Fatigue load of current tibial intramedullary nail designs: a simulated study

Comminuted tibial shaft fractures are traditionally treated with statically locked intramedullary nailing and protected weight bearing until fracture callous is evident. The purpose of this study was to demonstrate that a simulation of immediate full weight bearing following intramedullary nailing of these fractures does not result in implant failure.A comminuted fracture model was created using 2 pieces of polyvinyl chloride (PVC) pipe. Ten-millimeter-diameter tibial nails (Synthes, Paoli, Pennsylvania; Styker, Mahwah, New Jersey; Zimmer, Warsaw, Indiana; Smith & Nephew, Memphis, Tennessee) were inserted within the PVC pipe and secured proximally and distally with 2 or 3 locking bolts. The constructs were cycled in axial compression for 500,000 cycles or until implant failure. The tests were conducted using a modified staircase method (200 N per step), and the fatigue strength was identified for each of the tibial nail designs. When 2 interlocking bolts were placed proximally and distally, the fatigue strength was between 900 and 1100 N for the Stryker nail, 1100 and 1300 N for the Zimmer nail, 1200 and 1400 N for the Synthes nail, and 1400 and 1600 N for the Smith & Nephew nail. Adding a third interlocking bolt proximally and distally to the Smith & Nephew nail increased the fatigue strength by 13% to between 1700 and 1900 N. In all cases, implant failures occurred through the proximal or distal interlocking bolts.Biomechanical tests suggest that current tibial nail designs may permit immediate full weight bearing of comminuted tibial shaft fractures with minimal risk of implant failure. This may facilitate mobilization in the early postoperative period, especially in the multiply injured patient.