VESSEL WALL HETEROGENEITY IS AN IMPORTANT FACTOR IN ABDOMINAL AORTIC ANEURYSM RUPTURE RISK PREDICTION

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Current indications for the repair of abdominal aortic aneurysms (AAA) emphasize aortic diameter as a marker for risk of rupture. However, the use of aortic size alone may overlook the role that vessel heterogeneity plays in assessing the risk of rupture. Biomechanics may provide a more nuanced approach to predict the behavior of AAA. The current study was undertaken as a feasibility trial to use pre-operative CT scanning to compute mechanical and fluid mechanical stress profiles on the aortic wall for patients undergoing open AAA repair, and to correlate these stress characteristics with aortic wall histopathology obtained via a sampling grid during aortic reconstruction. One patient with non-ruptured AAA (maximum diameter of 57mm) underwent routine CTA scan examination two weeks prior to the scheduled surgery. The pre-operative mechanical wall stress (MWS) and the time average wall shear stress (TAWSS) fields were predicted by means of finite element analysis (FEA) and computational fluid dynamics (CFD) simulations, respectively. The derived stress maps served as a guide during surgery to collect specimens from patient diseased aorta. Elastin fragmentation and collagen content was evaluated on the collected samples and the correlation between MWS, TAWSS and the underlying histological structure was investigated. The macroscopic MWS and TAWSS were distributed non-uniformly over the wall and large variability of collagen content and elastin fragmentation was observed within the aortic sampling grid.There was a moderate negative correlation between thrombus thickness (TT) and MWS (Pearson’s ρ=-0.43, p=0.03) and between TT and TAWSS (ρ= -0.47, p=0.02). Elastin fragmentation was strongly correlated with the percentage of collagen content in the media (ρ=0.94, p=0.02). The observed regional differences are a consequence of local response of tissue to both mechanical and biological triggers. Therefore, the use of aortic size alone is insufficient to characterize the large degree of heterogeneity present in AAA walls even when considering a single patient. Non- invasive methods to infer local properties would greatly increase the reliability of the pre-operative risk of rupture prediction.