Page 307 - Read Online

Page 2 of 12 Pejcic et al. Vessel Plus 2019;3:32 I http://dx.doi.org/10.20517/2574-1209.2019.18
[2]
replacements when compared to non-elective replacements . Therefore, early predictions of aneurysm
rupture risk can be of great benefit to patients. Currently, an aortic aneurysm diameter greater than
5.5 cm is an indication for intervention. However, in patients with connective tissue disorders, where
structural changes in the aortic wall are well defined, a much lower threshold of dilatation is warranted for
intervention; in Marfan Syndrome, a diameter of greater than 5.0 cm is an indication for surgery with a
[3]
lower threshold of 4.5 cm in the presence of further risk factors .
[4]
Aneurysm diameter and growth may not accurately predict the risk of aneurysm rupture and might lead
to undertreatment of those who have other contributing factors such as aortic stiffness or a connective
[5]
tissue disorder. Alternatively, overtreatment is also a possibility: Trabelsi et al. found that even at a
diameter of 6 cm only 31% of patients with aneurysms developed complications. Moreover, in the general
population, there is a large number of individuals with aortic diameters between 5.0 and 5.5 cm whose
[6]
risk of complications are not well understood . The Laplace Law is the theoretical basis of the diameter
guidelines; however, this law is valid for simple spheres and uniform cylinders and might not adequately
[1]
appreciate the complex geometrical nature of the native vessels .

The biomechanical analysis of diseased aortas could therefore represent an intriguing opportunity to
further elucidate the mechanisms leading to rupture even in the absence of connective tissue disorder.
It has long been recognized that a blood vessel cannot be considered as a passive conduit for blood flow.
Rather, a blood vessel is a continuously adapting, dynamic element with the purpose of maintaining
[7]
optimal function in response to changing hemodynamic conditions . Therefore, a more thorough
understanding of the biological composition and biomechanics of the vascular system is warranted.
There is a need for experimental data with an effort to determine the response of the aorta to a pulsatile
waveform. Biological tissues, though subject to conservation of mass, momentum and energy, have unique
constitutive equations that differentiate them from inorganic materials, and thus make them harder to
characterize. This review aims to present, in brief, the current state of the biomechanical characterization
of arterial walls, particularly the aorta, through discussion of testing methods and their findings. Moreover,
overarching concepts such as histological effects, age dependent effects, segmental effects, hemodynamic
effects, viscoelastic modelling and torsion will be briefly explored.

CURRENT TESTING METHODS
Uniaxial tensile test
Uniaxial tensile tests have been widely used to test biological tissues in vitro due to their simplicity and
ability to give precise information regarding local properties of soft tissues. While extending a piece of the
sample (either rectangular in shape, or dog-bone shaped), the displacement and resulting force are recorded
until fracture [Figure 1A]. This data can be used to obtain a variety of stress-strain relations, and depending
on the constitutive model chosen, authors may make use of different stress and strain parameters outlined
throughout Continuum Mechanics such as Cauchy stress, engineering stress, Second Piola-Kirchoff stress,
Green strain, true strain, and engineering strain.

With uniaxial tensile testing, one can obtain the ultimate tensile strength (strength most often recorded
at failure), the yield strength, as well as the strain at failure. A single value of Young’s modulus, which is
useful for defining non-biological materials, cannot be adequately used for blood vessels since they are
characterized by a non-linear elastic behaviour. A common method to define this parameter for biological
tissues is to calculate the modulus incrementally and report it for a specified range of stress. Additionally,
most authors make use of the maximum tangential modulus to describe vessel stiffness and make succinct
comparisons between intra-study specimens. Currently there is no standard for testing protocol, and
variations in experimental parameters such as the force range and number of cycles for preconditioning,