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. At least one of the condyles is part-helical in shape and the respective bearing surface of the tibial component is correspondingly part-helical in at least a medio-lateral direction. In addition, 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.
