Constitutive Behavior of Coronary Artery Bypass Graft

Abstract

Implantation of coronary artery bypass graft (CABG) is still one of frequent revascularization methods. Biomechanics of a CABG wall may significantly affect its successful function, nevertheless, papers which concern with a constitutive modeling of the CABG are lacking in literature. The purpose of this study is to describe a constitutive behavior of the CABG tissue. A sample of the CABG underwent three years of remodeling under arterial conditions. This sample was analyzed in an inflation–extension test (vertically aligned closed-end tube was loaded by an internal overpressure and by varying axial force). Displacements of the specimen were recorded by CCD cameras and subsequently evaluated by digital image cross-correlation. The experiment comprised preconditioning cycles and measurement periods. Mechanical response of the CABG was studied using four different values of an axial force (weight). A pressure – circumferential stretch and an axial force – axial stretch data are presented. Presence of the so called inversion point in the pressure – axial stretch data was observed, manifested by an elongation of a pressurized tube under some critical value of axial stretch and by a shortening as soon as the axial stretch exceeds the critical value. This interesting phenomenon was previously reported for iliac arteries. Now we may confirm it for saphenous vein graft also. Selected data were used to fit a material model. The tissue was modeled as a one-layered composite reinforced by two families of helical fibers. The material was assumed to be locally orthotropic, nonlinear, incompressible and hyperelastic. Material parameters were estimated for the strain energy function based on a limiting fiber extensibility assumption. Model parameters are fitted by optimization based on radial and axial equilibrium equation in the thick–walled tube. Material model fits selected data successfully. Further work will be aimed at extension of material model domain on all measured data.