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 (kneecap) and femur (thighbone)

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.

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

In knee arthritis, 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 just in a single area of the knee.

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 metal and plastic implants. The healthy cartilage and bone, as well as all of the ligaments in the rest of the knee, are preserved.  Often this is in patients who have had longstanding problems with their kneecap dislocating or maltracking, in these patients additional procedures are often required to restore the alignment and path the kneecap runs it.