Biomechanical Properties of Human Ascending Thoracic Aortic Dissections

(a) CT image shows the ascending aorta obtained from the patient prior to surgery. The box indicates the approximate location which was dissected and mechanically tested in this study. (b) The excised tissue sample was attached to the biaxial testing instrument and stretched using various force controlled biaxial testing protocols. (c) Raw load–unload data from one representative experiment were obtained followed by tissue preconditioning and are shown for a sample subjected to loading ratio of 2C:2L and 2C: 1L in the circumferential and longitudinal directions, respectively.

Cauchy stress and Green strain data were obtained from multiple biaxial experiments for: (a) TAD tissues from group A (≤50 yr) and (b) those belonging to group B (>50 yr). Individual experiments are shown for each sample using 100 g base force in the circumferential or longitudinal directions that are represented by 1C:1L in the legend.

Equibiaxial stress–strain results were plotted for each individual sample in groups A and B in the circumferential and longitudinal directions. These were obtained by fitting the experimental results to a microstructurally motivated form of SEF as described earlier (Table 2).

Coefficients to the SEF were used to model the data from nonequibiaxial stress–strain experiments which were not used in the fitting procedure to assess the predictive capability of the model. Comparisons between the experimental results and the model are shown for samples in the two groups. The r² values indicate the goodness of fits and are shown for each sample.

Slopes to the elastin-dominated (E_E) and collagen-dominated (E_C) regions were obtained for equibiaxial stress–strain results and are shown for a sample. Transition points (T₁, T₂) were determined using the intersection of these lines.

To assess the possible differences in material properties within each group and those between the two groups: (a) Green strains were obtained at 25 kPa and 100 kPa for the circumferential and longitudinal directions and (b) tissue stiffness, measured as slope E_c to the curve at the high stretch range, was compared between groups. We do not see any differences in the tissue stiffness between the groups.