Unicompartmental Knee Replacement

Arthritis is an extremely common condition that develops in the knee over time most often due to wear and tear on the knee joint. Arthritis results in a loss of articular cartilage, the gliding surface of the knee. This results in a progressive roughening of the surfaces within the knee. Pain and swelling results, especially with increased activity.

There are three compartments in the knee. Medial (inner) and lateral (outer) compartments serve as the articulation between femur (thigh bone) and tibia (leg bone). The patello-femoral compartment is in the front of the knee. Arthritis is classified based on severity (amount of cartilage damage and joint space narrowing) as well as which compartments are involved. Tricompartmental arthritis is treated surgically with a total knee replacement. Arthritis that is localized only to the inner (medial) compartment of the knee can be treated with a unicompartmental knee replacement. This type of knee replacement replaces only the damaged compartment of the knee while preserving the lateral and patellofemoral compartments of the knee that have normal cartilage.

There are many cases of arthritis that can be managed with conservative measures. These conservative measures include, rest, icing, NSAIDs (anti-inflammatories), physical therapy, bracing, assistive devices, and activity modification. When these measures fail to control pain and the patient is unsatisfied with the quality of life allowed by the knee, surgery can be an effective treatment for the pain associate with arthritis.

The following case example demonstrates a patient with isolated medial compartment arthritis that was treated with a unicompartmental knee replacement (Figure 1.)

Figure 1. a. The right knee has narrowing of the joint space between the femur and tibia only on the inner compartment of the knee. The left knee has normal cartilage and no joint space narrowing. b. The patellofemoral joint has normal cartilage. c. The right knee has undergone a unicompartmental knee replacement restoring alignment to the knee and relieving pain.

Am I a Candidate For Unicompartmental Knee Replacement?

Several criteria must be met for unicompartmental knee replacement to be effective and successful:

• The arthritis in the knee and pain in the knee must be limited to the medial (inner) compartment of the knee.
• The knee must have less than 5 degrees of flexion contracture meaning the knee must come very near completely straight.
• The knee must have intact ligaments and no evidence of instability
• Varus (bow-legged) deformity should be correctable on exam
• The knee must not have inflammatory arthritis such as rheumatoid arthritis

When these criteria or met, a unicompartmental knee replacement is a very good option. If any of the criteria are not met, a total knee replacement is a safer option.

Surgery

• General or epidural anesthesia is required.
• Preoperative templating plans exactly the size implant required for the knee knee.
• A four inch incision centered over the inner aspect of the knee is made.
• Surgical time is two hours to perform the unicompartmental knee replacement.
• Hospital stay averages 2 days
• Weight bearing is allowed immediately with crutches or a walker. An assistive device is usually used for walking for 2-4 weeks.
• A continuous passive motion machine (CPM) will be used for 2-4 weeks to restore range of motion of the knee.
• Icing 20-30 minutes 3-4 times per day helps reduce swelling.
• The incision is kept dry for 7-10 days.
• A follow-up visit is required at 2, 6, 12, and 24 weeks after surgery to check range of motion and healing of the knee.
• You may need between 2-12 weeks off from work depending on the physical requirements of your job.

Do I need to have a Unicompartmental Knee Replacement and what are the risks?

If a patient has isolated arthritis in the medial compartment and has tried all conservative measures to control pain, a unicompartmental knee replacement is an effective option to relieve pain and improve function. Prior to undergoing a knee replacement procedure, one must have exhausted all conservative options and must be in severe pain. All surgery that involves artificial components has the risks of loosening, hardware failure, infection, mal-position of components, stiffness, injury to nerves and vessels, and scar tissue formation. The surgery has the risks of bleeding, blood clot formation, and other severe medical complications. Fortunately these risks are very rare but the risk of surgery must be weighed against the severity of the pain and disability of the knee.

How long does a unicompartmental knee replacement last?

Long term studies document 90-95% well functioning prostheses at 15 years. Unicompartmental knee replacements that loosen or fail are converted to total knee replacements.