Identifying indices of vulnerability in the aneurysmal abdominal aorta: The interplay between mechanics and morphology

An abdominal aortic aneurysm is the pathological dilatation of the abdominal aorta that can grow silently and rupture without warning. Over time, the vessel wall becomes weaker as inflammatory processes take over and the microarchitecture is compromised. Understanding the behaviour of the aneurysm wall at the macro and microscopic level can help elucidate the rupture potential of the vessel. This study proposes a novel method in assessing regions-specific differences by which we section the aorta into patches that can be traced back to specific areas on the aneurysm. The present study is thus an exploratory approach at assessing the aneurysms of multiple patients to establish differences between aneurysms, within aneurysms, and compare against healthy tissue. We established these differences, among others, using a variety of methods that assess the tissue microstructure, inflammation, composition, and mechanical response to loading. We also demonstrated the mechanical and structural heterogeneity in case studies exploring region-specific differences within the same patient. Coupling exploration into the pathophysiology of the aneurysm with its mechanical behaviour allowed us to paint a better picture of the disease, with mechanics often explaining biology and vice versa. We conclude that the pathological abdominal aorta exhibits a disruption in its extracellular matrix, profound inflammation, stiffer behaviour, and increased energy loss when compared with non-aneurysmal tissue. Ultimately, rupture risk assessment strategies need to utilize patient-specific parameters, and region-specific considerations need to be made

Heterogeneity of Ex Vivo and In Vivo Properties along the Length of the Abdominal Aortic Aneurysm

The current clinical guidelines for the management of aortic abdominal aneurysms (AAAs) overlook the structural and mechanical heterogeneity of the aortic tissue and its role in the regional weakening that drives disease progression. This study is a comprehensive investigation of the structural and biomechanical heterogeneity of AAA tissue along the length and circumference of the aorta, by means of regional ex vivo and in vivo properties. Biaxial testing and histological analysis were performed on ex vivo human aortic specimens systematically collected during open repair surgery. Wall-shear stress and three-dimensional principal strain analysis were performed to allow for in vivo regional characterization of individual aortas. A marked effect of position along the aortic length was observed in both ex vivo and in vivo properties, with the central regions corresponding to the aneurysmal sac being significantly different from the adjacent regions. The heterogeneity along the circumference of the aorta was reflected in the ex vivo biaxial response at low strains and histological properties. Present findings uniquely show the importance of regional characterization for aortic assessment and the need to correlate heterogeneity at the tissue level with non-invasive measurements aimed at improving clinical outcomes