A constrained prosthetic knee having a modular hinge post and a rotating bearing. A cannulated hinge post is rotatably connected to the femoral component of the knee prosthesis so that a hinge post extension may be anteriorly positioned through the hinge post and into the tibial component of the knee prosthesis, after positioning of the femoral component in the femur and the tibial component in the tibia. The hinge post is preassembled to the femoral component so that such assembly is not required during the implantation procedure. A meniscal component forming the rotating bearing of the knee prosthesis is packaged together with the hinge post extension so that the appropriate hinge post extension is readily available. The meniscal component includes a mechanism for preventing lift off of the meniscal component from the tibial component, while allowing rotation of the meniscal component relative to the tibial component.