Protective biomechanical and histological changes in the false lumen wall in chronic type B aortic dissection

The outer false lumen wall (FLW) changes from thin/compliant to thick/rigid as aortic dissection transitions from the acute to chronic phase. This study investigates biomechanical stiffness and histological changes of the FLW as the dissected aorta ages.

The free outer FLW from human tissue was analyzed from chronic type B dissection (chronic-FLW) n = 10, acute type A dissection (acute-FLW) n = 10, and transplant donor descending aorta that was manually peeled into 2 layers (control-FLW) n = 17. Biaxial tension testing in the longitudinal and circumferential directions was performed and stress-strain curves were obtained. A lower and higher tangent modulus was determined to assess stiffness. Quantification of collagen and elastin was performed by calculating the fibers' volume fraction from Z-stack scans.

The higher tangent modulus of chronic-FLW is larger (P < .01) than the acute-FLW and control-FLW in longitudinal (5.09 ± 0.9 MPa vs 1.72 ± 0.56 MPa and 1.17 ± 0.22 MPa) and circumferential (4.16 ± 0.67 MPa vs 1.04 ± 0.24 MPa and 1.07 ± 0.16 MPa) directions. The lower tangent modulus of chronic-FLW is larger (P < .05) than acute-FLW and control-FLW in both directions (longitudinal: 0.72 ± 0.24 MPa vs 0.13 ± 0.02 MPa and 0.27 ± 0.03 MPa circumferential:0.44 ± 0.13 MPa vs 0.12 ± 0.01 MPa and 0.21 ± 0.02 MPa). The volume fraction of collagen was increased (P < .01) and the volume fraction of elastin was decreased (P < .001) when comparing chronic-FLW, acute-FLW, and control-FLW (collagen-volume fraction: 0.24 ± 0.03 vs 0.12 ± 0.03 and 0.08 ± 0.02; elastin-volume fraction: 0.09 ± 0.03 vs 0.28 ± 0.03 and 0.39 ± 0.04).

As the acutely dissected aorta transitions to the chronic phase, the FL remodels by increasing collagen, decreasing elastin, and increasing aortic stiffness and thickness. This change in the chronic-FLW may be a protective adaptation to prevent FL enlargement and rupture in type B aortic dissection.