Doctor Thomas P. Sculco, Surgeon-in-Chief at the Hospital for Special Surgery in New York, notes the many causes of avascular necrosis what also admits that they still have much to learn about the causes.

Dr. Sculco For probably half of the patients who have it, we don't have a specific cause. Well Avascular Necrosis has many causes. It's associated with things like use of cortisone. Steroids. So patients who need to take steroids for inflammatory problems... rheumatoid arthritis, lupus, for example. Certain cancer conditions. People with sickle cell disease can get it. Trauma can cause it. An injury to the hip can cause it. Where the blood supply to the ball is interrupted as part of that injury, the bone will die and collapse as well. For probably half of the patients who have it, we don't have a specific cause.