The composition and mechanical properties of abdominal aortic aneurysms.

PURPOSE: The composition and mechanical properties of abdominal aortic aneurysms (AAAs) were studied.
METHODS: Stereologic study was used to measure volume fractions of the components of the aortic wall. Histochemical methods with picrosirus red and safranin O were developed to differentiate collagen from ground substance because they are difficult to distinguish from each other on histologic sections. Uniaxial tensile stress tests were carried out on a tensile-testing machine, and a stress-strain curve was plotted for each sample to study the mechanical properties of AAAs. The curves were fitted exponentially so sigma = aeb epsilon, where sigma is stress, epsilon is strain, and a and b are parameters.
RESULTS: In aneurysms (n = 8) the volume fraction of elastin was decreased from 22.7% +/- 5.7% to 2.4% +/- 2.2%, and the volume fraction of smooth muscle cells was decreased from 22.6% +/- 5.5% to 2.2% +/- 2.0%, whereas the volume fraction of collagen and ground substance combined was increased from 54.8% +/- 4.5% to 95.6% +/- 2.5% compared with nonaneurysmal aortas (n = 8). There was no significant difference (p > 0.05) in the ratio of collagen to ground substance (2.1 +/- 0.5 vs 2.0 +/- 0.4) between AAAs and nonaneurysmal aortas. The elastic diagrams showed that AAAs (n = 7) are less distensible and stiffer than nonaneurysmal aortas (n = 5). Parameter a was unchanged (p > 0.5), but parameter b was significantly greater (p < 0.002) for aneurysmal aortas.
CONCLUSIONS: Both the composition and mechanical properties of AAAs are different from those of nonaneurysmal aortas. The aneurysms were stiffer, and the volume fractions of collagen and ground substance were increased, whereas the volume fractions of elastin and muscle were decreased in aneurysms.