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The Oxford unicompartmental knee replacement (Biomet Orthopedics, Inc, Warsaw, Ind) was designed by John Goodfellow and John O'Connor and was fi rst used in a patient as a unicompartmental device in 1982.1 The design comprises a spherical metal femoral component and a fl at tibial base plate (Figure 1). Between the two, an unconstrained polyethylene bearing is inserted. It is the only unicompartmental device to use a fully congruous mobile bearing.2
In 1976, when the concept of mobile bearings was introduced into the practice of prosthetic knee replacement, the designers of the Oxford unicompartmental knee replacement identifi ed four principles to guide knee replacement:3
l) The prosthetic components should be shaped to allow for all distracting, sliding, and rolling movements between the bones, without restriction. This contrasts with constrained designs such as a hinged knee joint that limit these freedoms of joint movement; a fi xed hinge being the most extreme example.
2) The components should apply only compressive stress to the juxta-articular bone. If the components constrain joint movement, shear stresses develop at the implant/bone interfaces and predispose to component loosening.
3) All surviving soft tissue should be kept and restored to its natural tensions. The ligaments and muscles must provide stability to the otherwise unconstrained implant.
4) The areas of contact should be large enough to maintain the contact pressure under load at a level that prosthetic materials can withstand. Small contact areas are known to produce high contact stresses predisposing to polyethylene damage.
Reducing polyethylene wear at the bearing surface was a major concern, because it had been linked to early implant failure, particularly in unicompartmental replacements. 4,5 The 1976 design aimed, therefore, at fully congruent bearing surfaces to increase contact area and reduce wear.6 However, congruent surfaces imply a high level of articular surface constraint, confl icting with principles 1 and 2 above.3 The designers wrote that "a knee prosthesis that aims to restore physiological movements while maintaining congruity must incorporate a free bearing or bearings analogous to the human knee," and, as a practical solution, the mobile meniscal-bearing knee replacement was introduced.3,6
The Oxford meniscal polyethylene bearing has a fl at underside and a spherically concave upper surface to exactly match the fl at tibial platform and the spherical...