Genin Guy M, Kent Alistair, Birman Victor, Wopenka Brigitte, Pasteris Jill D, Marquez Pablo J, Thomopoulos Stavros
Department of Mechanical Aerospace and Structural Engineering, Washington University, St. Louis, Missouri 63130, USA.
Biophys J. 2009 Aug 19;97(4):976-85. doi: 10.1016/j.bpj.2009.05.043.
Attachment of dissimilar materials is a major challenge because high levels of localized stress may develop at their interfaces. An effective biologic solution to this problem exists at one of nature's most extreme interfaces: the attachment of tendon (a compliant, structural "soft tissue") to bone (a stiff, structural "hard tissue"). The goal of our study was to develop biomechanical models to describe how the tendon-to-bone insertion derives its mechanical properties. We examined the tendon-to-bone insertion and found two factors that give the tendon-to-bone transition a unique grading in mechanical properties: 1), a gradation in mineral concentration, measured by Raman spectroscopy; and 2), a gradation in collagen fiber orientation, measured by polarized light microscopy. Our measurements motivate a new physiological picture of the tissue that achieves this transition, the tendon-to-bone insertion, as a continuous, functionally graded material. Our biomechanical model suggests that the experimentally observed increase in mineral accumulation within collagen fibers can provide significant stiffening of the partially mineralized fibers, but only for concentrations of mineral above a "percolation threshold" corresponding to formation of a mechanically continuous mineral network within each collagen fiber (e.g., the case of mineral connectivity extending from one end of the fiber to the other). Increasing dispersion in the orientation distribution of collagen fibers from tendon to bone is a second major determinant of tissue stiffness. The combination of these two factors may explain the nonmonotonic variation of stiffness over the length of the tendon-to-bone insertion reported previously. Our models explain how tendon-to-bone attachment is achieved through a functionally graded material composition, and provide targets for tissue engineered surgical interventions and biomimetic material interfaces.
不同材料的附着是一项重大挑战，因为在它们的界面处可能会产生高水平的局部应力。在自然界最极端的界面之一存在着解决这个问题的有效生物学方法：肌腱（一种柔顺的、结构性的“软组织”）与骨骼（一种坚硬的、结构性的“硬组织”）的附着。我们研究的目标是建立生物力学模型，以描述肌腱与骨骼的连接处是如何获得其力学性能的。我们检查了肌腱与骨骼的连接处，发现有两个因素赋予了肌腱与骨骼过渡区域独特的力学性能分级：1）通过拉曼光谱测量的矿物质浓度梯度；2）通过偏振光显微镜测量的胶原纤维取向梯度。我们的测量结果促使人们对实现这种过渡的组织——肌腱与骨骼的连接处——形成一种新的生理学认识，即它是一种连续的、功能梯度材料。我们的生物力学模型表明，实验观察到的胶原纤维内矿物质积累的增加可以使部分矿化的纤维显著变硬，但前提是矿物质浓度要高于“渗流阈值”，该阈值对应于在每根胶原纤维内形成机械连续的矿物质网络（例如，矿物质连通性从纤维一端延伸到另一端的情况）。从肌腱到骨骼，胶原纤维取向分布的分散性增加是组织刚度的第二个主要决定因素。这两个因素的结合可能解释了先前报道的肌腱与骨骼连接处长度上刚度的非单调变化。我们的模型解释了肌腱与骨骼的附着是如何通过功能梯度材料组成来实现的，并为组织工程手术干预和仿生材料界面提供了目标。
