Dynamic testing of old and young baboon cortical bone with numerical validation

Cortical bone tensile mechanical properties at quasistatic and high rates (∼300 s^-1) were determined ex vivo using the right femurs of 12 female baboons, (Papio hamadryas spp.) from the Texas Biomedical Research Institute/Southwest National Primate Research Center in San Antonio, Texas. The animals were divided into two age groups: a young age group (6.63 ± 0.6 years) and an old age group (26.96 ±1.3 years). Seven specimens per group were monotonically loaded to failure to determine their mechanical properties. The quasistatic strength of the bone for the old group was just a little (but not significantly) lower than the young group. High strain rate tests performed with the Hopkinson bar indicate that baboon bone from the older group was significantly weaker under impact loads than that from the younger group. This observation is particularly important due to the similarities between baboon and human bone tissue. Typical strain rates for these tests ranged from 130 s ^-1 to 250 s^-1. A full-size 3-D simulation of the Hopkinson bar test was performed to confirm that the bone specimen was under stress equilibrium and to evaluate the consistency of the modulus and strength inferred from the tests. Simulations were performed in which the modulus, strength and failure strain were varied to see the sensitivity of the results. Additionally, simplified simulations were performed to estimate the strain rate environment of a femur during a fall at an impact velocity of 5 m/s, similar to a free fall velocity from a height of 1.3 meters. The simulations confirm that strain rates obtained in the Hopkinson bar are relevant because they are similar to those expected inr such a fall.