cells in the bone and bone marrow to die. Small lesions may heal without permanent damage to the joint, but large lesions may progress quickly to severe knee osteoarthritis and the eventual collapse of the joint.

In the knee, the knobby protrusion (condyle) on the inner part of the joint (the medial femoral condyle) is most often affected. In some cases, however, the interruption of blood flow can affect the outside of the joint (the lateral femoral condyle) or the flat top of the tibia (tibial plateau).

Possible Causes
The exact cause of osteonecrosis is unknown. However, experts generally agree that it can stem either from trauma—such as a fractured knee—that interferes with blood supply to the joint or as a consequence of conditions such as obesity, anemia, and lupus.

“Another likely cause is alcoholism,” says Dr. Marks. “People who drink heavily on a daily basis are particularly susceptible to osteonecrosis. So are people who are being treated with high doses of corticosteroid drugs.” The disease is known to affect three times as many women as men and to be most prevalent in people over the age of 60.

Early Warnings
Although a small patch of necrotic bone tissue may yield no symptoms, Dr. Marks cites “moderate to severe pain” as the most frequent early sign of the disease. The pain may be triggered by a specific activity or injury and may limit motion during the day, yet be most intense at night when the patient is trying to sleep.

Diagnosis And Treatment
Diagnosis is achieved through physical examination and review of the patient’s medical history plus the use of X-rays or magnetic resonance imaging (MRI). “An X-ray may be totally negative,” says Dr. Marks, “but you can see necrosis very clearly on a bone scan or MRI.”

Early diagnosis is critically important, he notes. “We will usually put the patient on crutches or a walker right away to reduce the load on the knee,” says Dr. Marks. “In the majority of cases, the pain will subside and the patient will have a functionally normal knee from then on.”

But if diagnosis is delayed, he adds, the area of necrotic tissue may enlarge, the joint will be become unable to bear weight, and the bone may collapse. “This can occur in a very short time,” he notes, “months, not years.”

Surgical Options
Several surgical methods are available to treat advanced osteonecrosis. One is osteotomy, in which dead tissue is removed. Another is core decompression, in which holes are drilled into the affected area to allow increased blood flow into it. The most successful option, says Dr. Marks, is total knee replacement, which is the treatment of choice in 10 percent to 15 percent of cases.