A knee joint prosthesis used in total knee arthroplasty having tibial components (5) and tibial bearing inserts (20) with predetermined shapes that allow for an additional degree of freedom in both an anterior-posterior direction and rotation. The tibial bearing inserts (20) include a predeterminedly-shaped extension (17), such as a sphere (8), disc (12) or trapezoid (13), that extends downwardly from the bearing (4). The tibial components (5) include a socket (9) having a corresponding predetermined shape of the extension (17) and retaining edges (10) located at a top of the socket (9) so the extension (17) may be "dropped into" the socket (9) and therein retained. In this manner, disengagement of the tibial bearing inserts (20) at extremes of anterior or posterior translation of the bearing relative to the tibial component (5) is prevented without affected the translation and rotation of the tibial component (5) and tibial bearing inserts (20).