An in vitro biomechanical investigation of an equine interlocking nail

OBJECTIVE: To determine the mechanical properties of Equine Interlocking Nail (EIN; JD Wheat Veterinary Orthopedic Research Laboratory, University of California, Davis) stabilized osteotomized tibiae and compare these variables with estimated in vivo loads. STUDY DESIGN: In vitro biomechanical investigation. ANIMALS: Twelve adult equine cadaveric tibiae. SAMPLE POPULATION: EIN-stabilized tibiae were tested monotonically under compression, 3- and 4-point bending, and torsion. Mechanical properties were compared with estimated in vivo loads. RESULTS: EIN-tibial composite mean compressive yield load (11 kN) and bending moment (216 Nm) were greater than loads expected postoperatively in vivo; however, the mean torsional yield load (156 Nm) was less than that expected in vivo. CONCLUSIONS: EIN-stabilized tibiae had compressive and bending strengths greater than those expected to maintain stability during walking in adult horses. Torsional yield strength did not appear sufficient to provide stability during walking in vivo. CLINICAL RELEVANCE: The EIN is not a feasible method of fracture repair for adult equine tibial fractures at this time, because its mechanical properties appear inadequate to withstand the postoperative torsional loads encountered during walking. Because this method of fracture repair may offer biological advantages, further modification of an interlocking nail for adult horses appears warranted.