Fracture biomechanics of screw-hole defects under various loading conditions – An ex-vivo feline femoral model

Abstract

Objective: Feline cortical bone has anecdotally been described to be brittle, tending to fissure and shatter readily under load. Evidence for appropriate screw size selection in feline patients appears limited with current guidelines of 25-33% bone diameter being extrapolations from other species. The study aims to evaluate the biomechanical properties of feline femora with screw-hole defects of increasing diameter, subjected to three-point bending and torsion to failure at two different loading rates. Study design: Eighty femoral pairs were harvested from adult cat cadavers. For each bending and torsional experiment, there were five groups (n=8 pairs) of increasing screw-hole defects (Intact, 1.5mm, 2.0mm, 2.4mm and 2.7mm). Mid-diaphyseal bicortical defects were created with an appropriate pilot drill-hole and tapped accordingly. Left and right femora of each pair were randomly assigned to a destructive loading protocol at low (10mm/min; 0.5˚/s) or high rates (3000mm/min; 90˚/s) respectively. Stiffness, load/torque-to-failure, energy-to-failure, and fracture morphology were recorded. Results: No significant differences in stiffness and load/torque-to-failure were noted with increasing deficit sizes in all loading conditions. Screw-hole defects up to 33% bone diameter had a maximum of 20% reduction in bending and torsional strength compared to intact bone at both loading rates. Higher loading rates showed significantly increased stiffness and load/torque-to-failure in bending and torsion compared to low loading rates (p<0.001). Conclusion: 2.7mm screw-hole defects did not significantly reduce feline bone failure properties in this ex vivo femoral study. These findings support current screw-size selection guidelines of up to 33% bone diameter as appropriate for use in feline fracture osteosynthesis.