What is osteonecrosis? The term literally means death of bone (osteo=bone, necrosis=death). It has been known by a number of other names including ischemic necrosis of bone, aseptic necrosis and AVN (standing for avascular necrosis). AVN has been quite popular in its use because it is shorter to say and write. More recently the term ON has been adopted.

ON can affect virtually any bone, but for practical purposes most cases involve only the hip, knee, shoulder or ankle joints in decreasing order of frequency. In fact, ON of the hip accounts for more than 90% of the cases.

There are two major forms of ON, post-traumatic and non-traumatic. Minor trauma is not believed to cause ON. Even major injury does not often result in ON. Certain kinds of fracture, where the blood vessels to part of the bone have been physically damaged, may result in ON.

Non-traumatic ON has been associated with a wide variety of diseases including gout, lupus, sickle cell disease, kidney or liver disease, and clotting disorders. In addition, high dosage steroid (cortisone) use is sometimes associated with ON, as well as high alcohol consumpution. Finally, as many as 30% of all patients with osteonecrosis are otherwise completely healthy with no associated risk factor. This is called "idiopathic," a medical term meaning "of unknown cause."

Who's at Risk?

If a person is completely healthy, the risk of getting osteonecrosis is quite small, probably less than one in 100,000. Another way to understand this is that most of the people who get ON probably have an underlying health problem. Children, as young as 4 and extending to the teens, get a form of ON which is called Legg-Calve-Perthe's disease (Perthe's for short) after the doctors who first described it. Most patients are between 30 and 50 with an average age of 38. Patients over the age of 50 are likely to have developed ON either by a fracture of the hip or more rarely in association with disease of the major blood vessels to the lower leg. Although the specific cause of the bone death is not precisely known except in the case of fracture, a number of conditions have been associated with ON. The most common includes a history of high dose steroid treatment for some medical condition (including Lupus, chronic lung disease, an organ transplant, etc). Low dose steroids (cortisone, prednisone, etc) commonly used for bee stings, poison ivy and acute allergies are not thought to cause ON. The next most common associated condition is a history of alcohol intake. The higher the intake the higher the risk.

The mechanisms by which these two risk factors (alcohol and steroids) cause ON are not well understood. The third most common group, are those patients who have no risk factors at all, and these patients are a true medical mystery. No matter what the cause, the symptoms and course of the disorder are remarkably similar.

First Symptoms

Unfortunately many patients with ON have had the disease for quite some time before symptoms are present. The initial symptoms are usually pain or aching in the affected joint with activity, which subsides after the activity has stopped. Symptoms usually begin slowly and may initially be intermittent. As the disease progresses, the pain increases and is associated with stiffness. Limping becomes common. In the hip, the most common joint affected, the pain is usually felt in the groin.

Diagnosis

The principle diagnostic tool is the x-ray. By the time that most patients have significant symptoms, the disease is advanced enough to be seen on standard x-rays. In most cases the x-ray will show the area of bone that is involved. However, the very earliest stages of the ON cannot be seen on a regular x-ray. A widely used and relatively new tool is called an MRI which stands for Magnetic Resonance Imaging. These special images are able to detect tissue changes that may not be seen on plain x-ray. Occasionally, your doctor may order a CAT scan which is a special series of x-rays, interpreted by the computer to show the three dimensional structure of the bone. Any of these tests will help the doctor to determine how advanced the disease is in your case.

Prevention

There are no known effective prevention measures. However steroids should only be taken as necessary and alcohol consumption should always be in moderation. Some experimental drug protocols are being evaluated which may have a place in treatment or prevention in the future.

Introduction

The concept of Risk/Benefit Ratio  Before entering into a description of some of the treatments available for ON, it is important that this concept be understood. Any surgical procedure has a certain element of risk involved. Even no treatment at all has the risk that the disease will progress, so doing nothing is not risk free. Some procedures may have a lower likelihood of success but may have a very little downside risk. Other procedures may have a higher degree of success, but also have a higher degree of risk. The physician must work with the patient in assessing all the factors that evaluate both risk and benefit for the patient in their particular circumstance. What is right for one patient may be absolutely wrong for another. This is particularly true for ON because each patient presents with a unique set of factors (age, associated disease, specific joint(s) involved, extent and progression of disease). Any treatment needs to be determined between you and your treating physician.

Extent of disease  The femoral head is the most frequent bone involved and will be used for this discussion. It is rare for the entire weight-bearing surface of the femoral head to be involved. However if more than half of the surface is involved, treatments designed to preserve the femoral head have a much lower chance of success.

Progression of the disease  In the earliest stage of the disease. It cannot be seen by a normal x-ray. Diagnosis is by MRI. Once it can be seen on x-ray, it is not actually the dead bone that can be seen but the response of the living bone to the area of necrosis. The advanced stages begin when the dead bone starts to fail mechanically through a process of microfractures of the bone. Eventually, this will result in damage to the other side of the joint, and need for a total joint replacement.

The greater the extent of the disease and the more advanced the progression, the less likely that the joint can be saved. Fortunately, joint replacement procedures today are highly successful, even in the relatively young patients affected by ON. It is always the physicians desire to preserve the normal joint whenever possible. Unfortunately many patients present with advanced, extensive disease.

Non-Surgical Treatement

Protected weight bearing  Crutches or a walker are very useful in alleviating the pain associated with ON. They can also be useful in protecting the joint between the time of diagnosis and scheduling of elective surgery. They may also play a role in limiting progression while associated medical conditions are managed. However, protected weight bearing alone is never adequate treatment for ON and will not result in cure of the condition, no matter how long it is maintained. Rarely, an associated medical condition may result in a patient not being able to have surgery. In this case, protected weight bearing may be an effective long-term solution for pain control.

Surgical Treatment

Core Decompression  This is a simple surgical procedure, which involves taking a plug of bone out of the involved area. It is applicable for mild to moderate degree of involvement that has not yet progressed to collapse. Because this involves creating a hole in the bone, six weeks of protected weight bearing is necessary to avoid fracture through the hole, one of the complications of the procedure. There is some controversy about this procedure with a few series that have been reported showing generally poor results. However, in centers that do this procedure frequently, most series have reported good results in the appropriate cases.

Bone Grafting  When a section of the bone has died, as is the case in ON, for some reason it doesn't seem to heal. One of the ways that can cause the bone to heal is to surgically remove the dead bone and fill the empty space with bone graft that is either taken from the patient or from the bone bank. The success of this approach depends upon the quantity of bone that has died. Another problem is that during the healing process, which can be very long (6-12 months) the patient must be on weight-bearing restriction.

Vascularized Bone Grafting  Regular bone graft, whether from the bone bank or from the patient is itself dead bone. It serves as a scaffold for the body to build new bone around but the body also has to grow a new blood supply. For this procedure, a bone with its blood vessels is taken from the patient and hooked up to blood vessels near the hip. The dead bone is removed from the femoral head and replaced with the grafted bone that carries with it it's own blood supply. The advantage of this approach is that the body doesn't have to rebuild a new blood supply and the bone graft retains its physical and mechanical properties.

Healing and complete filling of the defect still has to take place, during which time crutches or a walker has to be used. The disadvantage also is that a substantial piece of bone has to be taken from the lower leg (the fibula, the smaller bone of the lower leg below the knee). Some patients will develop symptoms in the area from which the bone graft is taken. The operation also takes several hours and requires a team experienced in these techniques.

Osteotomy  Usually it is the main weight-bearing area of the bone that is involved with ON. In some cases the bone can be cut below the area of involvement and rotated or turned so that another portion of the bone that is not involved in the ON can become the new weight-bearing area. These operations are not very common anymore, but may apply to special cases.

Femoral Head Resurfacing  Initially only the femoral head is involved, not the socket of the hip joint. FHR involves implanting a metal hemisphere over the femoral head, which exactly matches the size of the original femoral head. This is similar to capping a tooth when the root is still good, as opposed to pulling the tooth and putting in a false tooth. It is known that over a period of many years, the metal head will gradually wear out the socket and will need to be converted to a total hip replacement. This procedure is designed to "buy time" for the younger individual whose extent of disease or degree of progression is such that one of the preservative procedures listed above cannot be performed. Most patients with ON are under 50. It is generally believed that total hip replacement today will not last the 30+ most of these patients will require. Therefore, if 2 procedures are likely to be necessary, it is important that the first procedure does not make the second procedure more difficult or less likely to succeed. A THR following a failed femoral head resurfacing is more likely to be successful than revision THR that follows a failed primary THR.

Femoral Head Replacement  This is basically half a total hip replacement. All comments about femoral head resurfacing apply to femoral head replacement. However, because a femoral head replacement also puts a stem inside the femoral bone (the femoral shaft) it is a little more extensive than the resurfacing procedure. If it needs to be revised, it is a little more difficult to convert to a total hip replacement than the resurfacing procedure.

Total Hip Replacement  When the ON is advanced to the point that there is involvement of the socket as well, then the only thing that will be effective is either a hip fusion (making the hip completely stiff) or a total hip replacement. Total hip replacement is one of the most successful surgical procedures ever devised.  Success rates are usually above 95%! The problem with total hip replacements for patients with ON is that it is not uncommon for the patient to have a life expectancy of more than 40 or even 50 years. With current technology we don't think that it is likely that a total hip replacement will last that long. For this reason, many physicians will want to try some procedure to put off total hip replacement for a few years even when it is known that that procedure will not in itself be successful forever. If your disease is advanced, and/or extensive, then THR may be the only thing that makes sense.

Future Directions

Although there are several hundred thousand patients alive in the U.S. with ON, this is largely due to the fact that the average patient will live 30-40 years. With only 15-20,000 new cases each year (this is about the same number of practicing orthopedic surgeons in the U.S.).