Cortical and trabecular mechanical properties in the femoral neck vary differently with changes in bone mineral density

Osteoporosis is an increasing burden for our aging society. Fracture risk assessment tool (FRAX) and areal bone mineral density (aBMD) have been mainly used as a surrogate, but only identify 46% of patients sustaining a hip fracture. Adding information about material and mechanical properties might improve the fracture risk prediction. In this study these properties were assessed of cortical and trabecular bone samples from the human femoral neck. In total, 178 trabeculae were obtained from 10 patients suffering a low-Trauma fracture and 10 healthy donors (from a previous study) and 141 cortical specimens were newly manufactured from 17 low-Trauma fracture patients and 15 controls. Cyclic tensile tests were performed to extract elastic, plastic, viscous, damage, and failure properties with a rheological model. No significant difference of any investigated property was determined. Interestingly, donor aBMD indicated a significant correlation with the post-yield behavior and damage accumulation (modulus degradation) of cortical bone. Cortical bone indicated a significantly larger apparent modulus (17.2 GPa), yield stress (50 MPa), viscosity (17.9 GPas), and damage accumulation (73%), but a decreased toughness (1.6 MJ/m3), than trabecular bone (8.8 GPa, 30 MPa, 9.3 GPas, 60%, 3.2 MJ/m3, respectively). Qualitatively, cortical bone displayed a linear-elastic phase, followed by a plastic phase with little post-yield hardening. In contrast, trabeculae yielded early, with a pronounced post-yield hardening phase and fractured at larger strains. Only a few correlations between donor mineral status and tissue mechanical behavior were found. It is suggested that the trabecularization of cortical bone with age and disease may not only result in a decreased bone mass, but further causes a transitioning from stiff elastic cortical to soft, viscous trabecular bone. This aspect warrants further investigation to determine its role in age-and osteoporosis-related bone fragility.