Multiscale theoretical model shows that aging-related mechanical degradation of cortical bone is driven by microstructural changes in addition to porosity.

This study aims to gain mechanistic understanding of how aging-related changes in the microstructure of cortical bone drive mechanical consequences at the macroscale. To that end, cortical bone was modeled as a bundle of elastic-plastic, parallel fibers, which represented osteons and interstitial tissue, loaded in uniaxial tension. Distinct material properties were assigned to each fiber in either the osteon or interstitial fiber "families." Models representative of mature (20-60 yrs.) bone, and elderly (60+) bone were created by modeling aging via the following changes to the input parameters: (i) increasing porosity from 5% to 15%, (ii) increasing the ratio of the number of osteon fibers relative to interstitial fibers from 40% to 50%, and (iii) changing the fiber material properties from representing mature bone samples to representing elderly bone samples (i.e., increased strength and decreased toughness of interstitial fibers together with decreased toughness of osteon fibers). To understand the respective contributions of these changes, additional models isolating one or two of each of these were also created. From the computed stress-strain curve for the fiber bundle, the yield point (ϵy, σy), ultimate point (ϵu, σu), and toughness (UT) for the bundle as a whole were measured. We found that changes to all three input parameters were required for the model to capture the aging-related decline in cortical bone mechanical properties consistent with those previously reported in the literature. In both mature and elderly bundles, rupture of the interstitial fibers drove the initial loss of strength following the ultimate point. Plasticity and more gradual rupture of the osteons drove the remainder of the response. Both the onset and completion of interstitial fiber rupture occurred at lower strains in the elderly vs. mature case. These findings point to the importance of studying microstructural changes beyond porosity, such as the area fraction of osteons and the material properties of osteon and interstitial tissue, in order to further understanding of aging-related changes in bone.

Trabecular Architecture and Mechanical Heterogeneity Effects on Vertebral Body Strength.

PURPOSE OF REVIEW: We aimed to synthesize the recent work on the intra-vertebral heterogeneity in density, trabecular architecture and mechanical properties, its implications for fracture risk, its association with degeneration of the intervertebral discs, and its implications for implant design. RECENT FINDINGS: As compared to the peripheral regions of the centrum, the central region of the vertebral body exhibits lower density and more sparse microstructure. As compared to the anterior region, the posterior region shows higher density. These variations are more pronounced in vertebrae from older persons and in those adjacent to degenerated discs. Mixed results have been reported in regard to variation along the superior-inferior axis and to relationships between the heterogeneity in density and vertebral strength and fracture risk. These discrepancies highlight that, first, despite the large amount of study of the intra-vertebral heterogeneity in microstructure, direct study of that in mechanical properties has lagged, and second, more measurements of vertebral loading are needed to understand how the heterogeneity affects distributions of stress and strain in the vertebra. These future areas of study are relevant not only to the question of spine fractures but also to the design and selection of implants for spine fusion and disc replacement. The intra-vertebral heterogeneity in microstructure and mechanical properties may be a product of mechanical adaptation as well as a key determinant of the ability of the vertebral body to withstand a given type of loading.