Page 142 - Read Online

Page 2 of 13 Sun et al. Vessel Plus 2020;4:13 I http://dx.doi.org/10.20517/2574-1209.2020.02

wall stress observed in males was higher than that in females.

Conclusion: To evaluate the risk of rupture of AAA more precisely and specifically, the present study proposes a
new prediction method (based on equations) that includes more indicators such as sex and morphology, based
on numerical biomechanical simulations, which were confirmed as such. This study provides a sex-specific clinical
reference to assess the aforementioned risk of AAA rupture.

Keywords: Abdominal aortic aneurysm, sex difference, morphology, peak stress, rupture risk assessment

INTRODUCTION
Abdominal aortic aneurysms (AAAs) are a high-risk vascular disease characterised by local expansion of
the abdominal aorta to more than 50% of its original diameter. If AAAs are not treated in time, it is likely
[1]
to expand and rupture eventually . Its prevalence in individuals older than 60 years is at least four times
[2]
[3]
higher in men than women . The mortality rate associated with a ruptured AAA is as high as 60%-80%
and about 15,000 such deaths occur in the United States every year . Clinically, the treatment for AAA
[4]
includes open repair and endovascular aneurysm repair.
Previously, indications for surgical treatment of AAA were based on the largest vessel diameter (greater
than 5.5 or 5 cm) . However, relying mainly on maximal transverse measurement as a criterion to
[5]
determine whether an AAA should be treated may delay the optimal timing for intervention in some
patients or even lead to serious consequences such as rupture. In recent years, other biomechanical risk
factors for AAA rupture have been used to more accurately predict its development, such as morphological
[8]
[6,7]
factors (maximum diameter, asymmetry index, wall thickness, etc.) . Vorp et al. established a three-
dimensional model of an AAA and found that an aneurysm which was asymmetric had a great influence
on the distribution of wall stress, and peak wall stress increased nonlinearly with greater asymmetry. From
a biomechanical point of view, a ruptured AAA occurs when local stress of the vascular wall exceeds the
mechanical strength of its material. Thus, the higher local stress is along the AAA’s wall, the higher the risk
[9]
of a ruptured AAA . Besides peak wall stress, blood flow rheology has also been considered. For example,
the wall shear stress induced by pulsatile flow, due to friction between blood flow and the inner wall of
an AAA, was found to influence the rupture risk and function of AAA by damaging the endothelium and
inducing AAA wall remodelling [10-13] .

Recent studies have found that the risk of AAA rupture has significant sex differences. The UK Small
Aneurysm Trial, after a 10-year (1991-2001) follow-up of 496 patients, found that 5% of 411 males died of
a ruptured AAA, while 14% of 85 females did, which suggests that the risk of rupture in affected women
[14]
was three to four times higher . The medical literature has also reported a myriad of publications about
the importance of sex differences [15-18] with poorer outcomes in women. This observation has also been
confirmed in a large number of observational studies and randomized controlled ones in recent years [19,20] .
[21]
As noted by Ash et al. , there has been significant debate among vascular specialists regarding AAA
[22]
between men and women. A much higher incidence is seen in men, with a male to female ratio of 4:1 .
Women though, are older at presentation, exhibit faster rates of AAA growth and thus have a higher risk of
[23]
rupture at lower diameters . As the debate surrounding the definition, diagnosis and treatment of AAA in
women continues, more specific guidelines are needed .
[24]

The mechanism underlying sex-dependent differences in AAA rupture is still unclear however. The reason
may be due to women having an aorta diameter that is generally smaller than in men. Therefore, for
similarly sized AAAs, the degree of expansion in females is greater, and consequently, the risk of rupture
increases accordingly, which suggests that the threshold for treatment of the maximum diameter in female