Osteonecrosis (also known as avascular necrosis) is defined as cell death in components of bone - the marrow fat and mineralized tissue. It can cause degeneration in hips, knees, ankles, shoulders, and even jaws.
Sometimes it is caused by long-term exposure to prednisone, but in a certain percentage of patients the doctors don't know why it happened. When someone has it in their hips, they often have to have a total hip replacement.