Osteonecrosis of the Knee

Osteonecrosis of the knee (also known as avascular necrosis) is a painful condition that occurs when the blood supply to a section of bone in the femur (thighbone) or tibia (shinbone) is disrupted. Without adequate nourishment, the affected portion of bone dies and gradually collapses. As a result, the articular cartilage covering the bone also collapses, leading to disabling arthritis.

Osteonecrosis of the knee can affect anyone, but is more common in people over the age of 60. Women are three times more likely than men to develop the condition.

Osteonecrosis of the knee is most commonly seen in the femoral condyle, usually on the inner side of the knee (the medial femoral condyle). However, the condition can also occur on the outside of the knee (the lateral femoral condyle) or on top of the shin bone (the tibial joint surface), known as the tibial plateau.

Cause

It is not always known what causes the lack of blood supply, but a number of risk factors that make someone more likely to develop osteonecrosis have been identified:

Injury - A knee injury (such as a stress fracture or dislocation) combined with some type of trauma to the knee can damage blood vessels and reduce blood flow to the affected bone.
Oral corticosteroid medications - Many diseases, such as asthma and rheumatoid arthritis, are treated with oral steroid medications. Although it is not known exactly why these medications can lead to osteonecrosis, research shows that there is a connection between the disease and long-term steroid use. Steroid-induced osteonecrosis frequently affects multiple joints in the body, and can be seen in younger patients.
Medical conditions - Osteonecrosis of the knee is associated with medical conditions, such as obesity, sickle cell anaemia, and lupus (SLE).
Transplants - Organ transplantation, especially kidney transplant, is associated with osteonecrosis.
Excessive alcohol use - Overconsumption of alcohol over time can cause fatty deposits to form in and around the blood vessels as well as elevated cortisone levels, resulting in a decreased blood supply to the bone.

Symptoms

Osteonecrosis develops in stages. The first symptom is typically sudden onset of pain inside the knee, which may be triggered by a specific activity or minor injury. As the disease progresses, it becomes more difficult to stand and put weight on the affected knee, and moving the knee joint is painful. Other symptoms may include:

Swelling over the front and inside of the knee
Sensitivity to touch and pressure around the knee
Limited range of motion in the joint.

It may take from several months to over a year for the disease to progress. It is important to diagnose osteonecrosis early, because some studies show that early treatment is associated with better outcomes.

Stages of Osteonecrosis

There are four stages of development of osteonecrosis of the knee that can be distinguished by symptoms and X-rays:

Stage I: The symptoms are most intense in the earliest stage, and may subside in 6 to 8 weeks. In this stage, the X-rays are normal; a positive bone scan or MRI may be required to make the diagnosis. Treatment for Stage I disease is non-surgical, focusing on pain relief and protected weight-bearing.
Stage II: This stage follows in a couple of months and the X-rays will show flattened edges of the once rounded thighbones. An MRI, CT, or bone scan may be utilized to diagnose the condition.
Stage III: At this stage the condition becomes clearly visible on X-rays and no other diagnostic tests are required. At this point, the bone segment itself begins to decay away and cartilage covering the bone avascular area loosens and may displace. Surgical treatments are considered at this stage.
Stage IV: In this stage, the bone collapses, joint space narrows, bone spurs form, and the articular cartilage surface is destroyed (bone on bone arthritis). The only treatment at this point is joint replacement surgery.

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Diagnosis

A general health and medical history is obtained, and a careful knee examination undertaken. Imaging studies will be used to confirm the diagnosis, including:

X-rays - X-rays are used to look for changes that occur in bone, especially in the later stages of osteonecrosis. In the early stages of the disease, X-rays often may appear normal.
Magnetic resonance imaging (MRI) scans - Early changes in the bone that may not show up on an x-ray can be detected on an MRI. These scans are used to evaluate how much of the bone has been affected by the disease. An MRI scan may also show early osteonecrosis that has yet to cause symptoms (for example--osteonecrosis that may be developing in the opposite knee joint).

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This MRI scan shows osteonecrosis in the medial femoral condyle. An MRI is often used to detect early symptoms of the disease.

Bone scans - In some cases, a bone scan may be ordered. During this test, a very small amount of radioactive dye is injected into your vein. Osteonecrosis may cause an increased uptake of the radioactive material in the bone due to bone activity.

Treatment

Treatment for osteonecrosis depends on a number of factors, including:

The stage of the disease
The amount of bone affected
The underlying cause of the disease.

Nonsurgical Treatment

In the early stages of the disease, non-surgical treatment options include anti-inflammatory medications, braces, limited or reduced weight-bearing on the affected leg, strengthening exercises for thigh muscles, and activity modification may be needed to reduce the knee pain.

Surgical Treatment

If a large portion of the bone surface is affected, or if your pain does not improve with nonsurgical treatment, surgery may be recommended. There are several different procedures used:

Arthroscopic debridement and microfracture - In debridement (cleansing), a small arthroscopic camera and miniature surgical instruments are used to remove loose bits of bone or damaged cartilage from inside the joint space. For small lesions, multiple drill holes or microfractures may be made in the underlying bone to help promote blood flow and induce a healing reaction
Core decompression - This procedure involves drilling one larger hole or several smaller holes into the bone to relieve pressure on the bone surface and create channels for new blood vessels to nourish the affected areas of the knee. When osteonecrosis of the knee is diagnosed early, core decompression is often successful in preventing collapse of the bone and the development of arthritis.
Osteochondral (bone and cartilage) grafting - Core decompression is often combined with bone and cartilage grafting to help regenerate healthy bone and support cartilage at the knee joint. A bone graft is healthy bone tissue that is transplanted to an area of the body where it is needed. The tissue may be taken from another bone in your body (autograft) or from a donor (allograft).
Autologous chondrocyte implantation (ACI) - This is a two-stage procedure. In the first stage, an arthroscopic procedure is performed to remove a small number of cartilage-producing cells (chondrocytes) from your knee. These chondrocytes are sent to a lab where they are cultured (multiplied) for up to 6 weeks to increase the number of cells. In the second stage, the cultured/grown chondrocytes are implanted into the area of your knee with cartilage loss. The cells then grow in the joint, replacing the damaged cartilage with healthy cartilage.
Osteotomy - In an osteotomy, the tibia (shinbone) or femur (thighbone) is cut and realigned to help shift your weight off the damaged area of the knee. Shifting your weight off the damaged side of the joint will help relieve pain and improve function.
Total or unicompartmental (partial) knee replacement - If the disease has advanced to the point where the bone has already collapsed or progressed to arthritis, you may need surgery to replace the damaged parts of your knee. In knee replacement, the damaged bone and cartilage is removed and replaced with artificial metal and plastic joint surfaces components to restore the function of your knee.