Mechanical behavior of human trabecular bone after overloading

With the etiology of osteoporotic fractures as motivation, the goal of this study was to characterize the mechanical behavior of human trabecular bone after overloading. Specifically, we quantified the reductions in modulus and strength and the development of residual deformations and determined the dependence of these parameters on the applied strain and apparent density. Forty cylindrical specimens of human L1 vertebral trabecular bone were destructively loaded in compression at 0.5% strain per second to strains of up to 3.0% and then immediately unloaded to zero stress and reloaded. (An ancillary experiment on more readily available bovine bone had been performed previously to develop this testing protocol.) In general, the reloading stress-strain curve had a short initial nonlinear region with a tangent modulus similar to Young's modulus. This was followed by an approximately linear region spanning to 0.7% strain, with a reduced residual modulus. The reloading curve always approached the extrapolated envelope of the original loading curve. Percent modulus reduction (between Young's and residual), a quantitative measure of mechanical damage, ranged from 5.2 to 91.0% across the specimens. It increased with increasing plastic strain (r2 = 0.97) but was not related to modulus or apparent density. Percent strength reduction, in the range of 3.6-63.8%, increased with increasing plastic strain (r2 = 0.61) and decreasing apparent density (r2 = 0.23). The residual strains of up to 1.05% depended strongly on applied strain (r2 = 0.96). Statistical comparisons with previous data for bovine tibial bone lend substantial generality to these trends and provide an envelope of expected behavior for other sites. In addition to providing a basis for biomechanical analysis of the effects of damage in trabecular bone at the organ level, these findings support the concept that occasional overloads may increase the risk of fracture by substantially degrading the mechanical properties of the underlying trabecular bone.