Veterinary Surgery 29:38-47, 2000 An In Vitro Biomechanical Investigation of an Equine Interlocking Nail LAURIE A. MCDUFFEE, DVM, PhD, Diplomate ACVS, SUSAN M. STOVER, DVM, PhD, Diplomate ACVS, JOEL M. BACH, PhD, and KENNETH T. TAYLOR 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. ©Copyright 2000 by The American College of Veterinary Surgeons A DULT HORSES with diaphyseal tibial fractures have a poor prognosis for survival, 1,2 reportedly as low as 9%. 3 Reduced tibial fractures must be stabilized to enable immediate loading. 4 External co- aptation is a poor treatment option, because the stifle joint cannot be immobilized. 5,6 Dynamic compression plates (DCP) and specialty plates, including the dy- namic condylar screw (DCS) and dynamic hip screw (DHS) plates (Synthes, Paoli, PA), are the strongest implants available for use in equine fracture repair 7 ; however, clinical outcome has been disappointing. 1 Despite noteworthy in vitro monotonic mechanical strength and stiffness of DCP repaired osteotomized equine tibiae, 8 similarly stabilized clinical fractures have failed. 9 In our experience, failure has occurred either immediately after surgery or 3 to 4 weeks later, suggesting that the mechanical properties of DCP- repaired tibiae are inadequate. Fixation techniques for From the J.D. Wheat Veterinary Orthopedic Research Laboratory, Department of Anatomy, Physiology and Cell Biology, School of Veterinary Medicine, University of California, Davis, CA, and UC Ergonomics Program, Department of Medicine, University of California, San Francisco, Richmond, CA. Supported by the Center for Equine Health with funds provided by the Oak Tree Racing Association, the State of California pair-mutual wagering fund, and contributions by private donors; and Smith and Nephew Richards, Memphis, TN. No reprints available. Address correspondence to Laurie A. McDuffee, DVM, PhD, Department of Health Management, University of Prince Edward Island, 550 University Ave, Charlottetown, Prince Edward Island, Canada C1A4P3. ©Copyright 2000 by The American College of Veterinary Surgeons 0161-3499/00/2901-0005$3.00/0 38