Conventional interlocking nails for the femur use an inclined proximal interlocking screw; the AO/ ASIF universal interlocking nail utilizes a transverse interlocking screw which is anchored in cortical bone. The transverse position of the screw allows one to employ the nail for either the right or left side. The yield strength of the thinner diameter transverse screw, (thread 4.5 mm, core 3.0mm) was compared with that of the Grosse-Kempf nail oblique screw (thread 6.3 mm, core 4.5 mm) by testing in cadaver femora to evaluate the risk of fatigue failure. The test, performed using slow loading rates (50 N/s), simulated either a diaphyseal or a subtrochanteric fracture situation. The test rig applied a load corresponding to body weight and pull at the trochanter. Taking the highest load without plastic deformation as a criterion, the mechanical strength of the two screws was evaluated for each fracture situation. The thinner but shorter transverse interlocking screw demonstrated higher strength (4500N vs. 3000N; P--<0.02) than the thicker inclined screw in the diaphyseal and the subtrochanteric fracture situation (2300N vs. 1100N; P --< 0.06). The biomechanical test showed the transverse screw to have sufficient and superior strength.
The AO/ASIF group recommends the use of internal fixation with straight and angled blade plates to treat femoral fractures not suited to conventional intramedullary nailing [36]. Plating requires less time than closed intramedullary nailing when operating on polytraumatized patients in the supine position on a regular operating table [17]. However, when conventional plating techniques [36] are applied to complex fractures,