Aneurysmal Diseases Presentation O-9 S11

VESSEL ASYMMETRY AS AN ADDITIONAL TOOL FOR ANEURYSM RUPTURE RISK ESTIMATION

Barry Doyle (1), Anthony Callanan (1), David Vorp (2) and Tim McGloughlin (1)

1. Centre for Applied Biomedical Engineering Research (CABER), University of Limerick, Ireland; 2. Dept. of Vascular Remodelling and Regeneration, University of

Pittsburgh, USA.

Introduction An abdominal aortic aneurysm (AAA) can be defined as a permanent and irreversible localised dilation of the infrarenal aorta. This localised dilation is a result of a degradation of the elastic media of the arterial wall. Approximately 500,000 new cases are diagnosed each year worldwide resulting in 15,000 deaths per year in the USA alone [Kleinstreuer, 2005]. Currently, surgical intervention is decided based on the maximum diameter of the AAA. Most repairs are performed when the diameter exceeds 50-60mm. It has been shown that maximum diameter may not be a reliable predictor of rupture, as smaller AAAs can also rupture. In this research, the asymmetry of the AAA is examined in ten realistic cases. Methods Computed topography (CT) scan data was obtained for ten patients from our AAA database. Seven male patients and three female patients were randomly selected. All ten patients were awaiting AAA repair, as AAA diameters had reached or exceeded the current 5cm threshold for repair. This CT data was then reconstructed using the commercially available software, Mimics v10.0 (Materialise, Belgium), before being exported to ABAQUS v6.6-2 (SIMULIA, Rhode Island, USA) for finite element analysis. This allowed stress distributions to be examined on the surfaces of each AAA, and relationships between stress and geometrical parameters to be analysed. Results The numerical computations produced detailed stress distributions on each of the AAA models under the pressure loading. The results showed that the regions of peak wall stress occurred at regions of inflection on the surface of the AAA models. These results are consistent with previous work, both experimental [Morris, 2005] and numerical [Vorp, 1998]. The von Mises wall stress varies with respect to the asymmetry of the AAA centreline. The region of maximum centreline asymmetry experienced a region of elevated wall stress. As the anterior region of the AAA bulges outwards, the posterior region is often constrained from radial expansion by the spinal column, and

results in elevated posterior wall stress. It has been reported [Darling, 1977] that 82% of AAAs rupture on the posterior wall, and therefore, posterior wall stress may have a significant impact on AAA rupture. The peak wall stresses recorded in this study ranged from 0.3157–0.9584 MPa, with a mean ± standard deviation value of 0.482 ± 0.197 MPa.

Figure 1: Posterior wall stress along the length of an example AAA. Proximal neck is on the left and distal region on the right. Wall stress increases with an increase in centreline asymmetry. Discussion In this study, ten patient-specific AAAs were reconstructed, and wall stress distributions in each aneurysm were estimated using the finite element method. In order to examine the affect the asymmetry of the AAA has on wall stress, a simple method of calculating asymmetry was established. Once the asymmetry was determined for each patient, the von Mises wall stresses are compared to it. These relationships clearly show how wall stress in the posterior region of the AAA is determined by the degree of asymmetry in the anterior region. Therefore, as posterior wall stress is regionally higher than anterior wall stress, and can be linked to anterior bulging, these findings may have clinical importance. References Darling et al, circulation, 56(II):161-164, 1977. Kleinstreuer et al, Biomed Eng Online, 5:19, 2005. Morris et al, Strain, 40: 165-172, 2005. Vorp et al, J Vasc Surg, 27(4): 632-639, 1998.

16th ESB Congress, Oral Presentations, Monday 7 July 2008 Journal of Biomechanics 41(S1)