In vitro comparison of the effect of parallel and divergent transfixation pins on breaking strength of equine third metacarpal bones.

Breaking strength (torque at failure) of equine third metacarpal bones, with transfixation pins placed parallel in the frontal plane and 30 degrees divergent from the frontal plane, was determined in vitro. Two transfixation pins were placed through the distal metaphysis, using a jig designed to drill the holes in the assigned configuration. Paired metacarpal bones II through IV from 12 horses were tested in torsion. The torsional moment of the force applied through the transfixation pins at failure was compared for each limb. Metacarpal bones with divergent pins were significantly (P = 0.030) stronger, compared with those with parallel pins. Metacarpal bones with parallel pins failed with longitudinal oblique fractures through a proximal bone-pin interface, whereas those with divergent pins failed with more comminuted fractures through multiple bone-pin interfaces.