Keywords
endovascular repair - abdominal aortic aneurysm - aneurysm - aortic valve disease
- endograft placement - repair - artery
The abdominal aorta is the most common site of an aortic aneurysm. The visceral and
most proximal infrarenal segment (aneurysm neck) are usually spared and considered
more resistant to aneurysmal degeneration. However, if an abdominal aortic aneurysm
(AAA) is left untreated, the natural history of the aortic neck is progressive dilatation
and shortening.[1] This may have significant implications for patients undergoing endovascular repair
of AAAs (EVAR) as endograft stability and integrity of the repair are dependent on
an intact proximal seal zone. Compromised seal zones, caused by progressive diameter
enlargement and foreshortening of the aortic neck, may lead to distal endograft migration,
type Ia endoleak, aortic sac repressurization, and, ultimately, aortic rupture.
Is There Evidence of Progressive Neck Dilatation?
Is There Evidence of Progressive Neck Dilatation?
Postoperative structural changes at the proximal neck of AAAs have been noted for
years and their occurrence is unsurprising, given the already compromised integrity
of the aneurysmal vessel wall. Illig et al reported significant dilatation of the
AAA neck in about one-third of patients undergoing open surgical repair (OSR),[2] and Falkensammer et al calculated the annual rate of dilatation to be 0.16 mm in
the infrarenal segment and 0.18 mm in the suprarenal segment following OSR.[3] This supports the theory of progressive structural deterioration but is of limited
clinical importance for patients who have undergone OSR. On the other hand, progressive
aortic neck diameter enlargement is more concerning in patients undergoing EVAR with
self-expanding endografts, which represent the vast majority of commercially approved
devices. The generally accepted belief is that proximal neck dilatation leading to
type Ia endoleaks is due to outward radial forces exerted by stent grafts on the aortic
wall and, as long-term follow-up data of patients after EVAR accumulates, cases of
aortic neck dilatation (AND) beyond the nominal proximal diameter of the endograft
are identified ([Fig. 1]). Setting a threshold of 2 mm to define AND, Oberhuber et al reported an increase
in diameter after both EVAR and OSR both at the infrarenal level (22.3 and 19.6%,
respectively; p = 0.87), and at the suprarenal level (20.4 and 30.4%, respectively; p = 0.26) with the former occurring more frequently after OSR and the latter after
EVAR.[4] Interestingly, there were no significant differences between the dilatation rates
at both levels when comparing EVAR with OSR. This raises a question regarding how
much additional stress the endograft outward radial force poses to the aneurysm neck.
The authors suggested that neck dilation is likely “multifactorial” and an intrinsic
element of the progression of aortic disease, rather than due to any one discrete
process. Several other reports also suggest the process to be much more complex than
pure mechanical outward force. In fact, Georgakarakos et al notes that endovascular
treatment of AAAs should not be considered a single step intervention based on preoperative
anatomical features alone, but rather that the aorta continues to remodel around the
endograft for years after implantation.[5] This concept is essential for the modern vascular surgeon to consider when planning
the initial AAA intervention and any subsequent adjunctive procedures to enhance longevity
of the repair.
Fig. 1 Progressive dilatation of the aortic neck at the level of the left (lowest) renal
artery captured in follow-up computed tomography (CT) angiograms obtained at 2 months
(A), 13 months (B), and 27 months (C) from the time of endograft implantation, with development of type Ia endoleak (D) and distal endograft migration (E).
When considering loss of seal at the proximal neck, two major factors come into play:
neck dilatation and endograft distal migration. These may occur independently or,
indeed, be caused by one another. There is no doubt that endografts without active
fixation may migrate distally even in the absence of significant neck dilatation.
More commonly, however, a gradual outpouching of the aortic wall causes loss of endograft
apposition and distal displacement resulting in pressure transmission onto the adjacent
unlined wall and, ultimately, further dilatation. It had previously been observed[6]
[7] and then proven via computational stimulation by Georgakarakos et al that the factors
most responsible for augmentation of displacement forces are the inlet (neck) diameter
and the inlet-to-outlet ratio, with greater neck angulations also appearing to influence
maintenance of an intact proximal seal zone. In fact, almost all AAAs repaired with
endografts undergo some degree of proximal neck deterioration, clinically significant
or not, with the one exception of extremely favorable anatomies (small diameter necks
and little to no angulation). These are becoming increasingly uncommon in current
clinical practice, as EVARs outside the manufacturer's instructions for use (IFU)
now represent a significant percentage of all endovascular AAA repairs. Several reviews
support the observations by Georgakarakos et al and show a significantly higher rate
of proximal seal zone complications and type Ia endoleaks in patients with “hostile”
neck anatomy when compared to those with “favorable” anatomy.[8] Hostile anatomic characteristics include short aortic neck length (< 15 mm), large
(> 29 mm) preoperative aortic neck diameter, high suprarenal and/or infrarenal neck
angulation, reverse taper or conical neck configuration, and larger amount of neck
thrombus and calcium. Using a 3-mm threshold, we recently reported that post-EVAR
dilatation of the aortic neck 5 mm distal to the lowest renal artery in patients with
hostile neck anatomy was seen in 12.5% of all patients 1 month post-EVAR, and in 8.1%
of patients between 1 month and 1 year following endograft implantation.[9] Independent risk factors predicting AND in our study included: larger preoperative
aortic diameter at the level of the lowest renal artery, use of endografts with suprarenal
stents, and higher degree of endograft oversizing. Kret et al also reported that endograft
oversizing was the most significant factor linked to AND.[10] Aggressive oversizing (> 25%) is frequently chosen to compensate for adverse neck
characteristics such as short or conical necks. Multiple device-specific studies documented
this result as well and noted that the maximum diameter reached by most dilated necks
was the same diameter as that of the implanted device.[11]
[12] The hypothesis of these studies is that the wall merely dilates to accommodate the
oversized graft and then stops once it is fully incorporated. Although this observation
may seem to support the notion that outward radial forces play the greatest role in
proximal neck dilation, another possible explanation is that of suboptimal apposition
of the aggressively oversized endograft to the aortic wall and more extensive endograft
pleating that may create an environment of local microleaks. These, in turn, lead
to increased pressure on the aortic wall at the proximal seal zone that may be equally,
if not more, important in the progressive enlargement of the proximal neck diameter.
Tsilimparis et al, in their investigation of the effects of the Zenith endograft on
neck dilation, observed continuous progression of neck diameter growth at the 30-day,
2-year, and 5-year mark, with the greatest rates of growth occurring in the immediate
postperiod and, to a lesser extent, at 3 years.[13] The early steep growth curve was attributed to the initial accommodation of the
aortic wall to the newly implanted graft and all subsequent change to the ongoing
degeneration of the diseased aortic wall. This would explain why and how proximal
neck dilation occurs both in endovascular and after open repair, albeit at different
rates.[14]
Can Aortic Neck Dilatation Be Prevented?
Can Aortic Neck Dilatation Be Prevented?
Most reports on post-EVAR AND have studied proximal neck diameter changes after EVAR
with self-expanding stents (SES), but one notable study looked at AND incidence after
repair using balloon expandable stent (BES) grafts.[15] The authors looked at AND following implantation of a homemade aortouniiliac stent
graft consisting of a polytetrafluoroethylene (PTFE) graft affixed to a proximal Palmaz
stent. This graft was chosen for implantation in patients who were excluded from commercially
available devices due to unfavorable neck anatomy (acute neck angulation > 60° or
short necks < 15 mm). There were no incidences of AND or endograft migration reported.
The authors suggested two possible explanations: First, the lack of stent incorporation
into the aortic wall with SES[16] as opposed to the Palmaz stent which becomes a part of the aortic wall with greater
success. Second, the fact that SES are subject to repetitive motion within the aorta
leading to material “fatigue.” This ultimately results in loss of tensile strength
and resultant outpouching causing dilation of the neck wall. Another potential consideration
is the accuracy of endograft deployment in the proximal neck. Zarins et al has suggested
that the further below the renal arteries a stent graft is deployed, the more likely
the adjacent wall is to undergo subsequent dilation. Hence, with precise deployment
of BES, the graft may be deployed much closer to the renal arteries without fear of
obstruction.[17] Similar results were observed with the VI-Datascope graft in a small series of patients
with difficult neck anatomy at an average follow-up period of 11 months.[18] There are, however, two notable factors that may confound these conclusions. Generally,
BES are only oversized by 5%, whereas SES are oversized by 10 to 20%. As explained
previously, this would greatly affect the rate of AND. It is also conceivable that
proximal seal may be better with BES. In addition, all EVARs with BES had an aortouniiliac
configuration. Prior studies have shown that downward force at the bifurcation plays
a role in distal migration, but this factor is limited with uniiliac devices and so
it is difficult to reasonably compare the two types of grafts.
In 2013, TriVascular released its Ovation stent graft, which contained a unique feature
specifically designed to enhance the proximal seal zone. The “O rings,” are a set
of two hollow rings within the graft main body at the proximal end that are filled
with polymer after deployment to expand to the exact size and shape of the aortic
lumen and create a tight seal; they can even establish seal with adjacent thrombus
or calcification and in challenging cases of reversed taper anatomy. Most importantly,
once the rings have been deployed during the initial repair, they do not expand further,
thereby preventing the progressive outward radial force on the wall of the aortic
neck observed with other stent grafts. The theoretical benefit of such a design was
supported in a study by de Donato et al which reported freedom from type Ia endoleak
at 3 years to be 98%.[19] However, the length of follow-up in that study was not long enough to capture the
progressive changes in aortic neck diameter that occur as a result of disease progression
alone, particularly when one considers the presence of the two long suprarenal bare
metal stents that are designed to provide main body fixation. The structure of this
endograft is such that even a small increase in aortic neck diameter at the site of
the O rings would result in loss of seal and a type Ia endoleak. Therefore, long-term
follow-up is essential to determine the rate of AND and proximal seal zone compromise
with this endograft.
Considering the incidence and clinical significance of AND and endograft migration,
the use of prophylactic proximal fixation devices during the initial repair should
be strongly considered—particularly in patients with hostile neck anatomies. Grisafi
et al in 2011 first published results for use of multiple proximal fixation adjuncts
(angioplasty, extension cuffs, and uncovered stenting) for assisted primary patency.[20] In the mid-2000s, the HeliFx Endoanchor fixation device was released in the market
as a novel tool that improves endograft fixation and seal at the proximal neck. The
driving force behind its development was the need for a device that could be deployed
endovascularly with the precision and strength of an open surgical suture.[21] It has been suggested that when Endoanchors are appropriately deployed, they improve
endograft fixation and apposition to the aortic wall, thus improving proximal seal
after EVAR. Muhs et al compared two well-matched EVAR patient cohorts (with and without
use of Endoanchors) and reported significantly higher rates of aortic sac regression
in patients who underwent EVAR with Endoanchors, but no significant differences in
the rate of type Ia endoleaks.[22] This supports the notion that Endoanchor use improves proximal endograft seal. ANCHOR
is a prospective, non-randomized, dual-arm, multicenter, postmarket registry of the
real-world use of the Heli-FX system with Endoanchors. The registry has over 800 EVAR
patients enrolled worldwide without any reports of endograft migration, indicating
the efficacy of the device in enhancing endograft fixation. We recently found Endoanchors
to be an independent predictor of prophylaxis against AND; our hypothesis is that
once the aortic neck diameter reaches the nominal endograft diameter, the Endoanchors
keep the aortic wall attached to the endograft, acting as a stabilizing structure
and preventing further dilatation of the neck. It is conceivable that decreasing endograft
oversizing with concomitant prophylactic use of Endoanchors could be an effective
strategy for preventing AND in both short and long term and should be considered in
EVAR patients with longer life expectancy.
Conclusions
Aortic neck dilatation with most commercially available self-expanding endografts
is well-documented, multifactorial, and an important parameter when planning endovascular
AAA repair, particularly in patients with “hostile” neck anatomy. Endograft selection,
degree of oversizing, use of adjuncts that enhance fixation and sealing, and the option
of OSR should be strongly considered especially in patients with longer life expectancy.
