Abstract: A method for localizing a femur head center of a knee using only a marker array attached to a tibia, wherein the knee is modeled as a joint having at least one degree of freedom includes: using a geometrical model to describe kinematical behavior of the joint, said geometrical model including joint elements and a geometrical description of a position and orientation of the joint elements; acquiring a range of motion of the tibia with a tracking system, wherein the femur head center is fixed relative to the tibia; calculating positions and orientations of the geometrical model to fit the acquired range of motion; and calculating a location of the femur head center from the calculated positions and/or orientations.