a knee joint prosthesis includes femoral and tibial components , a bearing and a control arm . the bearing includes an inferior surface in sliding bearing engagement with the tibial component and a superior surface in articular bearing engagement with the femoral component . a notch extends into the posterior end of the bearing and a groove extends anteriorly from the notch in the inferior surface of the bearing . the posterior portions of the femoral component define a cam box having medial and lateral walls and a cam extending therebetween . the control arm is slidably engaged in the groove of the bearing and pivotally engage on the tibial component . the control arm further includes a post that extends into the cam box . the post includes a cam surface that engages the femoral cam to generate roll back of the femoral component on the bearing during flexion .