Knee prosthesis

A knee prosthesis comprising:

• a femoral component for securement to a femur, the femoral component defining medial and lateral J-shaped condyles and an intercondylar groove;
• and
• a fixed bearing tibial component for securement to a tibia, the tibial component having respective bearing surfaces which are fixed with respect to both a tibial engaging component and a stabilising peg for securing the tibial component to a tibia, the respective bearing surfaces being shaped to engage with said condyles both during extension and also over a range of flexion, wherein:
• the femoral and tibial components together comprise interengaging features, for stabilising against antero-posterior relative movement of the femoral and tibial components at flexion above a first predetermined angle of 110 degrees, wherein both the medial and lateral condyles are incongruent and unconstrained in an antero-posterior direction with the respective bearing surfaces at angles of flexion above the first predetermined angle of 110 degrees to allow for roll-back of the femoral component relative to the bearing surfaces, tibial engaging component, and stabilising peg;
• one of the condyles and the respective bearing surface of the tibial component are fully congruent at angles of flexion below a second predetermined angle of at least 70 degrees;
• the other bearing surface is incongruent with the other condyle and is unconstrained in at least the antero-posterior direction at angles of flexion below the second predetermined angle of at least 70 degrees, so as to allow for antero-posterior movement of the femoral component relative to the bearing surfaces of the tibial component;
• and
• wherein a transition zone is provided at angles of flexion between the second and first predetermined angles, in the transition zone the interengaging features begin to contact and the one condyle becomes unconstrained at a posterior side of the one condyle while an anterior side of the one condyle remains in contact with the respective bearing surface, which encourages external rotation and posterior translation of the other condyle until the first predetermined angle is reached, wherein both internal and external rotation is permitted.