Method and device for determining the mechanical axis of a femur

Method for determining the mechanical axis of a femur attached to a pelvis via a hip joint, comprising the steps of:

• moving the femur about the hip joint,
• following the movement of the femur via a navigation system by means of a marking element on the femur that passes over a surface area during said movement,
• storing position data of the femur obtained from the navigation system, and
• calculating the position of the mechanical axis of the femur from the stored position data of the femur in various positions,
• wherein:
• the femur is pivoted from an initial position only through a maximum pivoting angle of 15° in various directions to prevent movement of said pelvis,
• the mechanical axis of the femur is calculated from the position data of the surface area passed over by the marking element when the femur is moved and from position data of the knee joint;
• only position data corresponding to a pivoting angle less than a predetermined critical angle smaller than the maximum pivoting angle covered by the femur are used for the calculation of the mechanical axis;
• only position data corresponding to a pivoting angle more than a predetermined minimum angle smaller than the critical angle are used for the calculation of the mechanical axis;
• and
• the minimum angle is more than 3°.