By David W. Murray, MD, FRCS(Orth)
When designing a unicompartmental knee replacement two conflicting requirements exist. To minimize wear it is necessary to have large contact areas so the contact pressures are small. This is best achieved by having congruous articular surfaces. Conversely to minimize loosening it is necessary to have low constraints and thus incongruous articular surfaces. This conflict is best solved by using a mobile bearing, which should prevent both loosening and wear.
Ideally, a mobile bearing should reproduce the function of the normal meniscus. The meniscus minimizes wear by having fully congruent contact in all positions with both the tibial and femoral condyles. As the condyles are polyradial and polycentric the meniscus has to change its shape to maintain fully congruous contact with the surfaces. An artificial mobile bearing cannot change its shape; therefore, to achieve fully congruous contact the femoral component must have a spherical surface and the upper part of the bearing must have a matching spherically concave surface. This is the key feature of the design of the Oxford Knee (Figure). A detailed overview of the Oxford Knee is given in a book written by the authors.
The Oxford Knee was first used bi-compartmentally. It was soon discovered that it only did well with an intact anterior cruciate ligament (ACL). It also was observed that if the ACL was intact then the disease was almost always unicompartmental. The device was then used as a uni-compartmental replacement when the ACL was intact…
…Unfortunately, not every surgeon has been able to achieve good results: although most published 10-year series have survival rates >90%, not all do. This variability in results is also seen in the National Registers. Data from the Swedish Registry showed that centers performing more unicompartmental knee replacement had better survival rates. Data from the New Zealand Registry showed that surgeons performing greater numbers had better survival rates: surgeons who implanted at least 10 Oxford Knees per year had a very low failure rate (approximately 0.5% per year)…
…Approximately 1 in 3 patients requiring knee replacement are appropriate for the Oxford Knee. With this device you can expect rapid recovery, excellent function, and good long-term survival even in the young and active. However, good results can only be achieved with appropriate indications and techniques.
Prof Murray is from Nuffield Orthopaedic Centre, Oxford, United Kingdom.