多尺度理论模型表明，除了孔隙率之外，皮质骨的与衰老相关的机械退化是由微观结构变化驱动的。
Multiscale theoretical model shows that aging-related mechanical degradation of cortical bone is driven by microstructural changes in addition to porosity.
机构信息
Mechanical Engineering, Boston University, 110 Cummington Mall, Boston, 02115, MA, USA; Center for Multiscale and Translational Mechanobiology, Boston University, 110 Cummington Mall, Boston, 02115, MA, USA.
Mechanical Engineering, Boston University, 110 Cummington Mall, Boston, 02115, MA, USA; Center for Multiscale and Translational Mechanobiology, Boston University, 110 Cummington Mall, Boston, 02115, MA, USA; Biomedical Engineering, Boston University, 44 Cummington Mall, Boston, 02115, MA, USA.
出版信息
J Mech Behav Biomed Mater. 2023 Sep;145:106029. doi: 10.1016/j.jmbbm.2023.106029. Epub 2023 Jul 18.
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 (ϵ, σ), ultimate point (ϵ, σ), and toughness (U) 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.
本研究旨在深入了解皮质骨微观结构的衰老相关变化如何在宏观尺度上产生力学后果。为此，将皮质骨建模为弹性-塑性平行纤维束，代表骨单位和间质组织，在单轴拉伸下加载。为了使每个纤维都具有独特的材料特性，我们将纤维分为骨单位纤维和间质纤维两类，并分别为这两类纤维赋予不同的特性。我们创建了具有代表性的成熟（20-60 岁）和老年（60 岁以上）皮质骨模型，通过以下方式改变输入参数来模拟衰老：（i）将孔隙率从 5%增加到 15%，（ii）将骨单位纤维数量与间质纤维数量的比例从 40%增加到 50%，以及（iii）将纤维材料特性从代表成熟骨样本改为代表老年骨样本（即，增加间质纤维的强度和韧性，同时降低骨单位纤维的韧性）。为了了解这些变化各自的贡献，我们还创建了单独改变其中一个或两个参数的模型。从纤维束的计算应力-应变曲线中，我们测量了整个纤维束的屈服点（ε，σ）、极限点（ε，σ）和韧性（U）。结果发现，只有改变这三个输入参数，模型才能捕捉到皮质骨机械性能与文献中报道的衰老相关下降一致的情况。在成熟和老年纤维束中，间质纤维的断裂导致了极限点后初始强度的丧失。而骨单位的塑性和更渐进的断裂则推动了其余的响应。在老年情况下，间质纤维断裂的开始和完成都发生在更低的应变下。这些发现表明，为了进一步了解与年龄相关的骨变化，除了孔隙率之外，还需要研究微观结构的变化，如骨单位的面积分数以及骨单位和间质组织的材料特性。
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