Comparison of torsional properties between a Fixateur Externe du Service de Santé des Armées and an acrylic tie-in external skeletal fixator in a red-tailed hawk (Buteo jamaicensis) synthetic tibiotarsal bone model.

OBJECTIVE: To compare the torsional mechanical properties of 2 external skeletal fixators (ESFs) placed with 2 intramedullary pin (IP) and transfixation pin (TP) size combinations in a model of raptor tibiotarsal bone fracture. SAMPLE: 24 ESF-synthetic tibiotarsal bone model (polyoxymethylene) constructs. PROCEDURES: Synthetic bone models were fabricated with an 8-mm (simulated fracture) gap. Four types of ESF-synthetic bone model constructs (6/group) were tested: a FESSA with a 1.6-mm IP and 1.6-mm TPs, a FESSA with a 2.0-mm IP and 1.1-mm TPs, an acrylic connecting bar with a 1.6-mm IP and 1.6-mm TPs, and an acrylic connecting bar with a 2.0-mm IP and 1.1-mm TPs. Models were rotated in torsion (5°/s) to failure or the machine angle limit (80°). Mechanical variables at yield and at failure were determined from load deformation curves. Effects of overall construct type, connecting bar type, and IP and TP size combination on mechanical properties were assessed with mixed-model ANOVAs. RESULTS: Both FESSA constructs had significantly greater median stiffness and median torque at yield than both acrylic bar constructs; FESSA constructs with a 1.6-mm IP and 1.6-mm TPs had greatest stiffness of all tested constructs and lowest gap strain at yield. No FESSA constructs failed during testing; 7 of 12 acrylic bar constructs failed by fracture of the connecting bar at the interface with a TP. CONCLUSIONS AND CLINICAL RELEVANCE: Although acrylic bar ESFs have been successfully used in avian patients, the FESSA constructs in this study were mechanically superior to acrylic bar constructs, with greatest benefit resulting from use with the larger TP configuration.

Torsional and axial compressive properties of tibiotarsal bones of red-tailed hawks (Buteo jamaicensis).

OBJECTIVE To describe the torsional and axial compressive properties of tibiotarsal bones of red-tailed hawks (Buteo jamaicensis). SAMPLE 16 cadaveric tibiotarsal bones from 8 red-tailed hawks. PROCEDURES 1 tibiotarsal bone from each bird was randomly assigned to be tested in torsion, and the contralateral bone was tested in axial compression. Intact bones were monotonically loaded in either torsion (n = 8) or axial compression (8) to failure. Mechanical variables were derived from load-deformation curves. Fracture configurations were described. Effects of sex, limb side, and bone dimensions on mechanical properties were assessed with a mixed-model ANOVA. Correlations between equivalent torsional and compressive properties were determined. RESULTS Limb side and bone dimensions were not associated with any mechanical property. During compression tests, mean ultimate cumulative energy and postyield energy for female bones were significantly greater than those for male bones. All 8 bones developed a spiral diaphyseal fracture and a metaphyseal fissure or fracture during torsional tests. During compression tests, all bones developed a crushed metaphysis and a fissure or comminuted fracture of the diaphysis. Positive correlations were apparent between most yield and ultimate torsional and compressive properties. CONCLUSIONS AND CLINICAL RELEVANCE The torsional and axial compressive properties of tibiotarsal bones described in this study can be used as a reference for investigations into fixation methods for tibiotarsal fractures in red-tailed hawks. Although the comminuted and spiral diaphyseal fractures induced in this study were consistent with those observed in clinical practice, the metaphyseal disruption observed was not and warrants further research.