Cardiovascular disease (CVD) is the leading cause of death in the United States and a major cause of disability worldwide. A common type of vascular diseases in the U.S. is aortic aneurysm. Surgical interventions, such as open surgery or endovascular aneurysm repair (EVAR), are often required to avoid the high risk associated with aneurysm rupture. To better understand the role of biomechanics in aortic diseases and develop their clinical interventions, there is a need to understand the mechanical behavior of healthy as well as diseased aortas and their effects on aneurysm expansion and rupture potential. While it is well known that the mechanical properties of a blood vessel vary with location and age, little attention has been paid to its circumferential variations. Therefore, the goal of this study is to investigate spatial variations in the mechanical behavior of the descending thoracic aorta. Toward this end, the following has been accomplished: 1) a biaxial experimental apparatus with a stereo-vision system, which allows to track the three dimensional (3D) motion of the aorta during the inflation test, was developed, 2) the inflation tests at a fixed longitudinal stretch ratio were performed for two longitudinal portions - the proximal and distal - and four circumferential regions - the anterior, left lateral, posterior, and right lateral - of the porcine thoracic aorta, 3) stress-strain analysis were developed based on the approximation of the aortic wall surface using a set of continuous base functions in a curvilinear coordinate system, 4) the variations of stretch, stress, stiffness defined as a change in the circumferential stress corresponding to a change in the circumferential stretch, and pressure-strain elastic modulus were statistically analyzed, and 5) material parameters were estimated by a parameter estimation method using a constitutive model based on the constrained mixture approach. The experimental results showed that the posterior region was much stiffer than the anterior region. However, the physiological stiffness represented by the pressure-strain elastic modulus did not show significant difference among the circumferential regions for the proximal and distal portions of the thoracic aorta. In addition, the stress showed significant difference among the circumferential regions and the stretch was relatively uniform. In the parameter estimation, material parameters of elastin and collagen were dominant in the mechanical response, but the role of the smooth muscle seemed to be insignificant. A significant difference was found in the parameters of the elastin and collagen fiber between the anterior and posterior regions.In conclusion, this study presents an experimental method and analysis to measure local deformation of a blood vessel. Furthermore, it shows that there exist consistent spatial variations in the mechanical properties of the thoracic aorta. These findings increase our understanding of vascular mechanics and adaptation, and will eventually help to improve clinical treatments and interventions of vascular diseases.
Read
