Proximal femur elastic behaviour is the same in impact and constant displacement rate fall simulation

Understanding proximal femur fracture may yield new targets for fracture prevention screening and treatment. The goal of this study was to characterize force-displacement and failure behaviours in the proximal femur between displacement control and impact loading fall simulations. Twenty-one human proximal femurs were tested in two ways, first to a sub-failure load at a constant displacement rate, then to fracture in an impact fall simulator. Comparisons of sub-failure energy and stiffness were made between the tests at the same compressive force. Additionally, the impact failure tests were compared with previous, constant displacement rate failure tests (at 2 and 100mm/s) in terms of energy, yield force, and stiffness. Loading and displacement rates were characterized and related to specimen stiffness in the impact tests. No differences were observed between the sub-failure constant displacement and impact tests in the aforementioned metrics. Comparisons between failure tests showed that the impact group had the lowest absorbed energy, 24% lower maximum force and 160% higher stiffness than the 100mm/s group (p<0.01 for all), but suffered from low statistical power to differentiate the donor age and specimen BMD. Loading and displacement rates for the specimens tested using impact varied during each test and between specimens and did not show appreciable viscoelasticity. These results indicate that constant displacement rate testing may help understand sub-failure mechanical behaviour, but may not elucidate failure behaviours. The differences between the impact and constant displacement rate fall simulations have important ramifications for interpreting the results of previous experiments.