Interstitial fluid-solid interaction within aneurysmal and non-pathological human ascending aortic tissue under translational sinusoidal shear deformation

ABSTRACT

The interaction shear force between internal interstitial fluid motion and the solid circumferential-longitudinal medial lamellae helps generate the shear stress involved in dissection of human ascending aorta aneurysmal or non-pathologic tissue. Frequency analysis parameters from the total shear stress versus time response to translational 1 Hz sinusoidal shear deformation over 50 cycles measure the interaction with respect to the three factors tissue type, sinusoidal deformation amplitude and direction of the shear deformation. Significant 1, 3, and 5 Hz components exist in this order of descending magnitude for shear deformation amplitudes of either 25% or 50% of the specimen length. Evaporation tests indicate that the amount of free water in both aneurysmal and non-pathological tissue is nearly the same. The interstitial fluid-solid interaction under shear deformation is visible in the shoulders of the total shear stress versus time response curve that are caused by the 3 Hz component. During a single deformation cycle, the ratio of the amplitudes of the 3 Hz and the 1 Hz components measures the normalized amount of interaction. Under translational sinusoidal shear deformation at 25% amplitude, this interaction ratio is statistically smaller in non-pathologic than in aneurysmal human ascending aortic tissue in the circumferential direction. The frequency analysis parameters provide evidence that the structural changes in aneurysmal tissue induce an increase in the interstitial fluid-medial solid interaction shear force which contributes to the propensity for aneurysmal rupture. STATEMENT OF SIGNIFICANCE: Circumferential shear force between the interstitial fluid and medial lamellae within the human ascending aortic wall is demonstrably greater in aneurysmal than non-pathologic tissue. This force likely increases with medial elastin degeneration and may facilitate the dissection propensity in aneurysmal tissue. The 3 Hz component in frequency analyses of the total shear stress versus time curve produced by 1 Hz sinusoidal translational shear deformation measures the fluid-solid interaction shear force that is otherwise difficult to isolate. This non-standard examination of the interstitial fluid interaction helps clarify clinical mechanical implications of structural differences between aneurysmal and non-pathologic human ascending aortic tissue. The aneurysmal dissection susceptibility does not appear to depend on the amount of interstitial fluid or the wall thickness compared to non-pathologic tissue.