Optimal entry point for antegrade femoral intramedullary nailing (IMN) remains controversial in the current medical literature. The definition of an ideal entry point for femoral IMN would implicate a tenseless introduction of the implant into the canal with anatomical alignment of the bone fragments. This study was undertaken in order to investigate possible existing relationships between the true 3D geometric parameters of the femur and the location of the optimum entry point. A sample population of 22 cadaveric femurs was used (mean age=51.09+/-14.82 years). Computed-tomography sections every 0.5mm for the entire length of femurs were produced. These sections were subsequently reconstructed to generate solid computer models of the external anatomy and medullary canal of each femur. Solid models of all femurs were subjected to a series of geometrical manipulations and computations using standard computer-aided-design tools. In the sagittal plane, the optimum entry point always lied a few millimeters behind the femoral neck axis (mean=3.5+/-1.5mm). In the coronal plane the optimum entry point lied at a location dependent on the femoral neck-shaft angle. Linear regression on the data showed that the optimal entry point is clearly correlated to the true 3D femoral neck-shaft angle (R(2)=0.7310) and the projected femoral neck-shaft angle (R(2)=0.6289). Anatomical parameters of the proximal femur, such as the varus-valgus angulation, are key factors in the determination of optimal entry point for nailing. The clinical relevance of the results is that in varus hips (neck-shaft angle <or=120 degrees) the correct entry point should be positioned over the trochanter tip and the use stiff nails is advised. In cases of hips with neck-shaft angle between 120 degrees and 130 degrees , the optimal entry point lies just medially to the trochanter tip (at the piriformis fossa) and the use of stiff implants is safe. In hips with neck-shaft angle over 130 degrees the anatomical axis of the canal is medially to the base of the neck, in a "restricted area". In these cases the entry point should be located at the insertion of the piriformis muscle and the application of more malleable implants that could easily follow the medullary canal should be considered.
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