Biomechanics of the interlocking nail. A study of the proximal interlock

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 3.0 mm) 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 (4500 N vs. 3000 N; P less than or equal to 0.02) than the thicker inclined screw in the diaphyseal and the subtrochanteric fracture situation (2300 N vs. 1100 N; P less than or equal to 0.06). The biomechanical test showed the transverse screw to have sufficient and superior strength.