Local wall thickness in finite element models improves prediction of abdominal aortic aneurysm growth

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Abstract

Objective:

Growing evidence suggests that peak wall stress (PWS) derived from finite element analysis (FEA) of abdominal aortic aneurysms (AAAs) predicts clinical outcomes better than diameter alone. Prior models assume uniform wall thickness (UWT). We hypothesize that the inclusion of locally variable wall thickness (VWT) into FEA of AAAs will improve its ability to predict clinical outcomes.

Methods:

Patients with AAAs (n = 26) undergoing radiologic surveillance were identified. Custom MATLAB algorithms generated UWT and VWT aortic geometries from computed tomography angiography images, which were subsequently loaded with systolic blood pressure using FEA. PWS and aneurysm expansion (as a proxy for rupture risk and the need for repair) were examined.

Results:

The average radiologic follow-up time was 22.0 ± 13.6 months and the average aneurysm expansion rate was 2.8 ± 1.7 mm/y. PWS in VWT models significantly differed from PWS in UWT models (238 ± 68 vs 212 ± 73 kPa;P= .025). In our sample, initial aortic diameter was not found to be correlated with aneurysm expansion (r= 0.26;P= .19). A stronger correlation was found between aneurysm expansion and PWS derived from VWT models compared with PWS from UWT models (r= 0.86 vsr= 0.58;P= .032 by Fisherrto Z transformation).

Conclusions:

The inclusion of locally VWT significantly improved the correlation between PWS and aneurysm expansion. Aortic wall thickness should be incorporated into future FEA models to accurately predict clinical outcomes.

Clinical Relevance:

There is growing evidence in the literature suggesting that peak wall stress derived from finite element analysis of abdominal aortic aneurysms is a superior predictor of clinical outcomes compared with traditional maximum diameter measurements. Almost all biomechanical analyses to date assume an aortic wall of uniform thickness. In this study, the inclusion of a variable wall thickness aortic wall into finite element studies not only significantly altered calculated peak wall stress, but it also improved its correlation to the clinically relevant outcome of aneurysm growth. These results suggest that stress analysis-derived rupture risk stratification may be further improved using variable wall thickness aortic reconstructions.

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