Three Types of Knee OsteonecrosisOsteonecrosis of the knee is a condition where a portion of the femur (thigh bone) loses its blood supply, dies, and collapses. Another term used for osteonecrosis is avascular necrosis. The term avascular means that a loss of blood supply to the area is the cause of the problem and necrosis means death.
This condition can affect other joints as well (e.g., hip, shoulder). When the knee is involved, the problem usually occurs on the medial femoral condyle -- that's the large round knob of bone (called a condyle) at the bottom of the femur. The medial condyle is affected most often. That's the side closest to the other knee.
There are three separate types of knee osteonecrosis. The first type is called secondary osteonecrosis. It means the bone necrosis is caused by some other health condition. People who abuse alcohol or who are taking high amounts of corticosteroids are at risk for developing secondary osteonecrosis.
Organ transplant recipients, cancer patients, and patients with chronic rheumatoid arthritis receiving immunosuppressants for their inflammatory disease are also at increased risk of developing secondary knee osteonecrosis.
There are some theories about why these risk factors trigger osteonecrosis but no one knows for sure the actual mechanism between cause and effect. It's likely that there is more than one factor at play. The bottom line is that something causes loss of blood to the bone and decreased blood circulation within the bone.
There is one form of secondary osteonecrosis that we have a greater knowledge and understanding of and that's the kind associated with sickle cell disease (a blood clotting disorder). With sickle cell disease, red blood cells curl up and clump together creating a block inside the blood vessels. That blockage prevents blood from getting to the bone where it's needed to supply the bone with oxygen and nutrients. the result can be osteonecrosis.
A second type of osteonecrosis is spontaneous. As the name implies, this type develops suddenly without warning. Most of the time only one knee is affected. Age may be a factor as most cases of spontaneous osteonecrosis occur in adults aged 50 and older.
Some experts suspect earlier knee trauma as the true underlying cause of spontaneous osteonecrosis. For example, mini-fracture of the layer of bone just under the knee cartilage (called the subchondral layer) may later become osteonecrotic.
Separately, there has been some suspicion that osteopenia (decreased bone density prior to developing osteoporosis) is also part of the initial process in spontaneous osteonecrosis. If it turns out that either of these conditions leads to osteonecrosis then this type of osteonecrosis won't be called spontaneous any longer.
The third type of osteonecrosis of the knee is called postarthroscopic osteonecrosis. As the name suggests, this type develops after a patient has had arthroscopic knee surgery for some other problem.
Like the two other types of osteonecrosis, the actual reason why this complication develops remains largely a point of speculation and debate. It may be the type of instruments used during the surgery. Or it could be some type of unseen damage occurs in the meniscus or subchondral bone (remember, that's the first layer of bone just under the joint cartilage).
No matter what causes knee osteonecrosis, the goal of treatment is always the same -- prevent bone destruction and collapse. The patient is given medications for pain and inflammation and put on weight-bearing restrictions. Early diagnosis is good but even with early treatment, many people end up with severe bone destruction that is widespread (i.e., not contained in one or two small places).
Spontaneous osteonecrosis is the most likely to respond to conservative (nonoperative) treatment. If the lesion does not improve and get smaller with nonsurgical management in the first three months after diagnosis, then the surgeon may scrape the bone clean of any dead cells and give the joint a chance to heal. Transferring bone cells from a healthy part of the joint to the osteonecrotic section is another treatment option for spontaneous osteonecrosis. This procedure is called mosaicplasty.
In the case of spontaneous osteonecrosis, a unilateral implant (replaces just one side of the joint) is possible because only one side of the joint is affected. A total knee replacement may be more appropriate when severe osteoarthritis has developed on both sides of the knee.
Secondary osteonecrosis doesn't seem to respond well to conservative (nonsurgical) care. Of the studies that have been done, 80 per cent of the patients went on to develop advanced stages of osteonecrosis.
Therefore, surgery is often recommended for secondary osteonecrosis in order to preserve the joint and prevent the need for joint replacement. The surgeon may remove the dead bone and replace it with bone graft material from a bone bank. This method may work well when there is only one small lesion. But there aren't enough evidence-based studies to show this approach is best or prove there is a better way to manage the condition.
As for the treatment of postarthroscopic osteonecrosis, there is good news and bad news. The good news is that this type of osteonecrosis is very rare. The bad news is that with so few patients involved, there are no large studies of the problem. So, an evidence-based approach is missing.
Surgeons are currently relying on the results of several small studies to guide treatment. Conservative care can be tried at first. If that fails, then joint-preserving techniques may be attempted (bone grafting, microdrilling, mosaicplasty). As with spontaneous and secondary osteonecrosis, if the condition deteriorates and there is severe joint destruction, then joint replacement (unilateral or total knee) is the next step.
In summary, our knowledge of the risk factors, cause, pathology, and successful treatment approaches for each type of osteonecrosis of the knee is fairly limited. It is a progressive disease that can lead to arthritis and disability. More research is needed to understand the true nature of this disease and find ways to either prevent it from happening in the first place or treat it successfully when it does occur.
Michael A. Mont, MD, et al. Osteonecrosis of the Knee and Related Conditions. In Journal of the American Academy of Orthopaedic Surgeons. August 2011. Vol. 19. No. 8. Pp. 482-494.
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