The shape of most of the intramedullary and interlocking nails commonly used has been determined empirically as a segment of a circle. The radius of curvature of the Kuntscher nail, e.g., has a length of 3500 mm, the Aesculap-Kuntscher 3500 mm, the Grosse-Kempf 3000 mm, the AO 4000 mm, and the Klemm-Schellmann 3500 mm. Despite the fact that all these implants do not match the geometry of the femoral cavity, the application of “physiologically” curved nails is reported by two groups (11,14,22). In clinical use, the exact location of the point of insertion, axial malalignement, additional comminution of the fractured area or perforation of the distal metaphysis are related to a misfit between the geometry of the femoral cavity and the shape of the implant (4,10,11,12,15,16,20,21). Although the anatomy of the femur has been studied extensively (5,8,9,13,17), limited data is available with respect to the geometrical properties pertinent for intramedullary nailing of this bone. Two groups (11,14,22 and Winquist personal communication) determined the radius of curvature of the femur in the antero-posterior plane by sagittal x-rays.