Biomedical Engineering Reference
In-Depth Information
11.2.4 SEM Imaging
Changes of tissue microstructure due to the propagation of the dissection were inves-
tigated observing the surfaces of delaminated halves of specimens from each group
at SEM. The aortic tissues were fixed in 2
.
5 % glutaraldehyde for one hour, dehy-
drated in a graded series of ethanol/water solutions, dried, and then sputter coated
with gold. The orientation of each specimen inside the microscope was aligned to
that of the dissection propagation before imaging.
11.3 Results
Figure
11.2
shows representative delamination curves for three separate LONG and
CIRC strips cut from an ATAA of a 56 yr old male patient with BAV and aortic
diameter of 46 mm. The initial 'ramp' phase of the loading curve corresponds to
stretching of the peel arms whereas the jagged plateau region shows the slow and
controlled propagation of the AoD. In delamination testing, the oscillation is typical
and is often referred in the rubber mechanics literature as unstable or 'stick-slip'
tearing; i.e., the delamination does not propagate at a steady rate, but is arrested and
re-initiated at irregular intervals.
A comparison of
S
d
between non-aneurysmal and ATAA with BAV and TAV
in both LONG and CIRC directions was performed (Fig.
11.3
). A significant dif-
ference was observed for the
S
d
(i.e. the resistance to AoD) in both LONG and
CIRC directions of the non-aneurysmal aorta (
S
d
,
LONG
=
149
.
0
±
7
.
6 and
S
d
,
CIRC
=
126
.
0
±
6
.
6,
n
=
7) with respect to that of BAV ATAAs (
S
d
,
LONG
=
100
.
0
±
4
.
1 and
S
d
,
CIRC
=
88
.
4
±
4
.
1,
n
=
16) and with respect to that of TAV ATAAs (
S
d
,
LONG
=
116
.
8
8). Furthermore, the significant dif-
ference between LONG and CIRC strengths for the non-aneurysmal group indi-
cates anisotropic (i.e., directionally-dependent) dissection properties of the human
ascending thoracic aorta. However, the aneurysmal aorta displays isotropic dissec-
tion properties since both BAV and TAV groups are not statistically difference in
LONG and CIRC orientations. The
S
d
of BAV ATAAs was significantly lower than
TAV ATAAs in both orientations.
Patient age or aneurysm diameter could affect the delamination strength of the
aneurysmal aorta. To determine if the difference in
S
d
between groups was age-
dependent, the
S
d
data for the non-aneurysmal and BAV groups as a function of the
patient age were fit by linear regression to calculate two new sets of extrapolated
S
d
for non-aneurysmal (non-aneurysmal
extr
) and BAV (BAV
extr, age
), respectively, at
the ages of each of the TAV patients, see Fig.
11.4
(A). To assess if the difference
in
S
d
between aneurysmal groups was diameter-dependent, the
S
d
data for the BAV
group as a function of the aneurysm diameter were fit by linear regression to obtain
a new set of extrapolated
S
d
for BAV (BAV
extr, dia
) at the aneurysm diameter of each
of the TAV patients, see Fig.
11.4
(B). For the latter analysis, the comparison was
performed only between aneurysmal groups since the
S
d
can not be extrapolated for
±
6
.
1 and
S
d
,
CIRC
=
109
.
1
±
5
.
2,
n
=
