Measurement and analysis of ultimate mechanical properties, stress-strain curve fit, and elastic modulus formula of human abdominal aortic aneurysm and nonaneurysmal abdominal aorta.

OBJECTIVE: The maximal strain, stress, elastic modulus, and stress-strain curve fitting of abdominal aortic aneurysms (AAA) and bidirectional nonaneurysmal abdominal aorta (NAA) were measured and analyzed to obtain the ultimate mechanical properties, the more approximate stress-strain curve-fitting, and the elastic modulus formula of AAA and NAA.
METHODS: Fourteen human AAA samples were harvested from patients undergoing elective aneurysm repair. Twelve NAA samples comprised of six longitudinal-circumferential pairs of NAA from six cadaveric organ donors were used as controls. Samples were mounted on a tensile-testing machine and force was applied until breakage occurred. The maximal strain, stress, and elastic modulus were calculated and a stress-strain curve was plotted for each sample. Exponential and second-order polynomial curves were used to fit the stress-strain curve, and the means were estimated by comparing the R2 (coefficient of determination that represents the strength of a curve fitting). Coefficients of elastic modulus were calculated and analyzed, and the incremental tendency of each modulus was evaluated by comparing the difference of coefficients.
RESULTS: There was no significant difference in maximal stress among AAA, circumferential aortic aneurysms (CAA), and longitudinal aortic aneurysms (LAA). However, AAA maximal strain was significantly less (P < .01) than that of bidirectional NAA. AAA maximal elastic modulus was significantly greater than that of CAA and LAA (P < .01 and .05, respectively). R2 of AAA for second-order polynomial curve was significantly greater (P < .05) than that for the exponential curve. For the elastic modulus formula from the second-order polynomial curve, E = 2ax + b, the average value of a for the AAA was significantly greater (P < .01) than that for the bidirectional NAA, but there was no significant difference (P > .05) among the three groups for the average value of b.
CONCLUSIONS: Tensile test measurements can successfully analyze ultimate mechanical properties of AAA and NAA. AAA is stiffer and less distensible than NAA under the same maximal stress. Second-order polynomial curve fitting provides a more approximate description for AAA stress-strain curve than exponential curve fitting does. Formula variables a of the elastic modulus formula from second-order polynomial curve fitting can determine the incremental tendency of the elastic modulus, while b has negligible effect on the incremental tendency of the elastic modulus.