Abstract: This invention relates to knee-joint endoprostheses, and provide an endoprosthetic member embodying condylar replacement surface means to simulate the natural slideway for the knee cap, said slideway defining a space for the cruciate ligaments of the natural joint, and also having a curved boundary to be brought into alignment with the transverse edge of the natural joint which forms the upper boundary of the space for the cruciate ligaments relative to the intercondylar eminence. The prosthesis for the inner tibial plateau is made concave and the prosthesis for the outer tibial plateau is made saddle-shaped to match the natural tibial plateaus, all the prosthetic parts being made from a wear-resistant plastics material such as polyethylene for example.