Joint, Ligament and Muscle Disorders

Knee Osteonecrosis

Anatomy

The knee is the largest joint in your body and one of the most complex. Three bones meet to form the knee joint: the thighbone (femur), shinbone (tibia), and kneecap (patella). The ends of these bones, where they touch, are covered with articular cartilage, a smooth substance that protects the bones and enables them to move easily. The kneecap sits in front of the joint to provide some protection.

Knee ligaments connect the thighbone to the lower leg. The four primary ligaments in the knee act like strong ropes, holding the bones together and keeping the knee stable. Between the femur and tibia are C-shaped wedges, called menisci, that act as shock absorbers to cushion the joint. Large ligaments holding the femur and tibia together provide stability and long thigh muscles give the knee strength. All remaining surfaces of the knee are covered by a thin lining, or synovial membrane, that releases fluid to lubricate the cartilage, reducing friction to nearly zero in a healthy knee. Normally, these components work together in harmony, but when disrupted by disease or injury the result can be pain, muscle weakness and reduced function.

Description

Osteonecrosis, which literally means "bone death," is a painful condition that develops when a segment of bone loses its blood supply and begins to die. Osteonecrosis of the knee most often occurs in the knobby portion of the thighbone, on the inside of the knee (medial femoral condyle). However, it may also occur on the outside of the knee (lateral femoral condyle) or on the flat top of the lower leg bone (tibial plateau).

A relatively common cause of knee pain in older women, osteonecrosis of the knee affects more than 3 times as many women as men, most over the age of 60 years. Early diagnosis and treatment is very important or the condition can develop into severe osteoarthritis.

Cause

The exact cause of the osteonecrosis of the knee is not yet known. However, some have theorized that it may be the result of a stress fracture combined with a specific activity or trauma that alters blood supply to the bone, or a build-up of fluid within the bone that puts pressure on blood vessels and diminishes circulation. Osteonecrosis of the knee has been associated with specific conditions and treatments, such as obesity, sickle cell anemia, lupus, kidney transplants, and steroid therapy. Steroid-induced osteonecrosis frequently affects multiple joints and is usually seen in young patients.

Symptoms

The following are among initial symptoms experienced by those with osteonecrosis of the knee:

• Sudden pain on the inside of the knee, possibly triggered by a specific activity or minor injury
• Increased pain at night and with activity
• Swelling over the front and inside of the knee
• Heightened sensitivity to touch in the area
• Limited motion due to pain

Diagnosis

By reviewing your symptoms and X-rays or other imaging tests of your knee, your orthopaedic surgeon can diagnose osteonecrosis of the knee and determine what stage it is in. Osteonecrosis of the knee develops in the following four stages:

• Stage I—Symptoms are most intense in the earliest stage and may continue for 6-8 weeks and then subside. Because X-rays are normal, a positive bone scan or MRI is needed to make a diagnosis in this stage. Treatment is nonsurgical and focused on pain relief and protected weightbearing.
• Stage II—After several months, the disease may progress to the second stage and X-rays will show that the rounded edge of the thighbone is beginning to flatten out. An MRI or bone scan can confirm the diagnosis, and a computed tomography (CT) scan may be used to measure the affected area.
• Stage III—Approximately 3-6 months after onset, the disease reaches the third stage and becomes clearly visible on X-rays and no other diagnostic tests are necessary. As the bone begins to die, the articular cartilage covering it will loosen. Surgical treatments may be considered at this point.
• Stage IV—In the final stage, the bone begins to collapse, articular cartilage is destroyed, the joint space narrows, and bone spurs may form. Severe osteoarthritis results and joint replacement surgery may be necessary.

Nonsurgical Treatment

When osteonecrosis is diagnosed in the early stages of the disease, nonsurgical treatment may be all that is needed. Options may include: medication and activity modifications to reduce pain, a brace to relieve pressure on the joint surface, and a conditioning program with exercises to strengthen the muscles in the thigh.

Surgery

If more than half of the bone surface is affected, you may need surgical treatment. Several different procedures may be used to treat osteonecrosis of the knee. Your orthopaedic surgeon will discuss the options with you and make a recommendation based on your individual situation.

• Arthroscopic cleansing (debridement) of the joint
• Drilling to reduce pressure on the bone surface
• Procedures to shift weightbearing away from the affected area
• Unicompartmental or total knee replacement

With any surgery, there are some risks, and these vary from person to person. Complications are typically minor, treatable and unlikely to affect your final outcome. Your orthopaedic surgeon will speak to you prior to surgery to explain any potential risks and complications that may be associated with your procedure.