A biomechanical comparison of composite femurs and cadaver femurs used in experiments on operated hip fractures.

Fourth generation composite femurs (4GCFs, models #3406 and #3403) simulate femurs of males <80 years with good bone quality. Since most hip fractures occur in old women with fragile bones, concern is raised regarding the use of standard 4GCFs in biomechanical experiments. In this study the stability of hip fracture fixations in 4GCFs was compared to human cadaver femurs (HCFs) selected to represent patients with hip fractures. Ten 4GCFs (Sawbones, Pacific Research Laboratories, Inc., Vashon, WA, USA) were compared to 24 HCFs from seven females and five males >60 years. Proximal femur anthropometric measurements were noted. Strain gauge rosettes were attached and femurs were mounted in a hip simulator applying a combined subject-specific axial load and torque. Baseline measurements of resistance to deformation were recorded. Standardized femoral neck fractures were surgically stabilized before the constructs were subjected to 20,000 load-cycles. An optical motion tracking system measured relative movements. Median (95% CI) head fragment migration was 0.8mm (0.4 to 1.1) in the 4GCF group versus 2.2mm (1.5 to 4.6) in the cadaver group (p=0.001). This difference in fracture stability could not be explained by observed differences in femoral anthropometry or potential overloading of 4GCFs. 4GCFs failed with fracture-patterns different from those observed in cadavers. To conclude, standard 4GCFs provide unrealistically stable bone-implant constructs and fail with fractures not observed in cadavers. Until a validated osteopenic or osteoporotic composite femur model is provided, standard 4GCFs should only be used when representing the biomechanical properties of young healthy femurs.