Biomedical Engineering Reference
In-Depth Information
despite advances in diagnostics and surgical management (Bonnichsen et al.,
2011
).
The exact prevalence of AoD is difficult to determine, and most estimates are based
on necropsy studies with evidence in 1-3 % of all cases. The incidence of AoD is be-
lieved to be 5-30 cases per 1 million people per year, typically presenting in elderly
patients, and in males more frequently than females (ratio 3
1). AoD occurs more
typically in elderly patients in the presence of a tricuspid aortic valve (TAV) and in
relatively younger patients if a bicuspid aortic valve (BAV) is present. During the
first 24 to 48 hours, the mortality in patients not treated surgically is as high as 74 %
(Knipp et al.,
2007
). Even among patients treated with emergent aortic reconstruc-
tion, operative mortality averages 24 % worldwide as reported by the International
Registry of Acute Aortic Dissection (Rampoldi et al.,
2007
).
Type A ascending aortic dissections originate with an intimal tear typically oc-
curring near the sinotubular junction where the wall stresses are believed to be el-
evated (Coady et al.,
1999
). The intimal tear allows blood to enter the aortic wall,
splitting the media and progressively separating the medial plane along the longi-
tudinal direction of the aorta. This creates a new 'false lumen' which runs paral-
lel to the true lumen. The false lumen can reenter the true lumen anywhere along
the course of the AoD or exit through the adventitia resulting in frank rupture. As
the dissection extends distally, its propagation and re-entry follows unpredictable
courses that can result in life threatening ischemia of the heart, brain, abdominal
viscera, spinal cord and extremities (Johansson,
1995
). There are several risk fac-
tors predisposing patients to AoD. Among these are severe hypertension, connec-
tive tissue disorders such as the Marfan and Ehlers-Danlos syndromes, and bicuspid
aortic valve (BAV). The congenital malformation, bicuspid aortic valve is distinctly
associated with the development of ascending aortic dilatation imparting a marked
risk of AoD (Cripe et al.,
2004
) and occurs in 1 to 2 % of the population, mak-
ing it the most common congenital heart malformation (Bonderman et al.,
1999
;
Ward,
2000
). In the clinical practice of ATAA reported by Gleason (
2005
), over
40 % of patients undergoing elective surgical replacement of the ascending aorta
indeed have BAV. Additionally, older necropsy studies shown a risk of fatal dissect-
ing aneurysm in BAV patients nine times higher than patients with tricuspid aortic
valve (TAV) (Edwards et al.,
1978
; Larson and Edwards,
1984
). The histopathologic
analyses of AoD and aneurysms appear strikingly different from those of abdominal
aortic aneurysms (AAA). Thoracic aneurysms have distinct histopathologic charac-
teristics classified as cystic medial degeneration, which is non-inflammatory and
in stark contradistinction to AAAs showing inflammatory characteristics (Davies,
1998
; El-Hamamsy and Yacoub,
2009
).
From a biomechanical point-of-view, the AoD of ATAAs involves a separation
(i.e. a delamination) of the elastic layers of the degenerated aortic wall that oc-
curs when the hemodynamic loads exerted on the aneurysmal wall exceed adhesive
forces that normally hold the mural layers together.
The purpose of this work was therefore to quantify the biomechanical properties
of ATAA samples relative to non-aneurysmal human ascending aorta and to distin-
guish specific differences in the biomechanical properties of ATAAs from BAV and
TAV patients. This was achieved performing biomechanical delamination tests, fol-
lowed by tensile tests on the delaminated halves to show the distinct strengths of the
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