Our findings contradict previous studies on ascending thoracic and abdominal aortic aneurysms, suggesting that the former might not cause weakening but rather only stiffening and reduction in tissue extensibility and elastin content. Marked heterogeneity was evident in healthy and aneurysmal aortas. The present data offer insight into the pathogenesis of aneurysm dissection. Information on directional and regional variations is pertinent because dissections develop circumferentially and bulging preferentially occurs in the anterior region.

No abstract

Ascending thoracic aortic aneurysms (ATAA) were resected from patients during graft replacement and non-aneurysmal vessels during autopsy. Tissues were histomechanically tested according to region and orientation, and the experimental recordings reduced with a Fung-type strain--energy function, affording faithful biomechanical characterisation of the vessel response. The material and rupture properties disclosed that ATAA and non-aneurysmal aorta were stiffer and stronger circumferentially, accounted by preferential collagen reinforcement. The deviation of microstructure in the right lateral region, with a longitudinal extracellular matrix and smooth muscle element sub-intimally, reflects the regional differences in material properties identified. ATAA had no effect on strength, but caused stiffening and extensibility reduction, corroborating our histological observation of deficient elastin but not collagen content. Our findings may serve as input data for the implementation of finite element models, to be used as improved surgical intervention criteria, and may further our understanding of the pathophysiology of ATAA and aortic dissection.

This study addressed layer-specific differences in the biomechanical response of ascending aortic aneusysms, obtained from patients during graft replacement. Tensile tests were conducted on pairs of (orthogonally directed) intimal, medial, and adventitial strips from the anterior, posterior, and two lateral quadrants. The experimental data were reduced by the Fung-type model, affording appropriate characterization of the material properties. Testing of individual layers beyond rupture disclosed their failure properties, namely their capacity to bear varying deformation and stress levels. Material parameters [Formula: see text] and [Formula: see text], specifying circumferential and longitudinal stiffness, received the highest values in the adventitia or intima and the smallest in the media, with [Formula: see text] > [Formula: see text] in every layer but the intima. Similar extensibility at failure was found among layers, whereas the adventitia was the strongest of all. Circumferentially and longitudinally directed strips from each layer did not show uniform material parameters and failure properties among regions, but most differences did not reach significance. Medial and adventitial but not intimal layers were stronger circumferentially than longitudinally. This is the first study to place emphasis on the biomechanical properties of the distinct layers of human aneurysmal aorta that may be expected to shed light into the mechanisms promoting aneurysm dissection and rupture.