Section 5. Bone, Joint, and Muscle Disorders

Chapter 64. Avascular Necrosis of the Bone

Topic: Avascular Necrosis of the Bone

Avascular Necrosis of the Bone

Avascular necrosis of the bone (osteonecrosis, aseptic necrosis, osteochondritis dissecans) is the death of bone tissue due to an impaired blood supply.

Avascular necrosis most commonly occurs in people between the ages of 30 and 60. It most commonly affects the thigh bone (femur) at the hip (the head of the femur). Often, both hips are affected. In people older than 50, avascular necrosis is usually caused by a hip fracture but may be caused by a disease that blocks the small blood vessels that supply to the ends of the long bones. For instance, emboli consisting of fatty material may block these blood vessels in people with alcohol-induced liver damage. The thigh bone at the knee is also commonly affected; the arm bone at the shoulder is sometimes affected.

Doctors classify avascular necrosis according to whether or not it is caused by an injury. Only serious injuries can cause the disorder; minor ones do not. Commonly, displaced fractures or dislocations, in which the blood vessels to part of the bone have been torn or physically damaged, are the types of injuries that can cause avascular necrosis. Nontraumatic causes include alcohol abuse, high doses of corticosteroids (especially when given for prolonged periods of time), decompression sickness (which occurs in divers who surface too quickly (see Section 24, Chapter 295)), and sickle cell disease (see Section 14, Chapter 172). Less commonly, Gaucher's disease, tumors (for example, lymphomas), and radiation therapy can cause avascular necrosis. Certain blood clotting disorders can also cause it. In about 25% of people with the disorder, the cause is unknown. Avascular necrosis of the knee can occur spontaneously, primarily in women older than 55 who have no risk factors for the disorder. Some experts dispute whether spontaneous avascular necrosis of the knee is really avascular necrosis or some other unknown disorder.

See the sidebar Causes of Avascular Necrosis.

Symptoms

In some people, severe incapacitating pain starts suddenly--these people can often remember the precise day and hour when the pain started. This sudden onset of pain probably occurs when the blood supply is cut off. However, most people have avascular necrosis for some time before symptoms appear. Pain occurs when the dead bone finally collapses. Such pain is brought on by standing, walking, or moving the affected bone and generally improves when the person is resting.

In avascular necrosis of the hip, groin pain may extend down the front and inner portions of the thigh or be felt in the buttocks. The person limps, trying to minimize all hip movements. As the disorder progresses, more and more tiny fractures of the hip occur, and the bone eventually collapses. The pain also increases, and the hip joint feels stiff and loses some of its range of motion.

Avascular necrosis of the knee often begins suddenly as severe constant pain and tenderness, usually over the inner part of the knee. In about one third of people, the knee joint is swollen because it contains excess fluid.

Avascular necrosis of the shoulder may produce mild or temporary symptoms and may be hardly noticeable. The person usually minimizes shoulder movements to avoid pain.

Diagnosis

Because avascular necrosis is often painless at first, it is often not diagnosed in its early stages. When avascular necrosis occurs because of a serious injury, the disorder cannot be detected microscopically for days to weeks and cannot be detected on x-ray until some months later.

Magnetic resonance imaging (MRI) is the best test to detect avascular necrosis early, so that complications (such as collapse of the head of the femur) can be avoided. X-rays or computed tomography (CT) is used to determine whether the bone has collapsed, how advanced the disorder is, and whether the person has osteoarthritis involving the unaffected side of the joint.

Treatment

Simple treatment measures include use of nonsteroidal anti-inflammatory drugs (NSAIDs) or other analgesics for pain (see Section 6, Chapter 78), and avoidance of weight-bearing or strain on affected bones and joints. Often, these treatments must be maintained for 6 months or more. Exercises to increase a joint's range of motion are useful. However, these treatments are rarely adequate by themselves and do not cure the disorder.

The simplest surgical procedure is called core decompression, which involves taking a plug of bone out of the involved area. This procedure is very effective for early disease that has not yet progressed to bone collapse; it may even prevent collapse. Core decompression may also be used for someone younger than 50 whose bone has collapsed; this treatment may delay the need for a total joint replacement by several years because the person's pain is relieved or decreased. The procedure takes less than an hour to perform. The person must use crutches for 4 to 6 weeks afterward.

Another procedure is bone grafting. For avascular necrosis at the hip, bone grafting consists of taking bone tissue with the blood vessels intact from elsewhere in the body and attaching the bone tissue and blood vessels near the hip. The graft serves as an infrastructure from which the body forms new bone; however, for the operation to be successful, the body also has to form a new blood supply. This operation takes several hours to perform. The person must spend several months on crutches afterward.

If core decompression or bone grafting is performed before the bone collapses, it can help to prevent serious joint damage, particularly when the hip or knee is affected. Early surgery is less often needed when the shoulder is affected because the shoulder does not bear weight and often does well without surgery.

A procedure by which the bone is cut (called osteotomy) is used to delay the need for joint replacement in people who are younger than 50, in whom collapse of the bone has occurred. People who undergo an osteotomy are those who are not eligible for core decompression or bone grafting because too much damage has occurred. Usually, the location of the avascular necrosis is in a weight-bearing area of the bone. In some cases, the bone can be cut below the area that is involved and rotated or turned so that an uninvolved portion of the bone can become the new weight-bearing area.

A total joint replacement (see Section 5, Chapter 62) is the only effective procedure when avascular necrosis has caused significant osteoarthritis on the other side of the joint. Although the success rate for this procedure is higher than 95%, doctors must carefully consider the decision to recommend a joint replacement because artificial joints do not last forever and in young people may have to be replaced at some later time. Therefore, for some younger people, many surgeons use a procedure called femoral head resurfacing if the socket of the hip joint is not involved. This procedure involves implanting a metal cap over the femoral head (similar to capping a tooth rather than pulling the tooth and putting in a false tooth). Some people undergo the femoral head resurfacing procedure followed later by total hip replacement.