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Which factors determine the wear rate of large-diameter metal-on-metal hip replacements? Multivariate analysis of two hundred and seventy-six components.
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Unicompartmental knee

Quite often (about 20 per cent of cases) the arthritis predominantly affects just the inside of the knee, so-called medial compartment of the knee - the inner side (link to unicompartmental knee arthroplasty case example). As a result the outer or lateral side of the knee and the articulation between the back of the kneecap and the femur, the patella femoral joint are reasonably well preserved. It therefore seems logical just to resurface the damaged portion of the knee. The early failures of the localised surface replacement were probably a product of poor design and relatively poor fixation. Although some surgeons continued with medial or unicondylar replacement they were in the minority. Over the last ten years this procedure has increased in popularity. This has mainly been because of published outcomes which are reasonable (although inferior to total knee in terms of lasting ten years). The realisation that patients often have better knee function with a unicompartmental knee has contributed to the growing popularity of this procedure. There are two main design types, the so-called "mobile bearing" implant where the polyethylene is allowed to move and slide on top of the metal tibial tray and the "fixed" design in which the polyethylene tibial tray is fixed onto the metal tibial tray. The advantage of mobile bearing type is to reduce stresses on the implant due to the congruent rticulation, therefore reducing wear of the polyethylene.

The principles of surgery are similar to total knee arthroplasty although it is possible to use a much smaller incision to insert the implants using jigs by removing just the articular surfaces of the femur and tibia.

The functional outcomes are almost certainly better in patients having localised replacements. The longer term outcomes defined as how long they last in large Registry series such as the Swedish Arthroplasty Register would indicate that the failure rate due to loosening is about double that of total knee replacements and benchmark figures for 10 year survivorship of total knees are in the region of 95 per cent. Therefore although the functional results are better they are slightly less robust. These issues need to be discussed with the patient and this potential trade off discussed in terms of implant selection.

The concept of replacing just the medial side of the knee has extended to replacing the outer side of the knee if there is a deformity on the outer side of the knee and also between the kneecap and the patella if the joint is localised, patello-femoral replacement.

link to unicompartmental knee replacement case example.

(last updated 03/07/08 TB)


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