Background. For the available systems, the two paired nail holes at the distal femoral portion were of identical size and shape. However, the higher incidence of the first nail-hole failure was reported at the medial than lateral side. In practice, the nail was initially stabilized by the screw insertion into the first hole from the lateral-to-medial side. If the lateral first hole was adequately strengthened, the larger nail-hole diameter could ease to insert the screws into the lateral hole. Methods. The finite-element method was used to investigate the effect of the configuration of the lateral first hole on the screw insertion and stress distribution of the distal nailing system. The distal nail-hole thickness was increased and its edge was intentionally chamfered to increase its effectively projected area onto the sagittal plane. There were eight variations of the nail-hole configuration in the parametric analyses. Findings. The deflected nail diagonally contacts the screws at the medial first and lateral second nail-screw sites. The medial first hole was the most stressed and the stress of the medial second hole was the least. The increase in the nail-hole thickness significantly reduces the concentrated stresses of all four holes, thus allowing the lateral first nail-hole diameter to be increased without the strength loss at that site. The enlargement and chamfer of the lateral first hole significantly increase the effective nail-hole projected area and potentially facilitate the screw insertion. Comparatively, the size and shape re-design of the lateral first hole induced the minor increase in stress values of the other holes and screws. Interpretation. In nature, the load-transferring mechanism between the distal nail-screw interfaces was the point-contact problem. If the wall-thickness, diameter, and chamfer of the lateral first hole were well-controlled, this re-design decreases the stress at the medial first distal hole where most failures occur and renders the overall system mechanically safer and more surgically effective.
