In vitro cyclic biomechanical properties of an interlocking equine tibial nail

OBJECTIVE: To determine cyclic biomechanical properties of gap osteotomized adult equine tibiae stabilized with an equine interlocking nail (EIN). STUDY DESIGN: In vitro experimental biomechanical investigation. SAMPLE POPULATION: Thirteen adult equine cadaveric tibiae. METHODS: Adult equine tibiae with transverse, midshaft, 1-cm gap osteotomies, stabilized with an equine interlocking nail, underwent cyclic biomechanical testing in vitro under axial compression, 4-point bending, and torsion. Different specimens were subjected to different load levels that represented estimated in vivo loads at 2 Hz for 740,000 cycles. Fatigue life and gap strain were calculated. RESULTS: Compression and bending, but not torsional, fatigue life were longer than time necessary for bone healing. Compressive, but not bending or torsional, gap strains were small enough to be compatible with fracture healing by primary bone formation. Gap strains for compressive, bending, and torsional loads were compatible with indirect, or secondary, bone formation. CONCLUSIONS: Further modification should be made to the equine interlocking nail to increase bending stiffness and torsional fatigue life. CLINICAL RELEVANCE: The stainless steel equine intramedullary interlocking nail is unlikely to provide appropriate long-term stability for fracture healing in adult equine tibiae without modifications in the nail design and material.