Residual stress and strain in aortic segments

In the study of stresses and strains in vascular segments, it is generally assumed that the traction-free configuration assumed by a segment when there is no axial force and there are no intravascular and extravascular pressures is stress-free. To investigate the degree of validity of this assumption, 286 oval shaped rings were excised from three bovine and six porcine aortas and photographed. Radial cuts were made in these rings which opened up into horseshoe shapes and were also photographed. Smoothed boundary lengths at intimal and adventitial levels in the rings and their cut open configurations were measured from the photographs and the residual strains in the annular configuration relative to the open configuration were computed. It was found that: the average maximum residual intimal engineering strain in the uncut configuration was -0.082 for all nine aortas and -0.096 and -0.077 for the bovine and porcine aortas alone, respectively; the average maximum residual adventitial strain was 0.085 for all aortas, and 0.102 and 0.078 for the bovine and porcine aortas alone, respectively; an estimated average beneficial compressive stress of -0.188 X 10(5) Pa (corresponding to a strain level of -0.082) is available at the intimal level to counteract the in vivo tensile stress due to the intravascular pressure; an estimated average initial tensile stress of 0.195 X 10(5) Pa (corresponding to a strain level of 0.085) exists at the adventitial level which adds to the in vivo tensile stress due to the intravascular pressure. Although these stress levels are not large in comparison with the in vivo stress in the arterial wall, a detailed stress analysis must take into account these initial stresses.