A constrained prosthetic knee having as its component parts a femoral implant, a tibial implant, and a meniscal plate disposed between the implants. Knee flexion and extension is permitted by compoundly curved condyle surfaces of the femoral implant, which resemble corresponding surfaces of a natural knee, and correspondingly shaped convex bearing surfaces in the meniscal plate and tibial implant. This interface is defined by a continuous, concave, spherically shaped surface in the upwardly facing plateau of the tibial implant and a corresponding, continuous, convex spherical surface of the meniscal plate. A post projects from the tibial implant into a cavity defined by the femoral implant and the free end of the post and the cavity have opposing, cooperating and overlapping ledges which maintain the implants constrained to each other. The ledges permit unrestrained flexion, extension and torsional movement of the knee and limited lateral rocking (adduction-abduction).