Mechanical study of the safe distance between distal femoral fracture site and distal locking screws in antegrade intramedullary nailing

OBJECTIVE: To determine the safe distance for distal femoral fractures relative to the distal locking screws in antegrade intramedullary femoral nailing using a currently available titanium alloy nail design. DESIGN: Cyclic (fatigue) mechanical testing study. SETTING: Biomechanics laboratory. INTERVENTION: Intramedullary nailing of left synthetic fiberglass composite femora with type 32/33-C fractures at 1, 2, 3, and 4 cm from the more proximal of the distal locking screws. MAIN OUTCOME MEASUREMENT: The number of loading cycles to failure of the nail. RESULTS: A load level of 700 N through the femoral mechanical axis was validated as adequate to cause fatigue failure within 200,000 cycles in slotted stainless- steel nails. In the nonslotted titanium alloy nails, this load level caused failure in only 1 of 3 nails with a fracture at 2 cm from the more proximal of the 2 distal locking screws and in 2 of 3 nails with a fracture at 1 cm from the more proximal of the 2 distal locking screws. All of the other nails did not fail >1 million cycles. CONCLUSIONS: Under laboratory conditions, it is safe to assume that an antegrade titanium alloy nail will survive 1 million compression/bending cycles when the fracture is ≥3 cm from the more proximal of the 2 distal locking screws. Copyright © 2005 by Lippincott Williams & Wilkins.