AN IN-VITRO BIOMECHANICAL STUDY OF A MULTIPLANAR CIRCULAR EXTERNAL FIXATOR APPLIED TO EQUINE 3RD METACARPAL BONES

The biomechanical characteristics of a 4-ring circular multiplanar fix ator applied to equine third metacarpal bones with a 5 mm mid-diaphyse al osteotomy gap were studied. Smooth Steinmann pins, either 1/8 inch, 3/16 inch, or 1/4 inch, were driven through pilot holes in the bone i n a crossed configuration and full pin fashion and fastened to the fix ator rings using cannulated fixation bolts. The third metacarpal bone fixator constructs were tested in three different modes (cranial-cauda l four-point bending, axial compression, and torsion). Loads of 2,000 N were applied in bending and axial compression tests and a load of 50 N . m was applied during testing in torsion. Fixator stiffness was de termined by the slope of the load displacement curves. Three construct s for each pin size were tested in each mode. Comparisons between axia l stiffness, bending stiffness, and torsional stiffness for each of th e three different pin sizes were made using one-way analysis of varian ce. There was no visually apparent deformation or permanent damage to the fixator frame, and no third metacarpal bone failure in any of the tests. Plastic deformation occurred in the 1/8 inch pins during bendin g, compression, and torsion testing. The 3/16 inch and 1/4 inch pins e lastically deformed in all testing modes. Mean (+/-SE) axial compressi ve stiffness for the 1/8 inch, 3/16 inch, and 1/4 inch pin fixator con structs was: 182 +/- 16 N/mm, 397 +/- 21 N/mm, and 566 +/- 8.7 N/mm; b ending stiffness was 106 +/- 3.3 N/mm, 410 +/- 21 N/mm, and 548 +/- 12 N/mm; and torsional stiffness was 6.15 +/- 0.82 N . m/degree, 7.14 +/ - 0.0 N . m/degree, and 11.9 +/- 1.0 N . m/ degree respectively. For s tatically applied loads our results would indicate that a 4-ring fixat or using two 1/4 inch pins per ring may not be stiff enough for repair of an unstable third metacarpal bone fracture in a 450 kg horse. (C) 1996 by The American College of Veterinary Surgeons.