A knee prosthesis comprises a femoral component for securement to a femur, the femoral component defining medial and lateral condyles and an intercondylar groove； and a tibial component for securement to a tibia, the tibial component having respective bearing surfaces shaped to engage with said condyles both when the knee, in use, is extended and also over a range of flexion. The femoral and tibial components together comprise interengaging means, for stabilising against antero-posterior relative movement of the femoral and tibial components. One of the condyles and the respective bearing surface of the tibial component are at least partially congruent, at least at angles of flexion below a predetermined angle. The other bearing surface is incongruent with the other condyle in at least the antero-posterior direction so as to allow for antero-posterior movement of the femoral component relative to the bearing surfaces of the tibial component.
