A knee prosthesis comprises a femoral component defining medial and lateral condyles 12, 13 and an intercondylar groove； and a tibial component 15 having respective bearing surfaces 14, 14 shaped to engage with said condyles both when the knee is extended flexed. At least one of the condyles is part-helical in shape and the respective bearing surface 14 of the tibial component is correspondingly part-helical in at least a medio-lateral direction, resulting in lateral translation of the intercondylar groove during flexion and ensuring patellar tracking. In addition, the other bearing surface 14 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 during flexion. Optionally, at increased flexion a peg 132 and cam 130 mechanism (see figures 16, 17) may engage to cause roll-back of the femoral component on the tibial component.
