An in vitro biomechanical study of a multiplanar circular external fixator applied to equine third metacarpal bones.

This study investigates the biomechanical properties of a circular multiplanar fixator—a type of surgical device used for bone repair—in horse third metacarpal bones. It indicates that the stability of the fixator under three different types of loading conditions was insufficient for larger horse breeds.

About the Research Methodology

• The study aimed to analyze the properties of a 4-ring circular multiplanar fixator when applied to equine third metacarpal bones with a 5mm mid-diaphyseal osteotomy gap.
• The experiment for this study utilized Steinmann pins of varying diameters (1/8 inch, 3/16 inch, and 1/4 inch) that were driven through the bone in a crossed configuration. These pins were then fastened to the fixator rings using fixation bolts.
• The fixator constructs were then put under three types of tests—cranial-caudal four-point bending, axial compression, and torsion. These represent different stress conditions that the bone might undergo in real situations.

Results and Findings

• The study indicated no observable damage to the fixator frame and no bone fractures resulting from the tests.
• However, the smallest pins (1/8 inch) implemented in the fixator underwent plastic deformation during all test types, implying they permanently deformed under the load.
• The larger pins (3/16 inch and 1/4 inch) experienced elastic deformation—they deformed under load but returned to their original shape afterwards.
• The research suggests that the axial compressive, bending, and torsional stiffness was not high enough, particularly in the fixator constructs using two 1/4 inch pins per ring, for reliably repairing a third metacarpal bone fracture in a heavy horse (approximately 450 kg).

Key Takeaway

• The results suggest that a 4-ring circular multiplanar fixator is pretty resilient but may not be sufficiently rigid for larger horses. By extension, it highlights the importance of taking into account the size and weight of the animal when deciding on an appropriate treatment method. This research is critical in veterinary surgical planning, design of medical devices, and care protocols for larger equine breeds where metacarpal bone fractures are often encountered.