Patent ID: 7925324

Claim:
A method for determining an antetorsion angle of a femur, said antetorsion angle γ being defined by an angle between a central axis through the femoral neck of said femur, said central axis running through the center of the femoral neck and being parallel to a cortical surface of a proximal part of the femoral neck, and a posterior condyle axis, said posterior condyle axis connecting the most posterior points of the femoral condyles of the femur if the central axis through the femoral neck and the posterior condyle axis are projected onto a plane normal to the femoral shaft axis, wherein the femoral shaft axis is the central axis of the approximately cylindrically shaped shaft of the femur, the central axis of the cylindrical shaft being parallel to the most anterior tangent to the distal femur, and the femoral shaft axis of said femur also being perpendicular to the posterior condyle axis; said method comprising: (a) obtaining via an imaging system a first image of the posterior condyles of the femur, such that the posterior condyles overlap laterally, wherein the posterior condyle axis is projected onto a point, and the normal to an image plane of the first image is parallel to the posterior condyle axis; (b) defining in the first image the most anterior tangent to the anterior cortex of the distal femur; (c) shifting said tangent parallel to itself until it includes the point of the posterior condyle axis in the first image, thereby determining a transcondylar plane; (d) determining a parallel to the femoral shaft axis in the first image; (e) obtaining via the imaging system a second image of the femoral neck of the femur such that the normal to an image plane of the second image encloses an angle β with the femoral shaft axis and an angle θ with the transcondylar plane; (f) defining in the second image the femoral neck axis of the femur enclosing an angle α with the transcondylar plane; (g) calculating, using a processor, the antetorsion angle γ from the equation tan γ=tan α′/(cos β·cos θ).