Osteonecrosis.

Figure 1 - Normal hip joint versus hip joint with osteonecrosis

Causes and effects

What causes osteonecrosis?

For unknown reasons, the blood flow slowly decreases over time, creating pressure within the bone. Eventually, the bone begins to develop tiny breaks and collapse (see figure 1).

An injury such as a fracture (broken bone) or a hip dislocation can cause a loss of blood supply to the area. Alcoholism, an infection, sickle cell disease, pancreatitis (inflammation of the pancreas), or an organ transplant may cause a loss of blood supply to the area.

Incidence and risk factors

The following groups of people are most at risk for developing this condition:

• People who have a rheumatic disease such as rheumatoid arthritis or systemic lupus erythematosus;
• People who are taking high doses of steroids (cortisone-like drugs);
• Alcoholics;
• Professional scuba divers who have the bends (a condition brought on by the rapid reduction of air pressure).

Osteonecrosis of the hip occurs most often between the ages of 30 and 50, and is slightly more frequent in men than in women. Osteonecrosis of the knee occurs most often between the ages of 50 and 60, or much earlier if other risk factors are present. Osteonecrosis of the knee occurs three to four times more often in women than in men. Osteonecrosis often involves more than one joint at a time.

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