Stella John A, Sacks Michael S
Engineered Tissue Mechanics Laboratory, Department of Bioengineering, University of Pittsburgh, Pittsburgh, PA 15219, USA.
J Biomech Eng. 2007 Oct;129(5):757-66. doi: 10.1115/1.2768111.
All existing constitutive models for heart valve leaflet tissues either assume a uniform transmural stress distribution or utilize a membrane tension formulation. Both approaches ignore layer specific mechanical contributions and the implicit nonuniformity of the transmural stress distribution. To begin to address these limitations, we conducted novel studies to quantify the biaxial mechanical behavior of the two structurally distinct, load bearing aortic valve (AV) leaflet layers: the fibrosa and ventricularis. Strip biaxial tests, with extremely sensitive force sensing capabilities, were further utilized to determine the mechanical behavior of the separated ventricularis layer at very low stress levels. Results indicated that both layers exhibited very different nonlinear, highly anisotropic mechanical behaviors. While the leaflet tissue mechanical response was dominated by the fibrosa layer, the ventricularis contributed double the amount of the fibrosa to the total radial tension and experienced four times the stress level. The strip biaxial test results further indicated that the ventricularis exhibited substantial anisotropic mechanical properties at very low stress levels. This result suggested that for all strain levels, the ventricularis layer is dominated by circumferentially oriented collagen fibers, and the initial loading phase of this layer cannot be modeled as an isotropic material. Histological-based thickness measurements indicated that the fibrosa and ventricularis constitute 41% and 29% of the total layer thickness, respectively. Moreover, the extensive network of interlayer connections and identical strains under biaxial loading in the intact state suggests that these layers are tightly bonded. In addition to advancing our knowledge of the subtle but important mechanical properties of the AV leaflet, this study provided a comprehensive database required for the development of a true 3D stress constitutive model for the native AV leaflet.
所有现有的心脏瓣膜小叶组织本构模型要么假定跨壁应力分布均匀，要么采用膜张力公式。这两种方法都忽略了各层特定的力学贡献以及跨壁应力分布隐含的不均匀性。为了开始解决这些局限性，我们进行了新的研究，以量化两个结构不同、承受负荷的主动脉瓣（AV）小叶层——纤维层和心室层的双轴力学行为。具有极高灵敏度力传感能力的条带双轴试验，进一步用于确定分离的心室层在极低应力水平下的力学行为。结果表明，两层均表现出非常不同的非线性、高度各向异性的力学行为。虽然小叶组织的力学响应主要由纤维层主导，但心室层对总径向张力的贡献是纤维层的两倍，且承受的应力水平是纤维层的四倍。条带双轴试验结果还表明，心室层在极低应力水平下表现出显著的各向异性力学性能。这一结果表明，对于所有应变水平，心室层主要由周向排列的胶原纤维主导，且该层的初始加载阶段不能被建模为各向同性材料。基于组织学的厚度测量表明，纤维层和心室层分别占总层厚度的41%和29%。此外，完整状态下双轴加载时广泛的层间连接网络和相同的应变表明这些层紧密结合。除了增进我们对主动脉瓣小叶细微但重要的力学性能的了解外，本研究还提供了开发天然主动脉瓣小叶真实三维应力本构模型所需的全面数据库。
