The Importance of Patient-Specific Regionally Varying Wall Thickness in Abdominal Aortic Aneurysm Biomechanics

Abstract

Abdominal aortic aneurysm (AAA) is a vascular condition where the use of a biomechanics-based assessment for patient-specific risk assessment is a promising approach for clinical management of the disease. Among various factors that affect such assessment, AAA wall thickness is expected to be an important factor. However, regionally varying patient-specific wall thickness has not been incorporated as a modeling feature in AAA biomechanics. To the best our knowledge, the present work is the first to incorporate patient-specific variable wall thickness without an underlying empirical assumption on its distribution for AAA wall mechanics estimation. In this work, we present a novel method for incorporating regionally varying wall thickness (the "PSNUT" modeling strategy) in AAA finite element modeling and the application of this method to a diameter-matched cohort of 28 AAA geometries to assess differences in wall mechanics originating from the conventional assumption of a uniform wall thickness. For the latter, we used both a literature-derived population average wall thickness (1.5 mm; the "UT" strategy) as well as the spatial average of our patient-specific variable wall thickness (the "PSUT" strategy). For the three different wall thickness modeling strategies, wall mechanics were assessed by four biomechanical parameters: the spatial maxima of the first principal stress, strain, strain-energy density, and displacement. A statistical analysis was performed to address the hypothesis that the use of any uniform wall thickness model resulted in significantly different biomechanical parameters compared to a patient-specific regionally varying wall thickness model. Statistically significant differences were obtained with the UT modeling strategy compared to the PSNUT strategy for the spatial maxima of the first principal stress (p ¼ 0.002), strain (p ¼ 0.0005), and strain-energy density (p ¼ 7.83 e-5) but not for displacement (p ¼ 0.773). Likewise, significant differences were obtained comparing the PSUT modeling strategy with the PSNUT strategy for the spatial maxima of the first principal stress (p ¼ 9.68 e-7), strain (p ¼ 1.03 e-8), strain-energy density (p ¼ 9.94 e-8), and displacement (p ¼ 0.0059). No significant differences were obtained comparing the UT and PSUT strategies for the spatial maxima of the first principal stress (p ¼ 0.285), strain (p ¼ 0.152), strain-energy density (p ¼ 0.222), and displacement (p ¼ 0.0981). This work strongly recommends the use of patient-specific regionally varying wall thickness derived from the segmentation of abdominal computed tomography (CT) scans if the AAA finite element analysis is focused on estimating peak biomechanical parameters, such as stress, strain, and strain-energy density.

Patient specific stress and rupture analysis of ascending thoracic aneurysms

“…While each of these factors plays a role in precision of the wall stress calculation, it is generally accepted that an accurate geometry is the most important. Other investigators have performed sensitivity studies to evaluate the effect of assuming constant thickness (Raut et al, 2013b;Shang et al, 2013), different degrees of material nonlinearity (Polzer et al, 2013;Reeps et al, 2010) and patient specific material properties (Doyle et al, 2013;Polzer et al, 2013). By far the use of patient specific material properties along with the use of a nonlinear material model has the greatest impact changing the peak wall stress by 70-170%.…”

Biomechanical rupture risk assessment of abdominal aortic aneurysms based on a novel probabilistic rupture risk index

“…For this work, we used a biomechanical model, which was as sophisticated as reasonably/practically possible, and regarded AAA wall thickness as the only uncertain input information. Not knowing the individual AAA wall thickness is constantly mentioned as a key limitation of AAA biomechanics simulations, and the wall thickness naturally affects PWS predictions, see among others [54][55][56]. Specifically, we applied lognormal-distributed and extreme value-distributed wall thicknesses (according to in vitro measurements [7]), which aimed at capturing distinctive effects of this input uncertainty but did not account for the known significant spatial wall thickness variation within rsif.royalsocietypublishing.org J. R. Soc.…”

The influence of intraluminal thrombus on noninvasive abdominal aortic aneurysm wall distensibility measurement

“…While it is known that wall thickness varies within and between patients [15] and may affect the resulting wall stress estimates, and there is currently no noninvasive technique available to measure it accurately. However, if the spatial average of our patient-specific variable wall thickness was known and used, the results would probably not change significantly, as far as the spatial maxima of the first principal stress, strain, strain-energy density, and displacement [28].…”