Local mechanical properties of human ascending thoracic aneurysms

Abstract

Ascending thoracic aortic aneurysms (ATAAs) are focal, asymmetric dilatations of the aortic wall which are prone to rupture. To identify potential rupture locations in advance, it is necessary to consider the inhomogeneity of the ATAA at the millimeter scale. Towards this end, we have developed a combined experimental and computational approach using bulge inflation tests, digital image correlation (DIC), and an inverse membrane approach to characterize the pointwise stress, strain, and hyperelastic properties of the ATAA. Using this approach, the pointwise hyperelastic material properties were identified on 10 human ATAA samples collected from patients undergoing elective surgery to replace their ATAAs with a graft. Our method was able to capture the varying levels of heterogeneity in the ATAA from regional to local. It was shown for the first time that the material properties in the ATAA are unmistakably heterogeneous at length scales between 1 mm and 1 cm, which are length scales where vascular tissue is typically treated as homogeneous. The distributions of the material properties for each patient were also examined to study the inter-and intra-patient variability. Large inter-subject variability was observed in the elastic properties.

Nondestructive mechanical characterization of developing biological tissues using inflation testing

“…This reduction was most evident in the fiberreinforced materials, where more parameters were to be estimated. Without such an initial estimate, an inverse FE analysis with 3D elements is computationally costly, therefore previous studies typically resorted to faster methods such as shell meshes [18,21,22,23,39,36] or analytical solutions as used in the initial estimate [20,37]. These methods can provide excellent estimates for thin membranes, but possibly lead to inaccurate material parameter estimates when the tissues become too thick or feature residual stretches.…”

“…US imaging is key to this setup, as it allows for measurements of the tissue deformation during bulge testing, a test that can now be performed inside the bioreactor. Previous studies have typically measured the tissue deformation during bulging using stereo camera setups [19,21,22,23,37,39] or advanced optical rigs [18,40], which are incompatible with the limited view of samples insides a bioreactor. In contrast, US is able to measure throughout the tissue cross-section, and can therefore be easily mounted on top of a bioreactor.…”

“…Finally, biological planar tissues are not always thin enough to be considered as membranes, leading to bending moments and out-of-plane stresses. Several studies have successfully incorporated tissue nonlinearity and anisotropy in parameter estimation schemes of bulge tests [18,19,20,21,22,23]. Yet, no study has, to the best of the authors' knowledge, addressed the effects of tissue thickness, nor has tissue prestretch been accounted for.…”

Investigation on the Regional Loss Factor and Its Anisotropy for Aortic Aneurysms

“…Strain energy functions have been used for detailed biomechanical characterization of the ATA and ATAA walls [6,7,9,11,12,13,14]. Energy functions have the advantage of providing information on the entire tensile stress and strain ranges.…”

“…The mechanical properties of the ATA wall show directional dependency [17,18,19,20] and that it is inhomogeneous with respect to region and the aortic layers [8,13,17,19]. The mechanical response of the ATA is governed by the properties of its layers, the intima, media, and adventitia, which could be associated with non-uniform tissue remodelling during the progression of aortic aneurysms [21,22].…”