An implantable knee joint prosthesis which comprises a tibial tray, an articulating tibial insert attached to the tibial tray, and a femoral condylar component adapted to cooperatively bear upon the tibial insert. The femoral component has an intercondylar recess having parallel lateral side walls and posterior and anterior bearing members defining the boundaries of the recess. The tibial insert includes an eminence extending superiorly into the intercondylar recess and dimensioned and shaped to provide defined stabilization limits for preventing posterior-anterior dislocation while allowing limited translation and essentially free rotation and angulation throughout most of the total range of articulation. Metal reinforcements serve to secure the insert to the tray. Bearing surfaces of the eminence and the intercondylar recess are so shaped as to increase the area of contact between them as the degree of flexion of the leg increases to thereby limit the contact stress transmitted to the eminence and to reduce wear of the eminence.