High Strain Rate Testing of Bovine Trabecular Bone

In spinal vertebral burst fractures, the dynamic properties of the trabecular centrum, the central region of porous bone inside the vertebra, can play an important role in determining the failure mode. If the failure occurs in the posterior portion of the vertebral body, spinal canal occlusion can occur and ejected trabecular bone can impact the spinal cord resulting in serious injury. In some cases these ejected bone fragments impinge the spinal cord and cause paralysis. About 15% of all spinal cord injuries are caused by such burst fractures. Unfortunately, due to the uniqueness of burst fracture injuries, post-injury investigation cannot always accurately assess the degree of damage caused by these fractures. This research makes an effort to begin understanding the governing effects in this important bone fracture event. Measurements of the dynamic deformation response of bovine trabecular bone with the marrow intact using a modified split-Hopkinson pressure bar apparatus are reported and compared to quasi-static deformation response results. Because trabecular bone is soft, typical Hopkinson pressure bar experimental techniques used for high strain rate testing of harder materials cannot be applied. Instead, a quartz-crystal-embedded, split-Hopkinson pressure bar developed for testing soft materials is used. Care is taken to account for the orthotropic properties in the bone by testing only along the principle material axes, determined through micro-CT. In addition, shaping of the stress wave pulse is used to ensure a constant strain rate and homogeneous specimen deformation. Results indicate that the bone fails catastrophically beyond a critical strain and that pressure build-up in the fluids contained within the porous structure may be the cause of more extensive bone damage when compared to failure at quasi-static strain rates.