Abstract
We wanted to know if a mobile bearing Total Knee Arthroplasty was able to cope with rotation of the tibial tray about the femoral prosthesis, by studying the tibio-femoral and patello-femoral joints.
This was a kinematic study that used a mobile bearing TKA mounted on a jig that allowed rotation of the tibial tray. The TKA was moved through a 90° range of flexion and we used photography to record the effects at the tibio-femoral and patello-femoral joints. We found that with a fixed tibia, increasing the degree of external rotation increased the degree of medial femoral condyle lift off from the polyethylene insert which was complete at 25° of tibial tray external rotation. The lift off increased with the degree of flexion. The patello-femoral joint remained congruent. If the rotated tibial tray was mounted on a tibia that was allowed to freely rotate, it led to congruity at the tibio-femoral joint. Now we found that there was medial facet impingement and lateral facet lift off of the patella button in extension and flexion.
We concluded that this mobile bearing prosthesis did not cope well with rotation of the tibial tray. The relatively low congruency at the tibio-femoral articulation meant that there was a reduced “driving force” at the tibio-femoral joint resulting in less than adequate rotation of the mobile polyethylene insert. We feel that the tibial tray must be placed in neutral to the femoral prosthesis and failure to do so will result in abnormal polyethylene loading that would increase wear and may culminate in early prosthesis revision.
The abstracts were prepared by Mr Roger Smith. Correspondence should be addressed to him at the British Association for Surgery of the Knee, c/o BOA, Royal College of Surgeons of England, 35–43 Lincoln’s Inn Fields, London WC2A 3PN.
