A tibial element is provided for a replacement knee prosthesis which comprises a tibial tray provided with a bearing component having medial and lateral compartments. Control means acting between the tray and the bearing component are included which provide free posterior and anterior movement of the lateral compartment which is greater than any allowed free posterior and anterior movement of the medial compartment in relation to the tray. Thus, there is no relative free posterior movement and anterior movement of the medial compartment or some may be allowed.