Occasionally, patients have arthritis that is confined to the portion of the knee joint behind the knee cap - the patellofemoral joint. In this case, it is possible to replace only the front surface of the end of the femur (the trochlea) and the back of the kneecap (patella).
Patients who can isolate their pain to just behind the kneecap may be suitable candidates. The pain is usually tolerable when walking on flat surfaces but becomes severe going up or down stairs. It is important to note that the vast majority of patients with pain behind the kneecap can be successfully managed without surgery.
There should be evidence of advanced arthritis on x-rays of the patellofemoral joint. This arthritis may be primary, secondary to previous trauma or secondary to patellar instability in the past.
The procedure is not suitable for patients who have inflammatory types of arthritis such as rheumatoid arthritis, ankylosing spondylitis or psoriatic arthritis due to the high likelihood that the rest of the joint will be involved in the future.
The major advantage over a total knee replacement is that because you still keep your cruciate ligaments and the main load bearing surface of your knee, the knee will have a much more natural feel to it.
Also as the procedure is somewhat smaller, you will most likely get discharged from hospital a day or two earlier, recover a little quicker and aim to return to work / activities sooner than with a total knee replacement.
Most types of knee replacements are performed under a spinal anaesthetic, where the legs are numbed from an injection of local anaesthetic into the back. This can be combined with sedation if you prefer not to be aware of the surgery. Occasionally a general anaesthetic is used for various medical reasons, but ultimately this is the decision of the Anaesthetist looking after you and can be discussed with them prior to surgery.
The incision runs down across the front of the knee as for a total knee replacement although the exposure required will usually be somewhat smaller.
The tissue on the inside of the kneecap is cut and the kneecap flipped over to allow access to the surface of both the trochlea and the kneecap. Using special jigs, the surface of both is then prepared for the components, both of which are usually cemented in place. The joint is then thoroughly washed out and closed in layers and dressings applied.
Patellofemoral arthroplasty has recently had a resurgence in popularity with so called second generation implants. These implants are better designed and can be implanted in a more reproducible manner. Outcomes from these are comparable to total knee replacements.
Aside from the risks of all arthroplasty surgery, the primary risk is progression of arthritis to the tibiofemoral joint i.e the rest of the knee. This would usually be treated by conversion to a total knee replacement.
The prosthesis can also loosen with time and this is usually treated with conversion to a total knee replacements.
There is also a small risk that the patella can fracture after the surgery.
The bulky wool & crepe bandage can be removed 24 hours after surgery. The dressing underneath can remain in place for several days & only if it leaks blood will the nurse change it. As a general rule, the less the wound is interfered with, the better. It is fine to shower but avoid getting too much water directly on the dressings. The paper stitches can remain in place for 2 weeks. At this point, you can shower without the dressing on & the paper stitches will peel off easily when wet.
At the end of the procedure, some local anaesthetic is injected around the knee joint. This will usually ensure minimal pain following the surgery. As the local anaesthetic begins to wear off (usually after 12 - 24 hours), discomfort can return. This can be minimized with a combination of simple analgesia such as paracetamol, an anti-inflammatory & a stronger morphine-like tablet or injection (depending on patient tolerances / allergies). If you are in significant pain, call the nursing staff to help get some more analgesia.
When patients return home, they often forget to continue using ice packs. These when combined with a few minutes elevation can really help reduce pain & swelling. Try to adopt the motto “work hard, rest hard”, by performing your exercises regularly as directed by the physiotherapist but then getting the leg up on a couple of pillows with an icepack. A frozen bag of peas wrapped in a towel works fine.
A scar will run down along the front of the knee, usually somewhat shorter than the scar for a total knee replacement. This is closed with a dissolvable suture under the skin with additional paper stitches (like pieces of tape) on top. It is normal to have an area of numbness especially on the outer part of the knee but this should diminish over 12 - 18 months. The scar itself is quite red and raised initially and matures over 6 - 12 months when it will be flat and slightly lighter than the surrounding skin.
As with most knee replacements, you will be allowed to put your full weight through the leg after the surgery. However you will be given crutches for support and can wean yourself off these over the next 3 to 6 weeks. The physiotherapist will demonstrate the exercises that you will need to perform and ensure that you can mobilize in a safe manner. You can expect to stay 2 or 3 nights after the surgery which is usually a little less than for a total knee replacement.
This obviously depends on the type of work that you do but as a guide, you will need at least 6 weeks off if you have a sedentary job and up to 3 months for a more physical job.
Following rehabilitation, patients could aim to return to a stationary bicycle, a stepper machine and low intensity sports such as golf, doubles tennis, hiking and swimming. Due to the strain on the prosthesis, a return to running is not recommended and will likely lead to premature failure. Likewise any high impact sport is not recommended.
Roughly 6 weeks following surgery is reasonable. If you have an automatic car and it is your left leg, then perhaps you can start earlier. Bear in mind that sitting in a car involves a significant knee bend and anything longer than a few minutes can be very uncomfortable.
Long term, this will be fine but because of the incision, the front of the knee will feel quite numb. Patients often find that it may be up to 12 months after a knee replacement before they can kneel on it.
You will usually be reviewed 4 weeks following surgery. This allows us to ensure that the incisions are healing and check the motion. It is often the best time to have a talk about the procedure and ask any questions regarding returning to normal activities.
You should seek advice if the pain dramatically worsens after discharge. Also call if the knee becomes a lot more swollen or you get a large amount of bleeding. Any symptoms such as calf swelling or tenderness or simply feeling unwell or shortness of breath should also be assessed.
Compiled October 2015
Review date: March 2016
Review date: March 2016