Fortunately, knee replacement surgery, a procedure that replaces the worn out joint with a metal and plastic joint, is very successful at relieving pain and improving function. However, many people who have only limited arthritis of the knee are concerned about replacing the entire joint. There is another option for people who have arthritis limited to only a part of the knee joint, and that is called a partial knee replacement.
Partial Knee ReplacementA partial knee replacement, also called a unicompartmental knee replacement, means that only the most worn out part of the knee is being replaced. Generally, the knee is separated into three compartments, and therefore partial knee replacements are called unicompartmental knee replacements because they replace one compartment. The three compartments are the inner side of the knee, the outer side of the knee, and the part under the kneecap. Most partial knee replacements replace the inner (medial) side or the outer (lateral) side of the knee. However, there are also partial knee replacement implants to replace the cartilage under the kneecap.
The compartment under the kneecap is commonly called the patellofemoral compartment. The undersurface of the kneecap is on one side, and the groove on the end of the thigh bone (the femur) is on the other side. The implants used to replace the patellofemoral compartment consist of a metal groove to fit on the end of the femur, and a plastic disc that attaches to the underside of the kneecap.
Patellofemoral Replacement Surgery & RecoveryDuring surgery to perform a patellofemoral knee replacement, the surgeon must create a surface on the underside of the kneecap and the groove on the end of the thigh bone to accommodate the artificial joint. The implanted joint is usually held in position with bone cement, similar to a standard total knee replacement. The most important part of surgery is to ensure that the kneecap will slide up and down in the groove on the end of the thigh bone normally. If this is not carefully addressed, the newly implanted partial knee replacement could wear out quickly.
Recovery after any type of partial knee replacement, including a patellofemoral replacement, is usually faster than a total knee replacement. The patient begins exercises to bend their knee immediately following surgery. While full weight can be placed on the knee, crutches or a walker are generally used for support for the first few weeks.
Results of Patellofemoral ReplacementIt is difficult to know exactly how long a patellofemoral replacement will last. The implant designs have changed significantly in the last decade, in hopes that better implants will lead to better results. Unfortunately, there is not much long-term data, and we are really left with shorter follow-up. We do know that in most of the studies have been done, that 80-90% of patients will have good results in the first decade after patellofemoral replacement, but whether or not they will hold up longer than that is not understood.
We also know that the most common reason that a patellofemoral replacement will not work well, is not because of a problem with the implant, but rather a problem in the rest of the knee joint--the part that was not replaced. About 25% of patients will eventually require conversion of their partial patellofemoral replacement to a total knee replacement as a result of worsening arthritis in the other compartments of the knee.
This is why the patients who have the best results from patellofemoral knee replacement are those patients who have specific problems with their kneecap, such as a previous patella fracture or problems with patellar tracking. These conditions can lead to early wear of the cartilage under the kneecap. On the other hand, patients who have generalized knee arthritis, not as a result of a specific kneecap problem, tend to eventually require a full knee replacement.
Lonner JH "Patellofemoral Arthroplasty" J Am Acad Orthop Surg August 2007 ; 15:495-506.