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

Objective: 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. Methods: 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 (controlFLW) 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. Results: 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 controlFLW (collagen-volume fraction: 0.24 +/- 0.03 vs 0.12 +/- 0.03 and 0.08 +/- 0.02; elastinvolume fraction: 0.09 +/- 0.03 vs 0.28 +/- 0.03 and 0.39 +/- 0.04). Conclusions: 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.