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Morphometric
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In: Aortic Aneurysms ISBN: 978-1-62618-458-9
Editors: Daniel Fischhof and Franz Hatig © 2013 Nova Science Publishers, Inc.
Chapter 5
ENDOVASCULAR ANEURYSM REPAIR IN
CHALLENGING ANATOMY
Pieter Broos
1
, Rutger A. Stokmans
1,2
,
Bianca L. W. Bendermacher,
1
Philippe W. M. Cuypers,
1
Marc van Sambeek
1
and Joep A.W. Teijink
1,2
1
Department of Vascular Surgery, Catharina Hospital,
Eindhoven, The Netherlands
2
Department of Epidemiology, CAPHRI Research School,
Maastricht University,
The Netherlands
ABSTRACT
Over the past two decades, endovascular aneurysm repair (EVAR) has been
demonstrated to be safe and effective, and is nowadays a generally accepted alternative to
conventional open surgical aneurysm repair (OSAR). The landmark randomized
controlled EVAR-1, EVAR-2, OVER and DREAM trials have concluded that EVAR is
associated with reduced perioperative mortality, morbidity, blood loss and
hospitalization, and provides surgical treatment options for patients in a poor physical
condition. Despite these convincing short-term benefits of endovascular treatment,
EVAR and OSAR have shown similar survival rates at longer follow up.
A disadvantage of EVAR is the continuous risk of device-related complications,
such as migration and endoleaks, and so persisting risk of aneurysm rupture. The
landmark trails mention the higher rate of complications and secondary interventions
associated with EVAR. Furthermore, these possible complications necessitate a life-long
follow-up in EVAR-patients. The more recent OVER trial showed a similar
reintervention rate after two years, probably due to the use of later generation devices and
a changed approach towards Type-II endoleaks.
Despite the lack of consensus, patients seem to prefer endovascular repair rather than
open repair. Even in cases where, according to the instructions for use, patients are not
eligible for endovascular repair. Severely angulated or short infrarenal aortic necks are
associated with the occurrence of adverse EVAR outcomes. To ensure limited risk on
possible complications, only those with ideal anatomy are considered candidates for
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EVAR. Probably the significantly higher rate of secondary interventions in the EVAR-1
and DREAM trials was partly due to the use of older-generation devices, less knowledge
about aneurysm morphology, and less sophisticated intraoperative fluoroscopic imaging.
The evolution of stent-grafts and endovascular delivery systems has improved the
application of EVAR with less technical complications and re-interventions, which
results in more extensive eligibility criteria for safe endovascular treatment. The
EUROSTAR and Lifeline registries have proved this.
The Endurant Stent-Graft System is a new generation stent-graft with increased
performance in sealing and fixation, durability, conformability in tortuous vessels, and
delivery profile. The ENGAGE registry, initiated to evaluate the Endurant Stent-Graft
System, shows promising results with less aneurysm-related mortality, technical
complications and re-interventions in the long term. The recently launched EAGLE
registry looks at the clinical success of more challenging anatomy cases. The aim of this
registry is to collect clinical information on the performance of the Endurant Stent-Graft
System in anatomically challenging aneurysms, and to critically assess whether the
current guidelines for anatomic eligibility to endovascular treatment with this system can
be expanded even more.
INTRODUCTION
Abdominal aortic aneurysms (AAAs) are a significant health challenge with an estimated
incidence of 20-40 cases per 100 000 population per year. Initial treatment is aimed to prevent
rupture [1, 2]. There is a generally accepted indication for treatment when the threshold
diameter of the AAA (5.5 cm, measured by ultrasonography, in males) is reached, rapid
aneurysm growth is observed (>0.5 cm/6 months), or when the aneurysm becomes
symptomatic [3].
Since its first introduction in 1952 by Dubost et al. [4], open aneurysm repair has
undergone many improvements and became the gold standard of AAA treatment. Open
surgical aneurysm repair requires laparotomy and clamping of the aorta, frequently
accompanied by substantial blood loss. According long-term follow-up, open repair remains a
safe and durable option for the management of AAAs [5]. A perioperative mortality rate of
2% to 5% and significant morbidity are still associated with open repair.
The management of AAAs has changed dramatically over the last 20 years.
Independently, Parodi et al. and Volodos et al. introduced endovascular aneurysm repair
(EVAR) in 1991 [6, 7]. Under x-ray fluoroscopic guidance, a stent graft is introduced in the
abdominal artery and excludes the aneurysm sac from circulation in a minimally invasive
approach. EVAR is an alternative treatment for patients with severe comorbidities that are not
suitable for open repair.
The success of EVAR, in terms of exclusion of the aneurysm and absence of
perioperative and postoperative complications
and dimensions [8, 9]. For this reason, the instructions for use for all commercially available
endografts have clear guidelines on AAA morphology and dimensions. In practice, this will
lead to exclusion from EVAR for approximately 40% of patients with AAA. However, it is
clear that patients with anatomy outside the recommend limits are being treated by EVAR.
The first generation stent grafts included significant problems with progressive neck
dilatation, distal migration, modular separation, thrombosis, and loss of integrity [10, 11].
Substantial progress has been made in recent years with second and third generation devices
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Endovascular Aneurysm Repair in Challenging Anatomy
121
to prevent these complications [12-15]. Fourth generation stent grafts continue this
progression showing good results in terms of perioperative and postoperative complications
in patients with challenging anatomy [
16, 17].
Companies work on the refinement of both the stent graft and its delivery system, and the
experience of physicians rises. Experienced vascular surgeons dare to make way on
aneurysms with short and angulated necks, therefore allowing a broader group of subject to
be treated with EVAR.
CHALLENGING ANATOMY
Many vascular surgeons have adopted a preoperative planning to allow them to be better
prepared for complex aneurysm anatomy. To achieve good results with EVAR, preoperative
imaging of the aortoiliac anatomy is important (Figure 1). Imaging also predicts whether any
ancillary procedures may be necessary before or during EVAR. Precise measurement of
aortoiliac lengths and diameters is necessary to select the most suitable endograft.
Ultrasonography is used for aneurysm detection because it is cheap, readily available, and
accurate in diameter measurements. Ultrasonography is, however not proficient to provide
detailed information about the aortic morphology [18]. Helical or spiral computed
tomography angiography (CTA) combines conventional angiography and CT with the
possibility of advanced image processing. A 3D-reconstruction of a CTA offers the
possibility to measure angulation of the arteries in all directions. Magnetic resonance
angiography (MRA) has the potential to replace CTA because it does not require nephrotoxic
contrast agents and ionizing radiation, but is unable to show calcifications.
Figure 1. CTA images of unfavourable proximal neck features: (A) Neck angulation >60°, (B) neck
length <10 mm, (C) Circumferential thrombus of >50%.
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Aneurysm
Aneurysm morphology may influence endograft delivery and deployment. Distal
embolization is a potential risk if the AAA is filled with laminar thrombus. Aortic tortuosity
and angulation can be calculated using 3D-reconstructions.
Proximal Neck
The proximal non-aneurysmal infrarenal artery is defined as the proximal neck or landing
zone. The proximal neck extends from the most caudal main renal artery to the onset of the
aneurysm. Adequate sealing and fixation of the endograft at the proximal neck are absolute
requirements for complete aneurysm exclusion. For this reason, proximal neck anatomy is
considered a major limiting factor when determining suitable for EVAR [
19-21]. Most
IFU) state that EVAR should be performed in patients
with an aortic neck greater than 10 mm length, less than 32 mm in diameter, and with an
angulation less than 60°.
Angulation can be divided in supra--renal angulation
angle between the proximal AAA neck and the supra-renal aortic axis respectively the angle
between the proximal AAA neck and the main axis of the AAA (Figure 2A). To minimize the
influence of angulation itself on angle measurements, the angulation should be measured
perpendicular to the aortic lumen in the middle of the flexure, the central lumen line (CLL).
The sharpest angle of the CLL is considered the true angle of the aortic axis [22].
The shape of the aortic neck is ideally a cylinder. However, this is a simplification of the
true morphology of the neck and many variances are seen (Figure 3). To ensure maximum
proximal fixation and sealing the diameter changes in the proximal neck should be minimal.
The shape or configuration of the neck is usually determined by visual examination of coronal
views and 3D reconstructions of the aorta at different angle projections.
Figure 2. (A) Determination of the suprarenal (α) and infrarenal (β) angulation. (B) Calculation of the
iliac tortuosity index as τ = L1/L2.
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Figure 3. Overview of possible aortic neck morphology. (A): cylindrical, (B): conical, (C): barrel, (D):
hourglass, (E): reverse conical.
Iliac Anatomy
Iliac artery morphology is crucial to obtain delivery device access into the aorta. The iliac
arteries also provide the distal landing and sealing of the endograft. Inadequate iliac access is
the second most common reason for patient exclusion from EVAR, and is twice as common
in female patients [
23, 24]. The stent graft introduction system are 16-28 French in diameter
and relatively inflexible. Small iliac artery diameter or a combination of diffuse calcification
and severe stenosis ensure a higher risk of perioperative iliac artery rupture [25, 26]. Severe
stenosis or iliac occlusion would clearly necessitate some sort of adjunctive procedure to
ensure safe delivery of the endograft into the aortic segment. As with calcification of the
proximal aortic neck, severe calcification of the iliac arteries may cause incomplete distal
sealing, predisposing to a distal Type-I endoleak.
An iliac tortuosity index can be calculated to estimate the level of tortuosity (Figure 2B).
The iliac tortuosity index is defined as τ = L1/L2 where L1 is the distance along the CLL
between the common femoral artery and the aortic bifurcation and L2 is the straight-line
distance from the common femoral artery and the aortic bifurcation [27].
Tortuosity in the external iliac artery can often be straightened with a stiff wire, while the
common iliac artery may be more difficult to manage due to anatomical fixation of this
segment by the internal iliac artery (IIA).
Iliac artery aneurysms (IAAs) are the second most frequent aneurysm after AAAs. 70
percent of iliac aneurysm are common iliac artery aneurysms (CIAAs) and develop in 20
percent of patients with AAA [28]. 20 and 10 percent of IAAs are found in the internal and
external iliac artery, respectively [29]. When iliac aneurysms are confined to the proximal
common iliac artery (CIA) alone and a landing zone of adequate length is present in the distal
CIA, EVAR can proceed in the usual fashion. When there is extension of the aneurysm to the
common iliac bifurcation, or into the IIA, extension of the endograft limb in the external iliac
artery (EIA) is necessary.
Endograft Selection
The basic design of all endografts is roughly the same; a tubular graft supported by
stents along its length excludes the aortic aneurysm. The tubular graft either bifurcates into
two smaller diameter stent grafts that continue in the iliac arteries, or it continues in one iliac
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Table 1. General characteristics of various endografts available worldwide
Device
Minimum access
diameter for main
body
Treatable
aortic neck
diameter
Minimum
aortic neck
length
Maximum
aortic
angulation
Suprarenal
fixation
Treatable
iliac
diameter
Minimum
iliac seal zone
Cook Zenith
Flex®
22-26 mm graft = 7.1
mm
28-32 mm graft = 7.7
mm
36 mm graft = 8.5 mm
18-32 mm
15 mm
Mandatory
7.5-20 mm
10 mm
Gore
Excluder®
23-8.5 mm graft = 6.8
mm
31 mm graft = 7.6 mm
19-29 mm
15 mm
Not available
8-25 mm
10 mm
Medtronic
Endurant®
23-25 mm graft = 5.7
mm
28-36 mm graft = 6.4
mm
19-32
10 mm
15 mm
Mandatory
8-25 mm
15 mm
Medtronic
Talent®
22-28 mm graft = 7
mm
30-36 mm graft = 8
mm
18-32
10 mm
Available
8-22 mm
25 mm
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Endovascular Aneurysm Repair in Challenging Anatomy
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artery and a femoro-femoral bypass is carried out. Each endograft is available in different
length and diameter so it can be altered to fit the anatomy of the patient easily [30].
Since the first endografts were placed, many changes have been made in endograft
design. Modern endovascular grafts are developed with the aim of expanding the indications
for endovascular aortic aneurysm repair. Improvements of a number of concepts relating to
device design involve either the delivery system or the stent graft itself. Delivery systems
have become more flexible, allowing access in more tortuous vessels, better proximal
alignment and improved accuracy of stent deployment. To reduce migration, supra- and infra
renal fixation systems at the proximal stent have evolved. Evolution of Nitinol based systems
has produced more conformable devices and will probably result in a reduction in long-term
stent fractures and stent kinking, especially in challenging anatomies. Patients with favorable
anatomy show good results with most modern devices, whereas those with challenging
anatomies may benefit from particular design, delivery and deployment features requiring
considerable experience and knowledge for adequate device selection [
31, 32]. Table 1 shows
some of the main stent grafts available on the worldwide market and their instructions for use.
COMPLICATIONS
The proximal Type-I endoleak is the most important cause of AAA rupture after EVAR,
and Type-I endoleaks and migration are the most common reasons for re-interventions.
Complications can occur early or late during follow-up, therefore long-term follow-up is
required after EVAR [33]. Severely angulated and short proximal necks seem to show more
complications during or shortly after the procedure [21, 34-37]. However, with the newest
generation stent grafts complication rates in challenging AAA anatomy tends to be similar
than in conservative AAA anatomy [38-42].
Table 2. Endoleak classification [1]
Type
Definition
I
Endoleak relate to the stent-graft itself
II
Retrograde flow from collateral branches
III
Fabric tears, disintegration or disconnection
IV
Endotension
Pressurization without visible endoleak
Endoleak Type-I
A Type-I endoleak is defined as blood flow into the aneurysm due to incomplete or
ineffective seal at the stent graft ends. The blood flow is direct and under systemic pressure.
The AAA is not excluded from circulation anymore and therefore at risk of continued
expansion and rupture. AAAs with short, angulated and heavily calcified infrarenal necks are
at increased risk.[44] When Type-I endoleaks are seen during the initial procedure, they
should be corrected before the patient leaves the operating theatre. Intraoperative and late
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126
Type-I endoleaks can be resolved through additional ballooning at the proximal attachment
site, placement of balloon expandable stents, or stent-graft extension.[45-48].
Endoleak Type-II
Branch flow through a patent inferior mesenteric artery (IMA], lumbar artery, accessory
renal artery, or hypogastric artery is defined as a Type-II endoleak and is the most common
form of endoleaks after infrarenal EVAR. It shows an incidence of 15-20% perioperative and
at one month postoperatively. A reduction to 5-10% is seen after one-year follow-up [43].
The most common source of Type-II endoleaks is through lumbar arteries and via the inferior
mesenteric artery (IMA).
Nowadays, a much more conservative approach is accepted regarding Type-II endoleak
management, because many of them seems to be relatively benign [49]. Some Type-II
endoleaks tending to resolve spontaneously, others persist without causing clinical symptoms
and a small proportion cause significant increase in sac size [50]. When the decision is made
to repair a Type-II endoleak, embolization may be executed with a transarterial approach or
with direct endoleak puncture by using a translumbar approach [51-53].
Endoleak Type-III
Type-III endoleaks are defined as mid-graft endoleaks that originate either from fabric
holes or an inadequate seal between endograft components. The incidence of Type-III
endoleaks is between 0.1 and 6.4 percent. To avoid inadequate seal, it is vital to use sufficient
component overlap during the primary procedure. Particularly when there is severe tortuosity
of the iliac arteries or extreme neck angulation, which result in significant increase of forces
on the overlap. Type-III endoleak can be eliminated with deployment of additional stent graft
components to bridge the defect [54].
Endoleak Type-IV
A Type-IV endoleak is present when there is passage of blood or fluid through the graft
fabric due to porosity of the stent graft, a phenomenon recognized in the early period of stent
grafting generally taking place immediately after implantation [55]. Type-IV endoleaks are
usually not seen anymore at the first surveillance CTA, consequently requiring no secondary
intervention.
Endotension
Endotension, is persistent or recurrent pressurization of the aortic aneurysm sac after
EVAR in the absence of evident endoleak [56]. The majority of patients with endotension are
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