Abstract: A knee endoprosthesis comprising: a tibial part comprising a tibial rod bearing a tibial plate, a first tibial insert (11) disposed on the tibial plate, a femoral part comprising a femoral rod bearing two condyles (8, 9) each having a first sliding surface co-operating and congruent with a respective second sliding surface formed on the first tibial insert (11), wherein the tibial part is coupled to the femoral part so that the femur can bend relative to the tibia from a position (&agr;≈0) where the knee is extended to a position (&agr;max) where the knee is bent and vice versa. A progressive locking element is provided for limiting the lateral rotation and proper rotation of the condyles (8, 9) relative to the tibial plate, starting from a knee-bending angle &agr;0 determined in advance and up to a zero angle, the amplitude of proper rotation, when the angle &agr;0 is decreased, varying down to zero amplitude (complete blocking) at zero angle (unbent knee).