My son, who is only in his early 20s, has been diagnosed with something called osteonecrosis of the hip. The doctor told him that part of his bone had died. What does that mean?

Osteonecrosis of the hip is a condition where part of the bone does not get the blood it needs for nourishment and the bone cells die. When this happens in the hip, it is the top or the head of the femur, or thigh bone, that is affected.

It is generally a disease that affects young people, but doctors don’t understand yet what causes it. They do believe that it can be caused by previous hip dislocations or injuries, alcohol consumption, high doses of corticosteroids, or a genetic history of similar diseases.