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The Oxford Knee
The Oxford Knee is a partial knee replacement meaning that only the damaged part of the knee is replaced and the remaining normal knee, including all the ligaments are preserved. The X-ray above shows an Oxford knee implanted and as can be seen only half of the knee is replaced. (As the X-ray was taken immediately after the operation there are small clips in the skin indicating the length of the incision, which are subsequently removed). During the operation great care is taken to ensure that the new components are accurately positioned so that they restore the normal tension and function in the ligaments and work normally with the retained surfaces of the knee. As a result at the end of the procedure the knee works exactly as it did before the arthritis developed.
With a total knee replacements much larger incisions are used and all the surfaces of the knee are removed. In addition one of the most important ligaments of the knee, the anterior cruciate ligament, is removed and other ligaments are released and lengthened. The surfaces of the knee are then replaced. As the ligaments have been removed or altered the shapes of the implant surfaces are changed to restore some stability to the knee. As a results, although total knee replacements usually work well they do not function normally.
The Oxford knee design
The Oxford knee consists of three parts: A femoral component, which replaces the damaged surface of the femur (thigh bone); A tibial component that replaces the damaged surface of the tibia (shin bone); and a polyethylene bearing, which we call a meniscal bearing which replaces the meniscus. The meniscus is an essential part of the normal knee which spreads the load and prevents the knee from wearing out. Similarly the meniscal bearing prevents the knee replacement from wearing out.
No other knee replacement has meniscal bearings so to decrease the risk of wear these other devices have thicker bearings, so more bone has to be removed. Also because they wear faster knee function tends to deteriorate with time which is not the case with the Oxford Knee.
The bearing of the Oxford knee is freely mobile so the load between the implant and bone is predominantly compressive which is ideal for advanced types of fixation. We now use a special coating on the implant to which the bone directly bonds.
The Oxford knee is small so can be implanted through a small incision with minimal damage to soft tissue. Sophisticated instrumentation is used to accurately position the implant so as to restore normal knee function.
Advantages of partial and total replacement
Partial Knee Replacement (PKR) advantages
PKR is a smaller operation than TKR so patients recover quicker and spend less time in hospital. They are less likely to have medial complications such as heart attacks, strokes, infections or blood clots. As a result, although it is extremely rare that patients dies after knee replacement, the chance of death within 3 months of surgery is twice as high for TKR than PKR.
Patients are more likely to have an excellent outcome after a PKR than a TKR and less likely to have a poor or bad outcome.
PKR provides better function than TKR, and patietns with PKR tend to be more active than those with TKR. The leg is more likely to have its normal alignment and contour restored and has a greater range of movement.
The chance of a patient needing further surgery to the knee is lower after PKR than TKR. In our practice the chance of a patient having revision surgery (meaning a further operation including insertion of a new implant) is the same for PKR as TKR but the revision surgery for PKR is a more minor operation than for a TKR.
Total Knee Replacement (TKR) advantages
TKR is a reliable and successful procedure that can be used can be used for every patient, whereas PKR is not appropriate for all. We use PKR for about two thirds of our knee replacements. Unfortunately surgeons have different views about which patients are appropriate. Many surgeons only do a few PKR and they tend to use it in inappropriate patients and get poor results. Therefore a patients considering a PKR should consult a surgeons who does many PKR.
Data collected nationally show that TKR are revised less often than PKR. The main reason for this is that a revision of a TKR is a difficult operation and the results are often not very good. As a result most patients with a bad result after TKR are not revised and continue to be in pain. If a patient has a bad result after a PKR this is usually converted to a TKR, which is a straight forward operation and usually solves the problem.
It is an advantage that a PKR can easily be revised, even though it results in more revisions. This is because if there is a bad result after a PKR it is usually fixed, which is not the case after TKR.
Who is appropriate for the Oxford Knee
Between one half and three quarters of patients needing a knee replacement are appropriate for an Oxford Knee. Usually the medial compartment of the knee is replaced. The medial compartment is the part of the knee adjacent to the other knee. The lateral compartment (outside of the knee) can be replaced but this is done much less frequently and the principles of the surgery are different so this will not be discussed in detail here.
The X-ray above shows a knee that would be ideally treated with a partial replacement. In the lateral compartment (right hand side of image) there is a gap between the bones , about 5mm thick where the normal cartilage is. However in the Medial compartment (left side of image) the cartilage is worn away and there is bone rubbing on bone which tends to be very painful. In order to be certain whether a partial knee is appropriate other X-rays usually need to be taken to ensure the ligaments are normal and the cartilage behind the knee cap and in the lateral side is satisfactory. At operation, before proceeding with an Oxford knee the surgeon will confirm that the knee is appropriate.