A knee prosthesis comprises a femoral component 310 defining medial and lateral condyles 313, 314 and an intercondylar groove； and a tibial component 311 having respective bearing surfaces 315, 316 shaped to engage with said condyles both when the knee is extended and flexed. The femoral and tibial components together comprise interengaging means, for stabilising against antero-posterior relative movement of the femoral and tibial components at flexion above a first predetermined angle. One of the condyles and the respective bearing surface of the tibial component are at least partially congruent through at least a plane extending through a contact line between said one condyle and the respective bearing surface, at least at angles of flexion below a second 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. The interengaging means may be a peg 332 and cam 330 arrangement and at least one condyle may be part helical in shape.
