Knee necrosis degenerative

by Nathan Wei, MD, FACP, FACR

Nathan Wei is a nationally known board-certified rheumatologist and author of the Second Opinion Arthritis Treatment Kit. It's available exclusively at this website... not available in stores.

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Osteonecrosis of the knee refers to a condition where a piece of bone in the knee actually dies.

This condition is also referred to as osteochondritis dessicans or OCD. In OCD, a focal area of bone underneath cartilage undergoes necrosis (death). The overlying cartilage remains intact. As the necrotic bone dissolves, the cartilage loses its supporting structure. As a result, the bony fragment may become loose in the joint space.

The knee is involved about 75 percent of the time. The most common location is the non-weight-bearing medial femoral condyle (inside of the lower part of the femur) in 85 percent of cases of OCD of the knee. Twenty percent of cases affect both knees.

Genetic predisposition, loss of blood flow to the bone, repetitive trauma and abnormal bone formation have all been theorized as causes of OCD. It is believed to involve multiple factors, with repetitive shear and compressive forces playing a major role.

Many patients have no history of significant trauma. Because of the high incidence of bilateral involvement as well as involvement of non-weight-bearing areas, factors not associated with trauma may be involved. Loss of blood flow to bone due to blood vessel spasm, fat emboli, infection or blood clots, may play a role.

Most patients have pain related to activity and stiffness after periods of rest. Common complaints include a sensation of "catching" and "giving way," as well as the inability to fully extend the leg. Persistent or intermittent pain is poorly localized and worsens with weight bearing. On examination, effusions (fluid in the joint), crepitus (crunchiness), and joint line tenderness may be present.

OCD can be diagnosed on x-ray. If a lesion is noted, the other knee should also be examined.

All OCD seen on x-ray should be staged for stability with MRI. MRI has a 97 percent sensitivity for detecting unstable lesions. MRI is the most accurate non-invasive method for staging lesions and is important for clinical management. Stages I and II are stable lesions, while stages III and IV are unstable lesions in which the cartilage and bone are separated by synovial (joint) fluid.

Once staging has been completed, unstable lesions are managed surgically Conservative treatment of stable lesions is generally accepted.

Non-surgical management starts with rest. The goal of activity modification is to allow symptom-free activities of daily living. Physical therapy may be started, including stretching and range-of-motion exercises. Conditioning exercises and quadriceps strengthening may be helpful. Complete immobilization is discouraged since it leads to quadriceps atrophy and stiffness, which worsen prognosis. Persistent symptoms in a conservatively treated patient or the onset of joint catching or grinding indicate a loose body, and are an indication for arthroscopy.

In these cases, arthroscopy is important to evaluate the stability of the lesion. Depending on surgical findings, a loose body may be removed, a fragment excised, cartilage debrided or a lesion drilled to promote revascularization. Following surgery, range-of-motion exercises should be started early. Quadriceps strengthening may promote overall knee stability. Patients should be followed at three-month intervals with a clinical history and physical examination until symptoms resolve. Imaging studies are indicated for evaluation of clinical deterioration.

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