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Your knee is divided into three major compartments:

• Medial compartment — the inside part of the knee
• Lateral compartment — the outside part of the knee
• Patellofemoral compartment — the front of the knee between the patella and femur

(Left) A normal knee joint. The medial, lateral and patellofemoral compartments are shown.
(Right) An X-ray of a normal knee showing healthy space between the bones.

Within the patellofemoral compartment, the patella lies in a groove on the top of the femur called the trochlea. When you bend or straighten your knee, the patella moves back and forth inside this trochlear groove.

(Left) The patella rests in a small groove at the end of the femur called the trochlear groove.
(Right) As you bend and straighten your knee, the patella slides up and down within the groove.

Reproduced and adapted from The Body Almanac. American Academy of Orthopaedic Surgeons, 2003.

A smooth substance called articular cartilage covers the ends of the femur, trochlear groove, and underside of the patella. Articular cartilage helps your bones glide smoothly against each other as you move your leg.

In knee osteoarthritis, the cartilage protecting the bones of the knee slowly wears away. As the cartilage wears away, it becomes frayed and the underlying bone may become exposed. Moving the bones along this rough surface is painful. This can occur throughout the knee joint or in just a single area of the knee.

Illustration shows osteoarthritis that is limited to the patellofemoral compartment of the knee. The patella (kneecap) has been removed to show the cartilage damage on the underside.

Advanced osteoarthritis that is limited to the patellofemoral compartment may be treated with patellofemoral replacement. During this procedure, the underside of the kneecap and the trochlear groove are resurfaced with plastic and metal implants, respectively. The healthy cartilage and bone, as well as all of the ligaments in the rest of the knee, are preserved.

(Left) This X-ray shows a normal knee from the side. The arrows point to the normal amount of space between the bones.
(Right) This X-ray shows narrowed joint space and bone rubbing on bone due to arthritis.

Advantages of Patellofemoral Replacement

Potential advantages of patellofemoral replacement over total knee replacement include:

• Less blood loss
• Smaller surgical incision/less surgical trauma
• Less pain and swelling
• Quicker recovery
• Decreased complications
• Increased knee function and activity

In addition, because the bone, cartilage, and ligaments in the healthy parts of the knee are maintained, many patients report that a patellofemoral replacement feels more natural than a total knee replacement.

One advantage of patellofemoral replacement over total knee replacement is that healthy parts of the knee are preserved, which helps to maintain more natural-feeling function of the knee.

If your osteoarthritis has advanced and nonsurgical treatment options are no longer relieving your symptoms, your doctor may recommend knee replacement surgery.

Careful patient selection is crucial when considering patellofemoral replacement. To be a candidate for the procedure, your arthritis must be confined to only the patellofemoral compartment of your knee.

In addition, if you have any of the following, you may not be a good candidate for the procedure:

• Knee stiffness
• Ligament damage
• Poor patellar (kneecap) tracking
• Major deformity of the leg
• Inflammatory arthritis (such as rheumatoid arthritis)
• Crystalline arthritis (such as gout)
• Morbid obesity

A thorough evaluation by an orthopaedic surgeon will determine if you are a good candidate for patellofemoral replacement.

Because patients undergoing partial knee replacement typically recover faster than patients undergoing total knee replacement, the procedure will most likely be performed on an outpatient basis.

During your initial consultation, your doctor will determine whether you are a candidate for outpatient surgery or will require a brief stay in the hospital.

Surgical Procedure

Inspection of the joint. Your surgeon will make an incision at the front of your knee. They will explore all three compartments of your knee, verifying that the damaged cartilage is, in fact, located only between the patella and the femur and that your ligaments are intact.

If your surgeon finds damaged cartilage outside of the patellofemoral compartment, they may instead perform a total knee replacement. Your surgeon will discuss this contingency plan with you before your operation to make sure that you agree with this strategy.

Patellofemoral replacement. There are two parts to the procedure:

• Prepare the bone. Your surgeon will use special tools to remove damaged cartilage and a small amount of bone from the patellofemoral compartment of your knee.
• Position the implants. A plastic button, or cover, is used to resurface the backside of the patella. This button will line up with a thin metal component that is used to resurface the trochlear groove at the end of the femur. These parts are typically secured to the bone with cement.

Patellofemoral replacement implants. The metal femoral implant (left) resurfaces the trochlear groove of the femur. The round plastic patellar implant (right) attaches to the underside of the kneecap.

(Left) This X-ray is taken from above the knee. The patella and the trochlear groove of the femur have become deformed due to osteoarthritis. There is now bone rubbing on bone. (Right) The same knee after patellofemoral replacement. The patellar implant on the underside of the kneecap does not show in an X-ray.

After Surgery

After surgery, you will be taken to the recovery room where you will be closely monitored as you recover from the anesthesia. You will then be either taken to your hospital room or discharged (if your surgery is being performed on an outpatient basis).

Front view of a knee after patellofemoral replacement.

As with any surgical procedure, there are risks associated with patellofemoral replacement. These risks are similar to those of total knee replacement. Your surgeon will discuss each of the risks with you and will take specific measures to help avoid potential complications.

The possible risks of patellofemoral replacement include:

• Infection and/or wound healing problems
• Blood clots
• Persistent pain
• Patellar instability (dislocating kneecap)
• Injury to surrounding structures, including blood vessels or nerves
• Reaction to anesthesia
• The need for additional surgery

Pain management. After surgery, you will feel some pain. Many types of medicines are available to help control pain, including opioids, nonsteroidal anti-inflammatory drugs (NSAIDs), and local anesthetics. Treating pain with medications can help you feel more comfortable, which will help your body heal and recover from surgery faster.

Opioids can provide excellent pain relief; however, they are a narcotic and can be addictive. It is important to use opioids only as directed by your doctor and stop taking them as soon as your pain starts to improve.

Weightbearing. You will begin putting weight on your knee immediately after surgery. You may need to use a walker, cane, or crutches for several days following your operation.

Rehabilitation exercise. A physical therapist will provide specific exercises to help restore strength to your quadriceps muscles and maintain range of motion in your knee. It is critical to perform these exercises as often as directed in order to achieve a good outcome.

Doctor visits. You will continue to see your orthopaedic surgeon for follow-up visits to evaluate your progress after surgery.

To help doctors in the surgical management of osteoarthritis of the knee, the American Academy of Orthopaedic Surgeons has conducted research to provide some useful guidelines. These are recommendations only and may not apply to every case. For more information: Surgical Management of Osteoarthritis of the Knee - Clinical Practice Guideline (CPG) | American Academy of Orthopaedic Surgeons (aaos.org)