A knee prosthesis comprising: a femoral component having two condyles (201, 202) with an opening disposed between the two condyles (201, 202), and an asymmetrical cam extending between the condyles (201, 202), the cam having a medial end generally conforming to a medial plane, and a lateral end generally conforming to a lateral plane, the lateral plane extending generally parallel to the medial plane, the cam having a longitudinal axis extending from the medial end to the lateral end perpendicularly to the medial and lateral planes (222, 224), and a central plane that extends substantially parallel to the medial plane and lateral plane, the central plane being substantially equidistant from the medial plane and lateral plane; and a tibial component having bearing surfaces (101, 102) to support each of the femoral component condyles (201, 202), and a post disposed between the bearing surfaces (101, 102) and extending superiorly from the tibial component; the femoral component and tibial component engageable by contact between the femoral condyles (201, 202) and tibial bearing surfaces (101, 102), and by contact between the cam and post during at least a portion of flexion between the femoral and tibial components (200, 100); characterized in that the cam comprises a first curvature defined by a first plane passing through the cam, and a second curvature defined by a second plane passing through the cam, the first and second planes (P1, P2) each extend parallel to the longitudinal axis and perpendicular to the central plane, the cam and post are configured so that the first curvature on the cam contacts the post at a lower degree of flexion, and the second curvature on the cam contacts the post at a higher degree of flexion, the first curvature comprises a concave curve having a first vertex, and the second curvature comprises a concave curve having a second vertex, the distance between the medial plane and the first vertex is greater than the distance between the medial plane and the second vertex, such that moving the femoral and tibial components (200, 100) in flexion from the lower degree of flexion to the higher degree of flexion causes the femoral component to rotate about the post. a femoral component having two condyles (201, 202) with an opening disposed between the two condyles (201, 202), and an asymmetrical cam extending between the condyles (201, 202), the cam having a medial end generally conforming to a medial plane, and a lateral end generally conforming to a lateral plane, the lateral plane extending generally parallel to the medial plane, the cam having a longitudinal axis extending from the medial end to the lateral end perpendicularly to the medial and lateral planes (222, 224), and a central plane that extends substantially parallel to the medial plane and lateral plane, the central plane being substantially equidistant from the medial plane and lateral plane; and a tibial component having bearing surfaces (101, 102) to support each of the femoral component condyles (201, 202), and a post disposed between the bearing surfaces (101, 102) and extending superiorly from the tibial component; the femoral component and tibial component engageable by contact between the femoral condyles (201, 202) and tibial bearing surfaces (101, 102), and by contact between the cam and post during at least a portion of flexion between the femoral and tibial components (200, 100); characterized in that the cam comprises a first curvature defined by a first plane passing through the cam, and a second curvature defined by a second plane passing through the cam, the first and second planes (P1, P2) each extend parallel to the longitudinal axis and perpendicular to the central plane, the cam and post are configured so that the first curvature on the cam contacts the post at a lower degree of flexion, and the second curvature on the cam contacts the post at a higher degree of flexion, the first curvature comprises a concave curve having a first vertex, and the second curvature comprises a concave curve having a second vertex, the distance between the medial plane and the first vertex is greater than the distance between the medial plane and the second vertex, such that moving the femoral and tibial components (200, 100) in flexion from the lower degree of flexion to the higher degree of flexion causes the femoral component to rotate about the post.