Biomechanical Comparison of Orthofix Pins and Cortical Bone Screws in a Canine Humeral Condylar Fracture Model

Abstract

To compare shear stability of simulated humeral lateral condylar fractures reduced with either a self-compressing pin or cortical bone screw. In vitro biomechanical tests. Bilateral cadaveric canine humeri (n=18) without evidence of elbow disease. Lateral condylar fracture was simulated by standardized osteotomy. Bone fragments were stabilized with a self-compressing pin or a cortical bone screw (2.7 or 3.5 mm) inserted in lag fashion. Specimens were mounted in a materials testing system and the condylar fragment displaced in a proximal direction until failure. Mechanical testing variables derived from load-deformation curves were compared between stabilization methods using a Student's paired t-test. There were no statistically significant differences for mechanical testing variables between pin and screw stabilized specimens at expected walk and trot loads. Three yield points subjectively coincided with yield of the interfragmentary interface (Y1), bone at the implant interface (Y2), and implant deformation (Y3). Displacements at Y1 were 48-156% greater for pin than screw stabilized specimens. Y2 and Y3 loads were higher for screw than pin stabilized specimens, but likely supraphysiologic for dogs convalescing after surgical repair. A self-compressing pin or a cortical bone screw inserted in lag fashion both provided adequate strength in applied shear to sustain expected physiologic loads through the repaired canine elbow during postoperative convalescence. Because self-compressing pins were easy to implant and mechanical properties were not significantly different than cortical screws at expected physiologic loads, pins should be considered for the repair of traumatic humeral condylar fractures.