The Knee’s Three Compartments
Let me briefly review certain features of knee anatomy. There are three compartments in a knee, a situation analogous to an apartment with three rooms. The names of the compartments simply go by the two bones that make up that compartment. For example, on the inside part of the knee you have the medial (meaning inside) femoro-tibial compartment. On the outside you find the lateral (meaning outside) femoro-tibial compartment. The third compartment consists of the kneecap and the underlying thighbone and is therefore called the patellofemoral compartment.
Between the medial and lateral femoro-tibial compartments lie the famous cruciate ligaments. Famous, or perhaps infamous, because anyone reading the sports pages has heard of the torn anterior cruciate ligament.
When only one compartment of the knee is arthritic, say the inside (medial) part of the knee, one can argue for just replacing that part of the knee and leaving the rest alone. It’s not a new idea. It goes back to the earliest days of knee replacement surgery. Such a partial knee replacement is called a “unicompartmental knee replacement” as it involves just one compartment of the knee. It is abbreviated UKR and colloquially in orthopedic circles is called a “uni” (u-nee).
If a part in your car engine is broken, do you replace the part or do you change the whole engine? Obviously you change the part. But if a quarter to a third of the engine is broken you start thinking about starting from scratch with a new engine. If only one compartment of a knee is worn down do you limit the replacement to that one compartment? It depends on your optic: do you see this as being just one little part of the knee or do you consider this to be an entire third of knee? If you replace just one compartment will the others eventually wear out, thus necessitating another operation? How long will it take for the other compartments to wear out? Is it possible that they will never wear out?
Minimally Invasive Unicompartmental Knee Replacement
Faced with a patient with arthritis in one compartment (usually the medial, inner one) the orthopedic surgeon has a choice between a total knee replacement and a unicompartmental knee replacement. Studies on this subject support both points of view, and there exist an equal number of articles on both sides of the fence. In the United States, partial knee replacements have gone in and out of fashion over the last forty years. They tend to be more difficult than total knee replacements, yet surgeons get paid less. Recently instruments have been developed allowing unicompartmental knee replacement to be carried out through a skin incision that is shorter than that of a total knee replacement, and interest in the procedure has been renewed.
I’m in the camp that believes that in carefully selected patients a unicompartmental replacement is good idea, and I’ve been implanting them for over 15 years.
The Patellofemoral Replacement
An unusual variation on the unicompartmental replacement is the so-called patellofemoral replacement, another procedure I’ve been a proponent of. Here the kneecap is resurfaced with a plastic button, and a metallic shield is placed in the “trochlear groove” of the thighbone. This procedure is reserved for patients whose arthritis is confined to the patellofemoral compartment, i.e. the kneecap and the underlying thighbone. The advantages and disadvantages of this replacement over the total knee replacement are similar to those discussed above in the section on unicompartmental replacements. There is less of a surgical dissection, the time spent in the hospital is shorter, the motion is better, but there is always the concern that another part of the knee might eventually wear out.
Such replacements have been available for the last 25 years, but much more so in Europe than in the United States. Reviews of this procedure have been mixed, as is the case for all unicompartmental replacements, and a surgeon can find any point of view backed up by at least one article in the literature. Most orthopedic surgeons have never seen this procedure and are not likely to offer it to you, but you should know that it exists.
*Excerpted from What Your Doctor May NOT Tell You About Hip and Knee Replacement Surgery, Warner Books 2004.