A tibial knee joint prosthesis for attachment to a suitably prepared tibial bone, providing bearing portions in the lateral and medial compartments. The lateral and medial bearing surfaces of the component are inclined at different angles in the anterior to posterior direction of the knee, so that when mounted to the tibia, the lateral bearing surface of the prosthesis is higher than the medial bearing surface to the posterior side of the knee. In this way the lateral ligament is tightened progressively more than the medial ligament as the knee moves from extension to flexion, resulting in increased stability in the lateral compartment.