Assessing the Homogeneity of the Elastic Properties and Composition of the Pig Aortic Media

Abstract

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.

Tensile Residual Strains on the Elastic Lamellae along the Porcine Thoracic Aorta

“…It is widely accepted that elastin dominates the tensile properties of arteries up to the point of collagen recruitment just below physiological pressures [28, 33], yet waviness is observed in arteries fixed at pressures up to 60 and even at 100 mm Hg [28, 34, 35]. We note that the mean WI values for FU tissue was 1.23 in our study and that the mean in vivo stretch of the pig thoracic aorta is 1.20–1.26 [22, 36]; if the lamellae had to be straightened before they could bear a load, they would be under no load at physiological distension. Our results clearly show that wavy elastin can bear a load: L 0 mech matched the perimeter value and was 22% shorter than the lamellar length.…”

“…Although some studies have indicated that a single cut of a fresh ring releases all residual strains [14, 30, 36], more recent studies indicate that further cutting releases additional strain [15, 16, 18, 41, 42]. We have found that purifying an FC ring also produces further, albeit small, dimensional changes in the elastin (compare λ θ for FU/FC in fig.…”

Determination of the layer-specific distributed collagen fibre orientations in human thoracic and abdominal aortas and common iliac arteries

“…Differences were observed in intimal and internal muscular layers between different ethnic groups [2], and are also expected (in all likelihood) among different age groups of a single species [7,33]. In the human aorta, the elastin-to-collagen ratio decreases in progressively distal regions making the aorta most flexible at proximal regions [3].…”

Growth and remodeling in a thick-walled artery model: effects of spatial variations in wall constituents

“…However, to improve model convergence, a smooth transition between layers is assumed. In the model, the volume fraction of constituent i has a radial distribution given by (25) so that (26) For the baseline model, the volume fractions of collagen and elastin in the media are uniform and equal (Stergiopulos et al, 2001), with cells making up the remainder. The adventitia is also considered uniform, with collagen constituting most of the volume.…”