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Research Papers

Trans-Thrombus Blood Pressure Effects in Abdominal Aortic Aneurysms

[+] Author and Article Information
Clark A. Meyer, Carine Guivier-Curien

 Equipe de Biomécanique Cardiovasculaire, IRPHE UMR 6594 CNRS, Marseille 13451, France

James E. Moore

Department of Biomedical Engineering, Texas A&M University, TAMU 3120,  Zachry Engineering Center, College Station, TX 77843-3120

J Biomech Eng 132(7), 071005 (May 18, 2010) (7 pages) doi:10.1115/1.4001253 History: Received November 23, 2009; Revised February 02, 2010; Posted February 11, 2010; Published May 18, 2010; Online May 18, 2010

How much and how the thrombus supports the wall of an abdominal aortic aneurysm (AAA) is unclear. While some previous studies have indicated that thrombus lacks the mechanical integrity to support much load compared with the aneurysm wall, others have shown that removing thrombus in computational AAA models drastically changes aneurysm wall stress. Histopathological studies have shown that thrombus properties vary through the thickness and it can be porous. The goal of this study is to explore the variations in thrombus properties, including the ability to isolate pressure from the aneurysm wall, incomplete attachment, and their effects on aneurysm wall stress, an important parameter in determining risk for rupture. An analytical model comprised of cylinders and two patient specific models were constructed with pressurization boundary conditions applied at the lumen or the thrombus/aneurysm wall interface (to simulate complete transmission of pressure through porous thrombus). Aneurysm wall stress was also calculated in the absence of thrombus. The potential importance of partial thrombus attachment was also analyzed. Pressurizing at either surface (lumen versus interface) made little difference to mean von Mises aneurysm wall stress values with thrombus completely attached (3.1% analytic, 1.2% patient specific) while thrombus presence reduced mean von Mises stress considerably (79% analytic, 40–46% patient specific) in comparison to models without it. Peak von Mises stresses were similarly influenced with pressurization surface differing slightly (3.1% analytic, 1.4% patient specific) and reductions in stress by thrombus presence (80% analytic, 28–37% patient specific). The case of partial thrombus attachment was investigated using a cylindrical model in which there was no attachment between the thrombus and aneurysm wall in a small area (10 deg). Applying pressure at the lumen resulted in a similar stress field to fully attached thrombus, whereas applying pressure at the interface resulted in a 42% increase in peak aneurysm wall stress. Taken together, these results show that the thrombus can have a wall stress reducing role even if it does not shield the aneurysm wall from direct pressurization—as long as the thrombus is fully attached to the aneurysm wall. Furthermore, the potential for porous thrombus to transmit pressure to the interface can result in a considerable increase in aneurysm wall stress in cases of partial attachment. In the search for models capable of accurately assessing the risk for rupture, the nature of the thrombus and its attachment to the aneurysm wall must be carefully assessed.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic of pressurization cases for AAA loading with thrombus shown in gray and pressure applied at locations indicated by arrows. All cases begin with the same unloaded radial dimensions for thrombus interior (rti), thrombus-wall interface (rwt), and wall exterior (rwe), which were equal to 1.5 cm, 2.85 cm, and 3 cm, respectively.

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Figure 2

Surfaces of lumen, thrombus, and wall illustrated by surface nodes. Lateral view best illustrates the thickness distribution of thrombus for these patients.

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Figure 3

Partial attachment models with the region of partial attachment at the interface located in the central 10 deg—denoted by the dark line. Otherwise, these models have the same characteristics as the analytical models.

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Figure 4

Von Mises stress as a function of deformed radial position. TLP is pressurization at the lumen, NTLP is no thrombus, and TIP is pressurization at the interface. The aneurysm wall without thrombus has higher stress and a larger loaded diameter.

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Figure 5

Von Mises stress maps for the two patient specific models illustrating the role of pressurization surface and thrombus presence. Stress is highest in the case without thrombus (NTLP) and is increased by the most (>100 kPa), where the removed thrombus was thickest.

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Figure 6

Von Mises stresses in a partial attachment model. The partial attachment leads to stress concentrations within the wall only when pressure is applied at the interface to the wall only.

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