Abstract: A knee prosthesis comprising a femoral component and a tibial component. The femoral component comprises a medial femoral condyle having a medial femoral condylar surface and a lateral femoral condyle having a lateral femoral condylar surface. The tibial component comprises a medial tibial condyle having a medial tibial condylar surface and a lateral tibial condyle having a lateral tibial condylar surface. The medial femoral condylar surface comprises a part-spherical convex surface and the medial tibial condylar surface comprises a part-spherical concave surface, the part-spherical surfaces being arranged to enable the medial femoral condyle to engage in sphere-in-sphere engagement with the medial tibial condyle. The sphere-in-sphere engagement provides anterior-posterior stability of the femoral component relative to the tibial component.