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.