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Research Papers

Dissection Properties of the Human Aortic Media: An Experimental Study

[+] Author and Article Information
Gerhard Sommer, Martin Auer

Institute for Biomechanics, Graz University of Technology, 8010 Graz, Austria

T. Christian Gasser

Department of Solid Mechanics, Royal Institute of Technology (KTH), 100 44 Stockholm, Sweden

Peter Regitnig

Institute of Pathology, Medical University Graz, 8036 Graz, Austria

Gerhard A. Holzapfel1

Institute for Biomechanics, Centre for Biomedical Engineering,  Graz University of Technology, 8010 Graz, Austria; Department of Solid Mechanics, Royal Institute of Technology (KTH), 100 44 Stockholm, Swedenholzapfel@tugraz.at

1

Corresponding author.

J Biomech Eng 130(2), 021007 (Mar 28, 2008) (12 pages) doi:10.1115/1.2898733 History: Received July 21, 2006; Revised October 11, 2007; Published March 28, 2008

Aortic dissection occurs frequently and is clinically challenging; the underlying mechanics remain unclear. The present study investigates the dissection properties of the media of 15 human abdominal aortas (AAs) by means of direct tension tests (n=8) and peeling tests (n=12). The direct tension test demonstrates the strength of the media in the radial direction, while the peeling test allows a steady-state investigation of the dissection propagation. To explore the development of irreversible microscopic changes during medial dissection, histological images (n=8) from four AAs at different peeling stages are prepared and analyzed. Direct tension tests of coin-shaped medial specimens result in a radial failure stress of 140.1±15.9kPa (mean±SD, n=8). Peeling tests of rectangular-shaped medial strips along the circumferential and axial directions provide peeling force∕width ratios of 22.9±2.9mNmm(n=5) and 34.8±15.5mNmm(n=7); the related dissection energies per reference area are 5.1±0.6mJcm2 and 7.6±2.7mJcm2, respectively. Although student’s t-tests indicate that force∕width values of both experimental tests are not significantly different (α=0.05, p=0.125), the strikingly higher resisting force∕width obtained for the axial peeling tests is perhaps indicative of anisotropic dissection properties of the human aortic media. Peeling in the axial direction of the aorta generates a remarkably “rougher” dissection surface with respect to the surface generated by peeling in the circumferential direction. Histological analysis of the stressed specimens reveals that tissue damage spreads over approximately six to seven elastic laminae, which is about 15–18% of the thickness of the abdominal aortic media, which forms a pronounced cohesive zone at the dissection front.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

Arterial layers prepared from an aortic patch (adventitia, media, intima—from left to right)—donor: male, 63yr. The arrangement of the test specimens with respect to the medial patch is shown schematically on the central photograph. Cylindrical (coin-shaped) specimens were prepared for the direct tension test, and circumferential and axial oriented strip specimens for the peeling tests. The circumferential and axial directions of the artery are indicated by θ and z, respectively. The photographed centimeter scale provides a dimensional reference.

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Figure 2

Macroscopic view of a representative specimen ready for a peeling test, and sketch showing the 3D nature of the figure

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Figure 3

Schematic experimental setup

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Figure 4

Schematic direct tension fixing tool with a detailed drawing of the specimen-rod connection. Dimensions are given in millimeters.

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Figure 5

Representative photograph of the end stage of a successful direct tension test

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Figure 6

Representative image of a circumferentially oriented strip specimen during a peeling test acquired from the videoextensometer, and sketch showing the 3D nature of the image

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Figure 7

Photograph of a mechanically fixed specimen in the specimen holder

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Figure 8

Force-displacement behavior for all 8 coin-shaped specimens during the direct tension test. The thick (solid) curve characterizes the arithmetic mean response.

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Figure 9

Force∕width versus dissection path for all five circumferential strip specimens during the peeling test. The thick (solid) curve characterizes the arithmetic mean response.

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Figure 10

Force∕width versus dissection path for all seven axial strip specimens during the peeling test. The thick (solid) curve characterizes the arithmetic mean response.

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Figure 11

Representative histological images of the microstructure of (a) stretched and (b) unstretched human aortic media. Elastica van Gieson (EVG) staining, 4μm thick sections. Original magnification 800×.

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Figure 12

Histological images of a representative aortic media during peeling in (a) circumferential and (b) axial directions. Original magnification 20×. Histological images (c) and (d) represent magnifications of the dissection tips and highlight the irreversible mechanism of separation at the micrographic level. Original magnification 400×. EVG staining, 4μm thick sections.

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Figure 13

Representative histological images illustrating the “roughness” of the generated dissection surface during peeling in the (a) circumferential and (b) axial directions. EVG staining, 4μm thick sections. Original magnification 100×.

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