Arbabi V, Pouran B, Weinans H, Zadpoor A A
J Biomech Eng. 2015 Jul;137(7). doi: 10.1115/1.4030070. Epub 2015 Jun 2.
Transport of solutes through diffusion is an important metabolic mechanism for the avascular cartilage tissue. Three types of interconnected physical phenomena, namely mechanical, electrical, and chemical, are all involved in the physics of transport in cartilage. In this study, we use a carefully designed experimental-computational setup to separate the effects of mechanical and chemical factors from those of electrical charges. Axial diffusion of a neutral solute Iodixanol into cartilage was monitored using calibrated microcomputed tomography micro-CT images for up to 48 hr. A biphasic-solute computational model was fitted to the experimental data to determine the diffusion coefficients of cartilage. Cartilage was modeled either using one single diffusion coefficient (single-zone model) or using three diffusion coefficients corresponding to superficial, middle, and deep cartilage zones (multizone model). It was observed that the single-zone model cannot capture the entire concentration-time curve and under-predicts the near-equilibrium concentration values, whereas the multizone model could very well match the experimental data. The diffusion coefficient of the superficial zone was found to be at least one order of magnitude larger than that of the middle zone. Since neutral solutes were used, glycosaminoglycan (GAG) content cannot be the primary reason behind such large differences between the diffusion coefficients of the different cartilage zones. It is therefore concluded that other features of the different cartilage zones such as water content and the organization (orientation) of collagen fibers may be enough to cause large differences in diffusion coefficients through the cartilage thickness.
溶质通过扩散进行运输是无血管软骨组织的一种重要代谢机制。三种相互关联的物理现象，即机械、电学和化学现象，都参与了软骨运输的物理过程。在本研究中，我们使用精心设计的实验 - 计算装置，将机械和化学因素的影响与电荷的影响区分开来。使用校准的微型计算机断层扫描（micro - CT）图像监测中性溶质碘克沙醇向软骨的轴向扩散，监测时间长达48小时。将双相溶质计算模型拟合到实验数据中，以确定软骨的扩散系数。软骨建模时要么使用单一扩散系数（单区模型），要么使用对应于软骨浅层、中层和深层区域的三个扩散系数（多区模型）。结果发现，单区模型无法捕捉整个浓度 - 时间曲线，并且对接近平衡的浓度值预测不足，而多区模型能够很好地匹配实验数据。发现浅层区域的扩散系数比中层区域的扩散系数至少大一个数量级。由于使用的是中性溶质，糖胺聚糖（GAG）含量不可能是不同软骨区域扩散系数存在如此大差异的主要原因。因此得出结论，不同软骨区域的其他特征，如水含量和胶原纤维的组织（取向），可能足以导致整个软骨厚度的扩散系数存在很大差异。
