Effect of layer heterogeneity on the biomechanical properties of ascending thoracic aortic aneurysms

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 c_θθ and c_zz , specifying circumferential and longitudinal stiffness, received the highest values in the adventitia or intima and the smallest in the media, with c_θθ > c_zz 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.