Aims: Should the tibial tray be rotated about the femoral component of a total knee arthroplasty? Literature review provides evidence commending rotation and neutral alignment. We wanted to provide evidence to help this debate.

Method: We developed a knee jig allowing full range of movement of a knee arthoplasty. Under compression, we studied the isolated effect of tibial tray rotation about the femoral prosthesis in þxed and mobile bearing prostheses. We photographed the tibio-femoral and patello-femoral articulations.

Results: A mobile bearing prosthesis at 15 degrees of tray rotation suffered posteromedial and anterolateral polyethylene impingement. At 25 degrees, the medial femoral component lifted off. The þxed bearing prosthesis showed similar polyethylene impingement, but no femoral condyle lift off. In both prostheses, tray rotation increased lateral patella facet loading, which increased with knee ßexion.

Conclusion: Mobile bearing prosthesis was less tolerant than þxed bearing prosthesis to tibial tray rotation. Rotation caused polyethylene impingement, which would generate wear debris. Patella tracking was not improved by tibial tray rotation. The mobile bearing prosthesis is less congruent at the tibio-femoral articulation. Therefore there is less Òdriving forceÒ to rotate the polyethylene to align it to the femoral component, when the tray is rotated. We recommend the tibial tray be aligned to the femoral prosthesis in neutral.