Abstract: A knee joint prosthesis of the type described in U.S. Pat. No. 3,868,730 has a pair of inwardly sloped radially merging extensions of its condyle runners forming a patella-guiding trough above and between the runners. The extensions merge smoothly into each other at an angle of about 150.degree.. The center of curvature of the base of the trough is about 1.25 inches about a center located a little behind and somewhat above the center of movement of the femoral component. The extension narrows towards its upper portion. The patellar component, which rides in the base of the trough, is made of ultra-high molecular weight polyethylene and has a circular cross section with two bearing surface facets on both sides of a concave arcuate ridge which tracks in the base of the trough, for substantially complete contact between the patellar component and the trough substantially over their full range of movement.