Assessing the homogeneity of the elastic properties and composition of the pig aortic media

Most previous studies of arterial wall elasticity and rheology have assumed that the properties of the wall are uniform across the thickness of the media and, therefore, that the relationship between stress and strain may be described by a constitutive equation based on a single strain energy function. The few studies where this assumption has been questioned, focussed on differences between the adventitia and the media rather than on differences within the media itself. Here, we report in vitro elasticity and residual strain measurements performed separately on the inner and outer half of the pig aortic media, together with a histomorphometric assessment of the radial distribution of elastin, collagen and smooth muscle cell numbers. Although we found that the pressure-diameter relationships of the two halves were dissimilar, when allowance was made for their different unloaded dimensions, their material properties agreed closely, a result in keeping with the observed uniform radial distribution of scleroprotein and vascular smooth muscle. We also found a difference in the opening angle (which is often taken as a measure of residual strain) between the inner and outer medial halves. However, strain analysis showed that the opening angle is an extremely sensitive measure of residual strain and that the difference in the actual magnitudes of residual strain between the two halves of the media was small. We conclude that the media of the porcine thoracic aorta has similar elastic properties throughout its thickness and that this uniformity is matched by a uniform distribution of matrix protein and vascular smooth muscle cells. Furthermore, the distribution of strain in the media can adequately be described by a single-layer model with uniform elastic properties throughout its thickness.