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Page 2 of 13 Pisano et al. Vessel Plus 2020;4:33 I http://dx.doi.org/10.20517/2574-1209.2020.21

INTRODUCTION
[1]
The two most widespread diseases of the thoracic aorta are aneurysms (TAA) and dissections (TAD) . In
the United States, TAA is the 18th most common cause of death. TAA has an incidence rate of 10 cases
[2,3]
per 100,000 patients per year and a prevalence of 0.16% to 0.34% in the general population . Men are
more like to have TAA compared to women; however, women tend to develop worse clinical outcomes
[4]
and have an increased risk of TAD . It is important to closely monitor TAA patients. At the same time,
optimal surgical timing is crucial to improve survival. Cardiac surgery aims to prevent TAD or rupture
[5]
of the aneurysm . As a predictor of adverse aneurysmal outcomes, aortic diameter is still the most used
[6-9]
criteria . However, several studies have found that in a particular group of patients, complications may
occur at smaller aortic sizes than we would predict [10-12] . In our opinion, it is necessary to investigate other
parameters that better identify these high risk TAA patients for which earlier surgical intervention is
[13]
necessary and at smaller aortic size . The aim of this review is to analyze the biological, morphological,
and biomechanical network as a potential useful tool to detect TAA subjects at higher risk of complications
behind the diameter.

Role of the ascending aorta diameter in predicting acute aortic dissection
The in-hospital mortality rate of TAD is nearly 30% . Until now, the only prevention is monitoring of
[14]
the ascending aorta dilation and performing prophylactic surgical replacement. Although hypertension
and specific genetic syndromes are well known risk factors of TAD, it is still difficult to predict this deadly
condition with accuracy [15,16] . Current guidelines recommended surgery when the ascending aorta size
reaches 5.5 cm for non-syndromic patients and 4.5 cm in syndromic patients . However, data from the
[17]
[18]
International Registry of Acute Aortic Dissections showed that aortas could dissect at smaller sizes than
that advocated in the guidelines. Among 591 type A TAD, 59% occurred at sizes less than 5.5 cm and 40%
occurred at < 5.0 cm. These data correspond with our center’s experience. Among 326 patients treated
for Type A TAD in our Cardiac Surgery Department from April 2005 to March 2018, 212 patients had
[20]
[19]
a maximal diameter less than 5.5 cm . Svensson et al. showed that 12.5% of 40 bicuspid aortic valve
(BAV) patients with TAD had aortic sizes < 5 cm at the time of surgery. The same aortic diameter has been
detected in Marfan population. In addition, several studies have showed that the aortic diameter before
TAD is much smaller than after TAD. In experimental studies of human and porcine cadaver specimens,
[21]
Williams et al. showed that the onset of TAD caused a significant increasing of the aortic diameter (140%)
in relationship to the hydrostatic pressure and to the percentage of the dissected aortic wall. Neri et al.
[22]
calculated pre-dissection aortic size from surgical specimens withdrawn from 220 individuals who
underwent surgery for acute type A TAD. Using a specific explant technique, they performed cylinders
of fresh aortic tissue and measured the inner layer of the true aortic lumen in the absence of perfusion
pressure. The median ascending aorta size was 41.4 mm for the entire cohort. These authors concluded that
that only 10% of the study population had aneurysms before TAD onset. It is very important to remember
that looking only at the number of people operated for TAD with small diameter is not sufficient to
determine the relative risk of TAD at sizes < 5.5 cm. That number has to be put into context by knowing
how many people at those smaller diameters exist so that an actual risk can be determined. Accordingly,
[23]
Paruchuri et al. calculated the relative risk of TAD at sizes < 5.5 cm by analyzing both the number of
occurring dissections (numerator) and the population at risk at each aortic size (denominator). They found
that in the general population a large percentage of subjects (79.2%) had an aortic diameter < 3.5 cm and
only the 0.22% of subjects had an aortic diameter ≥ 4.5 cm. Yet, while the majority of TAD may occur at
aortic diameters below the surgical threshold, it is also true that the vast majority of aortas within this
population are considerably smaller than this threshold. Thus, the true statistical risk of TAD at small aortic
diameters may well be negligible given the anticipated enormous patient pool in the small aortic size range.
However, there is a group of patients in which TAD may occur at smaller aortic sizes than the guidelines
predicted. This questions the true prognostic value of the absolute aortic diameter and emphasizes the need
for optimal timing of surgical intervention, especially in those patients under surveillance who do not meet