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Research Papers

Strain Transfer Through the Aortic Valve

[+] Author and Article Information
Afshin Anssari-Benam, Himadri S. Gupta

 School of Engineering and Materials Science,  Queen Mary, University of London, Mile End Road, E1 4NS London, UK

Hazel R. C. Screen1

 School of Engineering and Materials Science,  Queen Mary, University of London, Mile End Road, E1 4NS London, UKH.R.C.Screen@qmul.ac.uk

1

Corresponding author.

J Biomech Eng 134(6), 061003 (Jun 08, 2012) (11 pages) doi:10.1115/1.4006812 History: Received November 23, 2011; Revised May 04, 2012; Posted May 11, 2012; Published June 08, 2012; Online June 08, 2012

The complex structural organization of the aortic valve (AV) extracellular matrix (ECM) enables large and highly nonlinear tissue level deformations. The collagen and elastin (elastic) fibers within the ECM form an interconnected fibrous network (FN) and are known to be the main load-bearing elements of the AV matrix. The role of the FN in enabling deformation has been investigated and documented. However, there is little data on the correlation between tissue level and FN-level strains. Investigating this correlation will help establish the mode of strain transfer (affine or nonaffine) through the AV tissue as a key feature in microstructural modeling and will also help characterize the local FN deformation across the AV sample in response to applied tissue level strains. In this study, the correlation between applied strains at tissue level, macrostrains across the tissue surface, and local FN strains were investigated. Results showed that the FN strain distribution across AV samples was inhomogeneous and nonuniform, as well as anisotropic. There was no direct transfer of the deformation applied at tissue level to the fibrous network. Loading modes induced in the FN are different than those applied at the tissue as a result of different local strains in the valve layers. This nonuniformity of local strains induced internal shearing within the FN of the AV, possibly exposing the aortic valve interstitial cells (AVICs) to shear strains and stresses.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

(a) An AV leaflet: 5-mm-wide strips are cut from the belly region, in either the circumferential or radial direction. (b) For macro-analysis, the specimen strips were ink-marked every 2 mm over a 10 mm length, resulting in 5 equidistance regions designated by C, RI, RII, LI, and LII, similar to the defined regions for analysis of FN strains. (c) Schematic of a strip specimen showing the 5 defined equidistance regions. Dots represent the boundary regions that were tracked by the confocal microscope. (d) A typical confocal microscopy image of cell nuclei within a boundary region.

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Figure 2

(a)–(f) A typical sequence of images of a group of cells tracked at each applied strain increment. The strain increases from (a) to (f), as given next to each image. (g) The frequency plot of nuclei displacement at each strain increment for the same group of cells. The strain increment is shown next to each distribution. Each distribution shows only the displacement occurring during that increment and is not cumulative.

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Figure 3

Schematic depicting the fibrous network within one of the defined regions of the specimen under tissue level deformation. Upon the application of strain to the sample (a), fibers rotate and displace, resulting in elongation of the entire network (b). The movement of the fibers can be inferred through monitoring the movement of the cell nuclei, as the cells (hollow circles) are attached to the fibers. Dashed boxes highlight the size of the field of view at either end of a specimen region.

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Figure 4

FN strains at different applied strains for specimens viewed from (a) the ventricularis layer and (b) the fibrosa layer, loaded in the circumferential direction. The network strains in the central region are significantly higher compared to those in other regions at applied strain levels above 8% and 10% in the ventricularis and fibrosa layers, respectively. FN strains for samples viewed from (c) the ventricularis layer and (d) the fibrosa layer, loaded in the radial direction. FN strains in central region become significantly higher than the other regions at strain levels above 18% and 24% for the ventricularis and fibrosa layers, respectively. No significant differences between microstrains in the symmetrical regions (RI-LI and RII-LII) were seen. Dashed lines indicate the 1:1 linear correlation between the applied strain and FN strain. The asterisks indicate statistical significance (p<0.05) between the central region and all other regions.

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Figure 5

Macrostrains across each sample region measured from (a) the ventricularis side and (b) the fibrosa side, loaded in the circumferential direction, and (c) the ventricularis side and (d) the fibrosa side, loaded in the radial direction. Dashed lines indicate the 1:1 linear correlation between macrostrains and applied strains.

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Figure 6

Difference in the FN elongation between the ventricularis and fibrosa layers in the circumferential direction: (a) the difference in each region that is unaffected by end effects (RI, C, LI) and (b) the summative difference in matrix elongation between the two layers from these 3 regions. The asterisks indicate a statistically significant difference (p<0.05) between the central region and RI or LI.

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Figure 7

Schematic showing the effect of the difference in elongation of the FN within the AV layers: the network (a) before and (b) after deformation. The FN is elongated more in the ventricularis layer (V) compared to the fibrosa (F). This will lead to internal shearing in the 1–3 plane.

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Figure 8

Internal shearing in the specimens at each applied strain increment: experimental data are compared to theoretical values

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