Aggarwal Ankush, Pouch Alison M, Lai Eric, Lesicko John, Yushkevich Paul A, Gorman Iii Joseph H, Gorman Robert C, Sacks Michael S
Center for Cardiovascular Simulation Institute for Computational Engineering & Sciences Department of Biomedical Engineering The University of Texas at Austin, Austin, TX, USA; Zienkiewicz Centre for Computational Engineering Swansea University, Swansea, UK.
Gorman Cardiovascular Research Group Department of Surgery University of Pennsylvania, Philadelphia, PA, USA.
J Biomech. 2016 Aug 16;49(12):2481-90. doi: 10.1016/j.jbiomech.2016.04.038. Epub 2016 May 6.
Residual and physiological functional strains in soft tissues are known to play an important role in modulating organ stress distributions. Yet, no known comprehensive information on residual strains exist, or non-invasive techniques to quantify in-vivo deformations for the aortic valve (AV) leaflets. Herein we present a completely non-invasive approach for determining heterogeneous strains - both functional and residual - in semilunar valves and apply it to normal human AV leaflets. Transesophageal 3D echocardiographic (3DE) images of the AV were acquired from open-heart transplant patients, with each AV leaflet excised after heart explant and then imaged in a flattened configuration ex-vivo. Using an established spline parameterization of both 3DE segmentations and digitized ex-vivo images (Aggarwal et al., 2014), surface strains were calculated for deformation between the ex-vivo and three in-vivo configurations: fully open, just-coapted, and fully-loaded. Results indicated that leaflet area increased by an average of 20% from the ex-vivo to in-vivo open states, with a highly heterogeneous strain field. The increase in area from open to just-coapted state was the highest at an average of 25%, while that from just-coapted to fully-loaded remained almost unaltered. Going from the ex-vivo to in-vivo mid-systole configurations, the leaflet area near the basal attachment shrank slightly, whereas the free edge expanded by ~10%. This was accompanied by a 10° -20° shear along the circumferential-radial direction. Moreover, the principal stretches aligned approximately with the circumferential and radial directions for all cases, with the highest stretch being along the radial direction. Collectively, these results indicated that even though the AV did not support any measurable pressure gradient in the just-coapted state, the leaflets were significantly pre-strained with respect to the excised state. Furthermore, the collagen fibers of the leaflet were almost fully recruited in the just-coapted state, making the leaflet very stiff with marginal deformation under full pressure. Lastly, the deformation was always higher in the radial direction and lower along the circumferential one, the latter direction made stiffer by the preferential alignment of collagen fibers. These results provide significant insight into the distribution of residual strains and the in-vivo strains encountered during valve opening and closing in AV leaflets, and will form an important component of the tool that can evaluate valve׳s functional properties in a non-invasive manner.
已知软组织中的残余应变和生理功能应变在调节器官应力分布方面发挥着重要作用。然而，目前尚无关于残余应变的全面信息，也没有用于量化主动脉瓣（AV）小叶体内变形的非侵入性技术。在此，我们提出一种完全非侵入性的方法来确定半月瓣中的异质应变——包括功能应变和残余应变，并将其应用于正常人体AV小叶。从心脏移植手术患者获取AV的经食管三维超声心动图（3DE）图像，在心脏切除后切除每个AV小叶，然后在体外以扁平状态成像。使用已建立的3DE分割和数字化体外图像的样条参数化方法（Aggarwal等人，2014年），计算体外和三种体内状态（完全打开、刚刚贴合和完全加载）之间变形的表面应变。结果表明，从小叶体外状态到体内打开状态，小叶面积平均增加20%，应变场高度异质。从打开状态到刚刚贴合状态的面积增加最大，平均为25%，而从刚刚贴合状态到完全加载状态几乎保持不变。从体外状态到体内收缩中期状态，基部附着附近的小叶面积略有收缩，而自由边缘扩张约10%。这伴随着沿周向-径向方向10°-20°的剪切。此外，在所有情况下，主拉伸大致与周向和径向方向对齐，最大拉伸沿径向方向。总体而言，这些结果表明，尽管AV在刚刚贴合状态下不支持任何可测量的压力梯度，但小叶相对于切除状态有明显的预应变。此外，小叶的胶原纤维在刚刚贴合状态下几乎完全被募集，使得小叶在全压力下非常僵硬，变形很小。最后，径向变形始终较高，周向变形较低，胶原纤维的优先排列使后者方向更僵硬。这些结果为AV小叶在瓣膜打开和关闭过程中遇到的残余应变和体内应变的分布提供了重要见解，并将成为能够以非侵入性方式评估瓣膜功能特性的工具的重要组成部分。
