The relationship of neck orientation to the shape of the proximal femur.

Neck angles and various dimensions measured from 70 normal cadaver femora and from clinical radiographs of 120 normal hips were analyzed. The neck-shaft angle and the inclination of the medial trabecular system varied over a range of approximately 35 degrees. The location of the femoral head center in relation to the femoral shaft and to the trochanters was related to neck orientation. As the neck became vertical, horizontal offset of the head center from the shaft decreased and its vertical level relative to that of the trochanter increased. The abductor muscle lever arm was measured with two methods. Values obtained when assuming that the joint resultant force follows the medial trabeculae demonstrated the closest relationship to the distance from head center to body midline. The normal shape of the proximal femur is variable, and variations in the absence of intrinsic bone disease appear to reflect adaptation to physiologic variations in the line of action of muscle forces.