Mechanical response of cortical bone in compression and tension at the mineralized fibrillar level in steroid induced osteoporosis

Nanocomposites like bone are used in vivo in a dynamic mechanical environment, and musculoskeletal fracture during traumatic events lead to pain and immobilisation, especially significant in elderly and patients with metabolic bone diseases. To mitigate against these occurrences, it is essential to understand the dynamic mechanical response of bone over short timescales, but the underlying matrix-level mechanisms are not well understood. Here, we studied the mechanical response of cortical bone at nano- and micro-scale in a mouse model with glucocorticoid induced osteoporosis and their wild type littermates using real-time synchrotron small-angle X-ray diffraction (SAXD) combined with in situ compression testing and micro-tensile testing under controlled strain rates. Under compression, the tissue modulus, yield stress, effective fibril modulus and fibrillar reorientation rate in osteoporotic bone is significantly lower than that in healthy bone. Under tension, when going from low to high strain rates, the effective fibril modulus of healthy bone increase by a factor of 4.8, but this tendency is suppressed in osteoporotic bone. Also, bone microstructure in osteoporotic showed large fraction of cavities with disrupted mineralization. Our results demonstrate how the nano- and microscale deformation mechanisms of bone ultrastructure change in osteoporotic bone under compression and tension. Our results suggest that material level changes of bone matrix contributed to the reduced mechanical competence of bone in metabolic bone diseases such as osteoporosis.