Narrator: This is Science Today. The exact cause of bone death, or osteonecrosis, is still relatively unknown. But Dr. Jay Lieberman, an assistant professor of orthopedic surgery at UCLA, says of the twenty thousand new cases of hip osteonecrosis each year, certain risk factors - such as trauma, the use of steroids or alcohol abuse - seem to be associated.

Lieberman: There are other diseases that are associated with it also - lupus, rheumatoid arthritis, Crohn's Disease. Some people think there's a high risk if you have diabetes mellitus.

Narrator: Lieberman says not knowing the exact cause and natural history of the disease is one of the major problems in treating osteonecrosis.

Lieberman: And what I mean by natural history is, we don't know how many people are walking around with osteonecrosis of the hip who never become symptomatic. Even when we talk about patients who take steroids or patients who have problems with alcohol abuse or these other diseases - the vast majority of these patients never develop osteonecrosis. So in general, we most often pick it up because a patient starts complaining of hip pain.

Narrator: For Science Today, I'm Larissa Branin.