Ask the Expert: Avascular Necrosis
Q1. What is avascular necrosis?
Avascular necrosis (AVN) is a disorder of bone that results from vascular injury. Bone is nourished by microscopic blood vessels that do not have cross circulation. In the femoral head, or ball of the hip joint, small microscopic parallel vessels provide blood supply to the bone of the femoral head. If those vessels become clogged, the area of bone supplied by that vessel dies. It appears that large circulating fat complexes or blood clots are most likely responsible for creating these clogs. Generally, many of the vessels become clogged at the same time causing a large infarct of the femoral head bone. Once the bone dies, the body responds by removing the dead bone and replacing it with new living bone. This is a slow process that occurs over many months. The disorder resulting from AVN is caused by the fact that the dead bone is resorbed faster than the new bone is formed so the structure of the bone is weakened. The weakening can be so extensive that the bone of the femoral head cannot carry normal weight bearing loads and fractures leading to collapse of the structure of the normally round femoral head. When this occurs, the perfect fit between the ball and socket of the hip is lost leading to arthritis. The collapse itself can also be painful. When collapse occurs, hip replacement is usually needed.
Q2. What are some of the causes of avascular necrosis?
Avascular necrosis of the femoral head (AVN) is associated with use of certain drugs and with certain diseases. Alcoholism and prolonged use of steroid medications can lead to AVN. Sickle cell anemia causes avascular necrosis and hip disease is a common morbidity associated with this blood disease. Rheumatologic diseases, most notably systemic lupus erythematous and the immune vascular diseases, are frequently associated with the development of AVN. HIV infection and AIDS is currently one of the most commonly associated diseases with this disorder. Finally, patients who recipients of organ transplants frequently develop AVN which may be associated with the transplant itself as well as the anti-rejection medications that are required. In my own experience, many patients present with no obvious risk factor for development of the disorder. I refer to these cases as “idiopathic AVN”. In these cases, it is important to rule out other diagnoses that can present similarly. These include insufficiency fractures and transient osteoporosis. These disorders are similar but usually result from calcium and vitamin D deficiency or some other metabolically caused bone weakness. Whenever I suspect AVN, I have the patient undergo a thorough diagnostic work-up to detect any deficiency or other cause of bone weakness.
Q3. What should you do if you are diagnosed with avascular necrosis?
If you are diagnosed with AVN, seek out treatment from a specialist interested in the disease. Endocrinologists, rheumatologists and orthopedic surgeons are the subspecialties that usually treat this disorder. Treatment remains very controversial and traditional surgical procedures such as core decompression have been prescribed. In my view, there is almost no scientific support for surgical treatment of the early stages of this disorder. From our understanding of the pathologic process, core decompression and vascularized grafting procedures make little sense and published clinical reports provide little evidence of consistent success. We have had excellent and consistent success treating our patients with bisphosphonate drugs which inhibit bone resorption. These drugs are also used to treat osteoporosis. These agents inhibit the bone resorbing cells called osteoclasts but allow the bone forming cells, osteoblasts, to continue the process of new bone formation unopposed. We believe this treatment is successful because it stops the bone resorption that weakens the bone yet allows repair to continue unopposed. Patients should seek out specialists familiar with this approach.
Q4. What happens if you do not diagnose avascular necrosis early?
Unfortunately, AVN is relatively asymptomatic in the early stages. Pain in the affected hip often does not present until the bone structure is weakened and collapse has begun. As a result, many patients don’t present until the hip is already damaged. If you are diagnosed with AVN, treatment from a specialist should be sought immediately. If you are diagnosed with AVN in the early stages, AVN can be successfully treated with a bisphosphonate drug.
Q5. Is there any current research available for avascular necrosis?
There is a lot of research being performed to improve the treatment of AVN. Most research concerns better drug treatment. There are new antiresorptive drugs being developed as well as drugs which stimulate the bone forming cells. It would make sense that applying a drug that stimulates the ingrowth of new bone would add to the effect of slowing bone resoprtion in preserving the intact bone structure before collapse occurs. Modifications of surgical procedures are also being developed in which stem cell implants are placed into the necrotic area to speed repair. I see this as a promising method to address the intermediate stages where some damage to the structure of the femoral head has occurred but before the collapse is bad enough to result in hip arthritis requiring hip replacement. Finally, research is also being performed to assess the outcome of total hip replacement when it is needed. In the 1980’s when hip replacements were usually implanted with cement, the long term durability of the artificial hips was noted to be poor. However, modern implants seem to perform well for AVN and current research is being done to verify this.
Dr. Charles N. Cornell is a Joint Replacement Surgeon and Clinical Director of Orthopedic Surgery at Hospital for Special Surgery. Dr. Cornell is board certified by the American Board of Orthopedic Surgeons and was admitted as a Fellow to the American Academy of Orthopedic Surgeons as well as the American College of Surgeons. Dr. Cornell’s areas of expertise include avascular necrosis, fractures and trauma, and total joint replacement, care of arthritis and geriatric orthopedics.