A method and an apparatus are disclosed for identifying a proper tibia bone rotation relative to the femur bone for orientating a prosthesis in a knee replacement surgery. The proper tibia bone rotation is identified from an external rotation of the tibia in extension and an internal rotation of the tibia flexion. A point located midway between the external and internal rotations and a Whiteside line of the femur defines a center axis of tibia rotation plane. The rotational angles of the tibia may be measured by a mechanical goniometer or a computer navigation apparatus.
