A knee joint prosthesis assembly including a femoral component and a unitary tibial component. The femoral component includes a first condyle and a second condyle. The unitary tibial component includes an engaging structure configured to rigidly secure a stationary bearing thereto. The stationary bearing is configured to articulate with the first condyle. The unitary tibial component further includes a planar tibial bearing surface configured to slidably support a mobile bearing thereon. The mobile bearing includes an inferior planar surface configured to slide along the planar tibial bearing surface and a superior surface configured to articulate with the second condyle.
