Page 313 - Read Online

Page 8 of 12 Pejcic et al. Vessel Plus 2019;3:32 I http://dx.doi.org/10.20517/2574-1209.2019.18

review is not intended to cover studies on hemodynamics and associated arterial mechanics; however,
to fully understand the complexity of characterizing the vasculature wall and the mechanisms that may
lead to cardiovascular complications, the fluid-structure interactions - for example, wave propagation in
arterial walls, local hemodynamics, and temporal wall shear stress - should be considered. Therefore, in
brief and without discussion of methods employed, several key concepts are mentioned. While the flow of
blood is generally laminar, in some cases this flow can be disrupted and result in transitional conditions
[Supplementary Figure 1], which may contribute to certain cardiovascular pathologies. Transitional flow
in the presence of hypercholesterolemia has been proven to prime the vessel wall for the pathogenesis of
[61]
atherosclerosis . Transitional blood flow has also been suggested as a cause of post-stenotic dilatation,
however this association may be due to the common occurrence of turbulence alongside stenosis of the
blood vessel wall [62,63] .

Transition has been proven to significantly increase pressure and shear stress within aneurysmal regions.
Khanafer et al. [11,64] suggest that this may result in a self-perpetuating mechanism of further dilatation and
subsequent increase of turbulence in the region. It has been suggested that the associated hemodynamics
through an aneurysm, such as recirculating flow, may result in the formation of thrombi [64,65] . The majority
of the literature; however, supports the hypothesis that the formation of an aneurysm is a multi-factorial,
[66]
degenerative process , not solely affected by hemodynamics and mechanical wall stress, but including
inflammation and immune response, molecular genetics, and degradation of surrounding connective
[64]
tissue .

Viscoelastic modelling
Large arteries are viscoelastic, which entails distinct mechanical behaviour compared to typical elastic
models and calls for analysis of time-dependent behaviour. Due to this viscous component, there is energy
retained within the arterial wall upon unloading, which is seen through hysteresis present in the stress-
strain, and pressure-diameter curves of arteries [67,68] . Hysteresis loops may be used to estimate damping
[69]
capacity, which is associated with the ratio of the dissipated energy to the stored energy . An interesting
[70]
study on strain-rate effect of mechanical properties by Delgadillo et al. revealed that at a stretch ratio of
1.5, the experienced load within the arterial wall is reduced by 20% when the strain rate is increased from
10 to 200 %/S. They suggested that: “this behavior might be a consequence of the faster fluidization and
small re-solidification that occurs in the cell at higher deformation rates”.

Torsion
While the response of arteries to axial stretching and circumferential stretching has long been studied
and used to quantify failure of the arterial wall, the effect of torsion has been explored to a lesser
extent, despite the fact that in vivo, arteries are often subject to twisting along the longitudinal direction
with body movement [71,72] . Klein et al. [72] found that there was a significant change in arterial length,
curvature and twist in the femoropopliteal arteries when subjects were cross legged compared to straight
legged. Furthermore, the abdominal aorta and common iliac arteries exhibit significant morphological
deformations from musculoskeletal motion. Hence, torsion is of particular concern since it has been
identified as a possible contribution to failure of stents in the more mobile arteries [71-74] . Further study on
the shortening, twisting and bending patterns of these arteries with stenting is required.

CONCLUSION
The inter-individual heterogeneity of the aorta’s geometry and composition, and the distinct differences in
[26]
regional mechanical properties , fuel the difficulty behind understanding the underlying mechanics of
the aorta. Uniaxial tests provide data regarding local mechanical properties and provide base comparisons
between diseased and healthy arteries [4,8-10,12,75] . However, biaxial tests provide a better estimate of the
multiaxial and anisotropic properties of arteries [14-18] . Both tests allow for data collection of incremental