The dependence on anatomic site of trabecular bone structure-function relationships

Trabecular bone is a primary load-bearing tissue of the musculoskeletal system. Density and trabecular architecture are the structural features that govern the apparent mechanical properties, and these features vary greatly between anatomic sites. Understanding the dependence of trabecular bone structure-function relationships on anatomic site is important for the study of age-related bone fragility, including fracture risk assessment and prophylaxis. Study of the site-specificity of these relationships also reveals to what extent the mechanics of the bone adaptation process differs throughout the skeleton. Through a combination of experimental and computational methods, relationships between density, architecture, elastic modulus, and tensile and compressive yield properties were investigated for human trabecular bone from four sites: the vertebra, proximal tibia, femoral greater trochanter, and femoral neck. Mechanical properties were measured along the principal trabecular orientation. Elastic modulus was defined to be consistent with the concave downwards nonlinearity present in the initial portion of the stress-strain curves. Characterization of this nonlinearity identified contributions of rate-dependence, damage accumulation, and large deformations of the trabecular structure. Relationships between density and each of elastic modulus and yield stress depended on anatomic site. This site-dependence was largely attributed to inter-site variations in architectural anisotropy. In contrast, yield strain was relatively insensitive to density and did not depend on the degree of anisotropy. Yield strain was remarkably uniform within each site, but differed across sites. Anatomic sites that displayed lower compressive yield strains were identified as those with a greater propensity to undergo large deformations. Statistical correlations between yield strain and quantitative architectural measures indicated that intra-specimen variations in architecture were as important as mean values for a specimen in explaining the site-dependence of yield strain. The combined ability of density and architecture to capture the inter-site differences in mechanical properties suggests that elastic and yield properties of trabecular tissue do not differ substantially across sites. The relationships developed in this work provide increased precision in predictions of mechanical properties from structural measures. The results also indicate that a complex set of microstructural deformation mechanisms, spanning multiple length scales, are involved in the elastic and yield behavior of human trabecular bone.