Geometrical analysis of the femoral neck torsion.

By the geometrical analysis of three cross sections the surface along the femoral neck, the second moments of area and the major axes of symmetry are obtained. The surface passing through these axes determined in each section, from the lateral to the medial third of the neck, is one of the two anatomical surfaces of symmetry of the neck. The geometric structure defined by this surface is a helicoid. No plane, in the ordinary sense of the word, has to be defined in the femoral neck. At the trochanteric junction, the major axis is inclined about 17 degrees on the anteversion plane, twists anteriorly and becomes more horizontal to finish with a mean inclination of 60 degrees at the cephalic junction. Surprisingly, this twist and then the neck torsion are in opposite directions to that suggested by the bony markers, often taken as a reference and reported in the anatomical descriptions.