Closed intramedullary nailing is an accepted method of treatment for femoral shaft fractures. Technical complications of this procedure include fracture instability, which may result in nail migration and malrotation, and femoral bursting during nail insertion. These complications were investigated in terms of geometric and mechanical parameters of the bone-implant system. Mechanical test results indicate that nails of the same nominal size from different manufacturers possess more than a twofold difference in flexural rigidity and a threefold difference in torsional modulus. These differences are due to variations on the cross-sectional shape and wall thickness. Measurements in insertion force and hoop stress in cadaver femora were significantly different for most of the nails tested due to differences in mechanical and geometric properties. Insertion forces in the distal fracture component were consistently lower than those in the proximal fracture component and decreased linearly with component length. Increasing the reamed diameter in the distal component 1 mm over the nail size decreased the insertion forces significantly. The most significant variable affecting femoral bursting was found to be the placement of the starting hole. Anterior offset from the neutral axis of the canal by more than 6.0 mm consistently resulted in excessive hoop stresses and femoral bursting. Case reports are presented to illustrate these biomechanical principles clinically.