Mapping local mechanical properties of human healthy and osteoporotic femoral heads

The complexity of bone tissue, its hierarchical multi-scale structure and the delicate mechanobiological equilibrium that regulates its architecture make the analysis of bone local mechanical characteristics highly challenging. The strong dependence between bone's mechanical properties and meso- and micro-architectural arrangement is particularly evident in the human femur, where loads are transmitted from the pelvis along the stress trajectories. An alteration in loads distribution generates a consequent deterioration in the structural pattern of trabecular groups, that worsen in presence of pathologies such as osteoporosis. Considering this complexity, it is crucial to map femur's local strength values, in order to capture their variability and to evaluate the possible causes of fragility and fracture. For this purpose, a portable micro-compression device compatible with a synchrotron is designed and realized: it allows to test millimetric samples cut along the stress trajectories from three healthy and three osteoporotic femoral heads and to real-time monitor the apparent stress-apparent strain curves. Intra-patients variability shows increased resistance to compression in the proximal sub-region, anatomically close to the acetabulum, where the loads are transmitted from the pelvis to the femur. Inter-patients variability show an interesting trend: non-osteoporotic femoral heads are characterized by a more uniform distribution in terms of Young moduli in the whole head, while non-uniform values are present in osteoporotic subjects.