Biomechanical Simulation of Evolving Aortic Aneurysms for Designing Intervention

Yale University, New Haven CT

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Abstract

DESCRIPTION (provided by applicant): Abdominal aortic aneurysms (AAAs) are focal, asymmetric dilatations of the infrarenal aorta that are increasingly responsible for significant mortality in our aging society. The primary clinical (global) metrics for deciding intervention remain the maximum diameter and rate of expansion of the lesion. Yet, many smaller lesions rupture whereas many larger ones may not rupture over long periods. Because rupture occurs when wall stress exceeds wall strength, it should not be surprising that calculated (local) wall stress can be a better predictor of rupture-risk than current (geometric) metrics. Our work is motivated, however, by the need to estimate much better both wall stress and strength, that is, to move beyond current stress analyses that are simply snap shots in time based on patient-specific geometric models of lesions that are assumed to have the same uniform, population-averaged material properties regardless of size. In contrast, we will build a novel biochemomechanical computational model to simulate the patient-specific evolution an AAA consistent with the well accepted hypothesis that intramural cells can remodel matrix in response to perturbed mechanical stimuli and thereby generate regional heterogeneities that can offset complexities in geometry and hemodynamics that would otherwise cause marked regional variations in wall stress (and thus perturbed cell function). Complicating this mechano-stimulated evolution, however, is a hemodynamically driven intraluminal thrombus (ILT) that can upset the local balance in matrix production and removal and thereby locally weaken the wall. The primary goal of this Competing Renewal of HL086418 (funded in 2008 at the 6.3 percentile) is to build on a successful first 4 years of funding (22 papers published, plus 6 submitted/in preparation) and extend our current mechano-biological model to include local chemo-biological effects of an evolving ILT on AAA enlargement and rupture-risk. Our three Specific Aims are (i) to build and verify, via parametric studies exploiting idealized geometries, a novel fluid-solid-growth model, (ii) to use a well established rat elastase infusion model to correlate, for the firs time, local effects of thin vs. thick ILT on histological, cell biological, and mechanical propertis of the evolving AAA wall, and (iii) to use serial CT scans retrospectively from 30 patients to trai and test our final model, with the ultimate goal of providing increased confidence in predictions based on standard clinical information of the probable expansion or rupture-risk over the subsequent 6 to 12 months. In summary, the fundamental hypothesis driving this work is - the complexity of AAA pathophysiology and biomechanics results from competing effects of a mechano-stimulated injury response and a chronic inflammatory response, the latter of which depends primarily on spatiotemporal changes in ILT that are driven by the changing hemo- dynamics. We suggest that the only way to understand such complexity is to build and use an appropriate computational model, which we submit will help resolve longstanding controversies regarding the roles of ILT.