Abstract
Current clinical practice for aneurysmatic interventions is often based on the maximum diameter of the vessel and/or on the growth rate, although rupture can occur at any diameter and growth rate, leading to fatality. For 27 medial samples obtained from 12 non-aneurysmatic (control) and 9 aneurysmatic human descending thoracic aortas we examined: the mechanical responses up to rupture using uniaxial extension tests of circumferential and longitudinal specimens; the structure of these tissues using second-harmonic imaging and histology, in particular, the content proportions of collagen, elastic fibers and smooth muscle cells in the media. It was found that the mean failure stresses were higher in the circumferential directions (Control-C 1474kPa; Aneurysmatic-C 1446kPa), than in the longitudinal directions (Aneurysmatic-L 735kPa; Control-L 579kPa). This trend was the opposite to that observed for the mean collagen fiber directions measured from the loading axis (Control-L > Aneurysmatic-L > Aneurysmatic-C > Control-C), thus suggesting that the trend in the failure stress can in part be attributed to the collagen architecture. The difference in the mean values of the out-of-plane dispersion in the radial/longitudinal plane between the control and aneurysmatic groups was significant. The difference in the mean values of the mean fiber angle from the circumferential direction was also significantly different between the two groups. Most specimens showed delamination zones near the ruptured region in addition to ruptured collagen and elastic fibers. This study provides a basis for further studies on the microstructure and the uniaxial failure properties of (aneurysmatic) arterial walls towards realistic modeling and prediction of tissue failure. Statement of SignificanceA data set relating uniaxial failure properties to the microstructure of non-aneurysmatic and aneurysmatic human thoracic aortic medias under uniaxial extension tests is presented for the first time. It was found that the mean failure stresses were higher in the circumferential directions, than in the longitudinal directions. The general trend for the failure stresses was Control-C > Aneurysmatic-C > Aneurysmatic-L > Control-L, which was the opposite of that observed for the mean collagen fiber direction relative to the loading axis (Control-L > Aneurysmatic-L > Aneurysmatic-C > Control-C) suggesting that the trend in the failure stress can in part be attributed to the collagen architecture. This study provides a first step towards more realistic modeling and prediction of tissue failure.
Highlights
Aneurysms and dissections of thoracic aortas are life threatening conditions with high mortality rates even though clinical procedures have been improved in recent years [1,2,3]
Current practice guidelines recommend surgical repair of large thoracic aortic aneurysms to prevent fatal aortic dissection or rupture, but observations have shown that adverse aortic events may already occur at smaller diameters [7]
More than 80% of aortic dissections develop in the absence of a pre-existing aneurysm, indicating that aneurysm formation and dissection are in general different pathophysiological conditions [8]
Summary
Aneurysms and dissections of thoracic aortas are life threatening conditions with high mortality rates even though clinical procedures have been improved in recent years [1,2,3]. The medial layer is responsible for the main structural and functional properties of the thoracic aorta [12,13] It consists of many medial lamellar units each having a thickness of about 13-15 μm, with smooth muscle cells in the lamellar unit interconnecting the elastic laminas, and interspersed collagen fibers (mostly of type I and III) [14]. The outer layer, the adventitia, consists mainly of thick bundles of wavy collagen fibers (mostly type I) in the unloaded state, and is very compliant at small strains but changes to a stiff ‘jacketlike’ tube at high strains so that the artery is prevented from overstretching and rupture [10,11,15]. Schriefl et al [16] described the media and the adventitia of aged thoracic aortas as displaying two collagen fiber families organized in separate sublayers with different orientations. This organization in layers is less clear with sometimes a third or a fourth family of fibers being apparent, and in general displaying a carpet-like structure [16]
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