P.08 Biomechanical Characterization of Ascending Thoracic Aortic Aneurysms in Humans: A Continuum Approach to in vivo Deformations

Abstract

Background Dysfunctional cellular mechanosensing appears central to aneurysm formation [1]. We aimed to derive material parameters of aneurysm tissue from in vivo deformations, which may increase insight into the underlying structural integrity of the pathological tissue. Methods Videos of tracking markers (example Video in supplement, screenshot in Figure) placed on ascending aortic segments were captured alongside radial arterial blood pressure in patients undergoing open-thorax ascending thoracic aorta aneurysm (ATAA) repair (n = 5) and coronary bypass (controls; n = 2). Normalised cross-correlation was used to determine marker displacements, resulting in estimates of systolic/diastolic diameters, distensibility, and cyclic axial engineering strain. A thinwalled, cylindrical geometry was assumed, with amorphous (Neo-Hookean) and fibrous (two-family) constitutive contributions [2]. This framework was fitted to individual patient measurements, by varying parameters c (amorphous material constant), k1 and k2 (fiber stiffness and strain stiffening parameter), β (fiber angle w.r.t. circumferential direction), unloaded intact length (L), and internal radius (Ri). Results Axial strain tended to be lower (expected) and distensibility larger (unexpected) in aneurysm than controls (Figure). However, the intrinsic pressure-dependence of distensibility must be considered when drawing conclusions related to differences in structural stiffness between both groups [3]. Material stiffness parameters (c and k1) appeared higher in aneurysm patients than in controls which is in line with previous studies in mice [4]. Conclusion We are developing a method to determine ATAA material properties from in vivo deformations and observed increased material stiffness in ATAA. Aneurysm Control Measured outcomes Diastolic diameter [mm] 40 ± 5 23 ± 3 DBP [mmHg] 58 ± 11 34 ± 2 SBP [mmHg] 90 ± 18 93 ± 7 Distensibility [MPa–1] 4.3 ± 3.0 3.7 ± 1.1 Axial strain [%] 4.3 ± 2.1 7.6 ± 3.5 Estimated properties c [kPa] 37 ± 29 15 ± 13 k [kPa] 43 ± 26 24 ± 24 R1 [mm] 17 ± 1 10 ± 1 β [degrees] 35 ± 3 36 ± 2 k2 – 34 ± 9 37 ± 3 L [mm] 24 ± 5 15 ± 2