Read about his disease here:
http://en.wikipedia.org/wiki/Avascular_necrosis
Here are the causes:
There are many theories about what causes avascular necrosis. Proposed risk factors include, chemotherapy,alcoholism,[4] excessive steroid use,[5] post trauma,[6][7] caisson disease (decompression sickness),[8][9] vascular compression,[10] hypertension, vasculitis, arterial embolism and thrombosis, damage from radiation, bisphosphonates(particularly the mandible),[11] sickle cell anaemia,[12] Gaucher's Disease,[13] and deep diving.[14] In some cases it isidiopathic (no cause is found).[
Other key points:
Avascular necrosis is especially common in the hip joint. A variety of methods are now used to treat avascular necrosis,[21] the most common being the total hip replacement, or THR. However, THRs have a number of downsides including long recovery times and short life spans. THRs are an effective means of treatment in the geriatric population; however, doctors shy away from using them in younger patients due to the reasons above. A new, more promising treatment is hip resurfacing or metal on metal (MOM) resurfacing. It is a form of a THR, however in this procedure, only the head of the femur is removed as opposed to a THR in which the entire neck is removed. MOM resurfacing is still experimental in America but has been endorsed in Great Britain as an excellent alternative to a THR.
Other treatments include core decompression, where internal bone pressure is relieved by drilling a hole into the bone, and a living bone chip and an electrical device to stimulate new vascular growth are implanted; and the free vascular fibular graft (FVFG), in which a portion of the fibula, along with its blood supply, is removed and transplanted into the femoral head.[27]
Prognosis
The amount of disability that results from avascular necrosis depends on what part of the bone is affected, how large an area is involved, and how effectively the bone rebuilds itself. The process of bone rebuilding takes place after an injury as well as during normal growth.[25] Normally, bone continuously breaks down and rebuilds—old bone is reabsorbed and replaced with new bone. The process keeps the skeleton strong and helps it to maintain a balance of minerals.[25] In the course of avascular necrosis, however, the healing process is usually ineffective and the bone tissues break down faster than the body can repair them. If left untreated, the disease progresses, the bone collapses,[2] and the joint surface breaks down,[15] leading to pain and arthritis.[
And you are talking about 40 HR? This is what ended Bo Jackson's career.
