AJR 2000; 175:289-302
© American Roentgen Ray Society
Endovascular Repair of Abdominal Aortic Aneurysms
Current Status and Future Directions
John A. Kaufman1,2,
Stuart C. Geller1,
David C. Brewster3,
Chieh-Min Fan1,
Richard P. Cambria3,
Glenn M. LaMuraglia3,
Jonathan P. Gertler3,
William M. Abbott3 and
Arthur C. Waltman1
1
Division of Vascular Radiology, Massachusetts General Hospital, Fruit St.,
Boston, MA 02114.
2
Present address: Dotter Interventional Institute, Oregon Health Sciences
University, 3181 S.W. Sam Jackson Park Rd., Portland, OR 97201-3011.
3
Division of Vascular Surgery, Massachusetts General Hospital, Boston, MA
02114.
Received February 23, 2000;
accepted after revision March 22, 2000.
Honoring Alred Gary, MD and Augustus W. Crane, MD
This is the eighth in a series of Centennial Dissertations that the
AJR is publishing this year in honor of the former presidents of the
American Roentgen Ray Society, Two of whom are pictured above.
Address correspondence to J. A. Kaufman.
Introduction
Endovascular repair of abdominal aortic aneurysms with stent-grafts is a
new catheter-based, imaging-guided procedure that has the potential to
redefine the traditional approach to the treatment of abdominal aortic
aneurysm. In addition, this technology crosses traditional practice boundaries
and is the focus of intense entrepreneurial activity by the
medical—industrial complex. Imaging plays a major role in the
preprocedural patient evaluation, implantation of stentgrafts, and patient
follow-up. This article will review the current status of stent-grafts for the
treatment of abdominal aortic aneurysm in the United States, with emphasis on
the clinical aspects important to the radiology community as a whole.
Abdominal Aortic Aneurysms
The definition of an abdominal aortic aneurysm is focal enlargement of the
abdominal aorta, usually involving the infrarenal portion of the vessel, to
more than 50% larger in diameter than the normal aorta or to greater than 3 cm
in its largest true transverse dimension
[1]
(Fig. 1). The pathophysiology
of abdominal aortic aneurysms is only partially understood, but the risk
factors are similar to those identified for symptomatic arteriosclerosis,
including age of more than 50 years, male sex, smoking, and chronic
obstructive pulmonary disease
[1]. The primary complication
of abdominal aortic aneurysm is acute rupture, a frequently lethal event
despite emergent surgical intervention
[2]. Aneurysm rupture is
directly related to aneurysm size, because the tension on the wall of the
aorta is the product of the radius of the artery and the blood pressure
(Laplace's law). As a rule, even large abdominal aortic aneurysms are
asymptomatic until rupture occurs. Prophylactic repair is therefore
recommended for aneurysms exceeding 4.5-5 cm in diameter
[3,
4]. In 1994, abdominal aortic
aneurysm was diagnosed in approximately 114,000 hospitalized patients (8470 of
these aneurysms were ruptured), with a male—female ratio of 2.3:1
[5]. On the basis of data from
the National Hospital Discharge Survey, Lawrence et al.
[5] determined that almost
39,000 operations were performed for repair of abdominal aortic aneurysm
during that year.
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Fig. 1. —73-year-old man with atherosclerotic abdominal aortic aneurysm.
Coronal maximum intensity projection of contrast-enhanced helical CT angiogram
shows infrarenal abdominal aortic aneurysm. Aneurysm starts well below renal
arteries (curved arrows) and ends at aortic bifurcation. True size of
abdominal aortic aneurysm is indicated by calcification in wall of aorta
(straight arrows) because mural thrombus deposited in abdominal
aortic aneurysm sac results in smaller opacified lumen.
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To understand the unique aspects of stentgraft repair of abdominal aortic
aneurysm, a brief summary of the principles of surgical aneurysm repair,
termed "endoaneurysomorrhaphy," is necessary. Surgical repair of
abdominal aortic aneurysm was first performed successfully in the early 1950s
[4]. The basic goal of surgical
repair is exclusion of the abdominal aortic aneurysm from the arterial
pressure with preservation of blood flow to the pelvis and legs via an
implanted vascular conduit (usually a synthetic fabric or expanded
polytetrafluoroethylene). This is usually achieved by incision of the aneurysm
sac; removal of mural thrombus; ligation of patent branch vessels arising from
the sac; selection of a graft of appropriate size and shape; suture
anastomosis of the graft to the vessel proximal and distal to the aneurysm;
and, lastly, closure of the decompressed aneurysm sac over the synthetic graft
material (Fig.
2A,2B,2C).
Because this is a major operation by any criteria, the overall operative
mortality for elective surgical repair performed in experienced centers is 4%
or less but can be as high as 8.4%
[1,
4,
5]. The major sources of
perioperative morbidity are cardiac, pulmonary, renal, hemorrhagic, and septic
complications [6]. Five-year
survival after surgery is approximately 65-70%, with the major source of
long-term morbidity and mortality attributable to cardiac disease
[6,7,8].
The long-term (i.e., 10-year) complications related to surgical repair of
abdominal aortic aneurysm are aneurysm formation at the surgical anastomoses
(4%); recurrent true aneurysms (5%); graft occlusion (3%); and graft
infection, aortoenteric fistula, or both (combined incidence 5%)
[8,
9].
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Fig. 2A. —Surgical repair of abdominal aortic aneurysm. (Reprinted with
permission from [100])
Drawing shows exposure of abdominal aortic aneurysm from anterior approach.
Dashed lines indicate site of incision in sac. Inferior mesenteric artery
(arrow) arises from anterior surface of abdominal aortic
aneurysm.
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Fig. 2B. —Surgical repair of abdominal aortic aneurysm. (Reprinted with
permission from [100])
Drawing shows abdominal aortic aneurysm has been opened and thrombus removed.
Orifices of lumbar arteries (arrow) are oversown to prevent
back-bleeding into sac.
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What is a Stent-Graft?
A stent-graft is an intraluminal device that consists of a supporting
framework (currently made of metal such as stainless steel or nitinol) and a
synthetic graft material (Fig.
3A,3B).
Stent-grafts can be either self-expanding or balloon-expandable, depending on
the type of metal in the stent. The stent may be located inside, outside, or
within the graft material, and it may be along the entire length of the graft
or restricted to the ends. To deliver the stent-graft through a small vascular
access, the device is compacted onto a catheter or compressed into a sheath.
With the use of imaging guidance, the device is advanced into an appropriate
location in the aorta from a remote access site and deployed.
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Fig. 3B. —Photographs of sample stent-graft. Partially deployed stent-graft.
Constrained device is delivered into body from remote access over guidewire,
after which stent-graft is allowed to reexpand.
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The ultimate goal of endovascular repair of abdominal aortic aneurysm with
a stent-graft is the same as for surgical repair: depressurization of the
aneurysm sac to prevent rupture. In the ideal situation, the stent-graft
excludes all blood flow from the aneurysm sac allowing thrombosis. However,
stent-graft exclusion of abdominal aortic aneurysm differs from open repair in
several important ways. First, determination of the appropriate graft size and
configuration for an individual patient is based on preprocedural imaging
rather than on direct inspection of the vessels as occurs during surgery.
Current stent-grafts cannot be recaptured or repositioned once deployed, so
the correct device must be inserted the first time. Accurate preprocedural
imaging and measurements are therefore paramount. Second, the
"anastomosis" between the stent-graft and the vessel wall is a
contact rather than a sutured attachment. The ends of the device must push
against the inner walls of the vessel with sufficient force to prevent blood
from flowing around the device into the abdominal aortic aneurysm. Third,
patent branch vessels arising from the aneurysm sac cannot be inspected and
occluded directly; they can be dealt with only using catheter or laparoscopic
techniques. Fourth, the skill sets for most of the elements of the procedure,
including online interpretation of fluoroscopic images and complex catheter
manipulations, are extensions of traditional interventional radiology rather
than surgical practice. Lastly, complete exclusion of the aneurysm sac does
not occur in all stent-graft procedures, as it does with open surgical repair
[10].
The concept of percutaneous placement of an endovascular graft is
attributed to Dotter [11], who
proposed the idea in 1969. Use of stent-grafts for abdominal aortic aneurysm
in humans was first described by Parodi et al.
[12] in 1991, who constructed
devices from Palmaz stents (Cordis Endovascular, Miami, FL) and a standard
woven polyethylene terephthalate surgical graft material. Two stent-grafts are
commercially available in the United States: the Ancure (Guidant,
Indianapolis, IN) and AneuRx (Medtronic, Minneapolis, MN) (Fig.
4A,4B,4C,4D,4E,4F).
In addition to the approved devices, a number of additional abdominal aortic
aneurysm devices are in various stages of clinical trials, including Excluder
(W. L. Gore, Sunnyvale, CA), Lifepath (Baxter Healthcare, Deerfield, IL),
Talent (Medtronic World Medical, Sunrise, FL), Vanguard (Boston Scientific,
Natick, MA), Zenith (Cook, Bloomington, IN), Endologix (C. R. Bard, Murray
Hill, NJ), and Quantum LP (Cordis Endovascular). Many of these stent-grafts,
plus others not mentioned here, are commercially available outside the United
States. Custom stent-grafts can be constructed for specific patients unable to
be treated with any of these devices
[13,
14].
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Fig. 4A. —Two commercially available stent-grafts. Photograph of one-piece
bifurcated stent-graft (Ancure; Guidant, Indianapolis, IN). Supporting metal
stents are located inside graft material at ends of device. Note exposed metal
attachment hooks (straight arrows). Radioopaque marker bands
(curved arrow) are visible on surface of graft. (Courtesy of
Guidant)
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Fig. 4B. —Two commercially available stent-grafts. Radiograph of 71-year-old
man with abdominal aortic aneurysm with implanted bifurcated stent-graft
(Ancure; Guidant) shows supporting metal localized to attachment sites
(straight arrows). Location of graft material is indicated by
radioopaque marker bands (curved arrow).
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Fig. 4C. —Two commercially available stent-grafts. Photograph of modular
bifurcated stent-graft (AneuRx; Medtronic, Minneapolis, MN). When assembled,
modular components telescope with sufficient overlap to form hemostatic seal
between components. (Courtesy of Medtronic)
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Fig. 4D. —Two commercially available stent-grafts. Radiograph of 76-year-old
man with abdominal aortic aneurysm with implanted bifurcated stent-graft
(AneuRx; Medtronic). Metal supports entire device.
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Fig. 4F. —Two commercially available stent-grafts. Digital subtraction
angiogram of same patient as E immediately after placement of
bifurcated stent-graft (arrows) shows exclusion of aneurysm.
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Stent-grafts for abdominal aortic aneurysm are available in three basic
configurations: tube, bifurcated, and tapered with concomitant
femoral-to-femoral bypass (Fig.
5A,5B,5C).
Stent-grafts can be grouped by generations, with the earliest devices
constructed from "off-the-shelf" stents and graft materials, to
devices that use materials and manufacturing techniques unique to their
purpose [15]. Features that
can be used to distinguish between devices are listed in
Table 1. For example, the
Ancure devices use barbed Elgiloy stents (Elgiloy Limited Partnerships, Elgin,
IL) within the ends of the graft that extend beyond the graft material, have
no supporting metal in the body of the grafts, and are single-piece devices
[16]. The AneuRx stent-graft
uses nitinol metal attached to the outside of the graft material along the
full length of the graft, has no exposed metal or barbs beyond the graft
material, and is a modular (multipiece) device
[17]. To our knowledge, there
is little proven difference in intermediate-term out-comes between generations
of commercial devices, although technical success (ability to insert the
device) tends to improve with each new generation of device
[17,
18].
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Fig. 5C. —Drawings of basic configurations of stent-grafts. Tapered
aortounilateral external iliac artery stent-graft with occluder (solid
straight arrow) in contralateral common iliac artery, embolization coils
in ipsilateral internal iliac artery (open arrow), and surgical
femoral-to-femoral cross-over graft (solid curved arrow). Occluder
and coils prevent retrograde perfusion of aneurysm sac.
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Patient Eligibility
Several factors determine whether a patient is a suitable candidate for
endovascular repair of abdominal aortic aneurysm: patient demographics, the
type of aneurysm, and the type of device. The demographics of the ideal
patient for a stent-graft are not well defined at this time because of the
lack of data on long-term outcomes and the rapid evolution in device design.
Conservative investigators urge conventional open repair for all low-risk
surgical candidates or those with long life expectancies
[19]. All practitioners are in
agreement that patients at high risk for complications during open abdominal
aortic aneurysm repair or with prohibitive anatomic barriers to surgical
access to the aorta are ideally suited for stent-grafts
[20,21,22].
The ready availability of lay information sources about stent-grafts, such as
the Internet, has resulted in a strong patient-driven demand for the
procedure.
The type of abdominal aortic aneurysm most appropriate for stent-graft
repair is open to interpretation. In general, simple unruptured
atherosclerotic abdominal aortic aneurysms that would otherwise qualify for
surgical repair (>4.5 cm in diameter) can be considered for stent-graft
repair, although a precise size criterion may vary with sex
[21,22,23].
Patients with complex anatomy, such as multiple accessory renal arteries
arising from the aneurysm sac, or with concomitant visceral artery occlusive
disease that requires correction are less desirable candidates. Application of
this technology to acutely ruptured aneurysms remains highly selective because
complete exclusion of the aneurysm sac cannot be ensured
[24,25,26].
Limited experience suggests that patients with inflammatory abdominal aortic
aneurysm respond well to endovascular repair
[27]. Despite a suggestive
report about stent-graft exclusion of mycotic thoracic aortic aneurysms,
surgical excision of infected abdominal aortic aneurysm should remain the
first-line therapy [28].
The dominant limiting factor in patient selection is the stent-graft itself
[29,
30]. Each device has specific
and relatively restrictive requirements with regard to the diameter, length,
and angulation of the proximal and distal attachment sites and to the ability
of the iliofemoral arteries to accommodate the stent-graft delivery systems.
Patients who do not fit the device cannot be treated. For example, both the
Ancure and AneuRx devices require a 1.5-cm length of normal-diameter aorta
distal to the renal arteries for adequate seal at the proximal attachment
site. Angulation between the infrarenal aortic neck and the abdominal aortic
aneurysm of more than 60° makes precise deployment and seal at the
proximal attachment difficult. A similar length of normal aorta distal to the
abdominal aortic aneurysm is required for the Ancure tube stent-graft. For
bifurcated stent-grafts, preservation of at least one hypogastric artery is
recommended to avoid potential pelvic or buttock ischemic complications
[31]. The Ancure device
requires a 2-cm length of normal-diameter iliac artery for distal attachment,
and AneuRx bifurcated stent-grafts require 1 cm.
The size of the stent-grafts should be 10-20% greater than the outer
diameter of the normal vessel at the proposed attachment sites to maximize the
chance of an effective seal. In addition, this allows potential enlargement of
the attachment site over time
[32,
33]. With this in mind, the
maximal luminal diameters of the proximal aortic and distal iliac attachment
sites that can be treated with current devices are approximately 26 and 18 mm,
respectively. The approval of modifications of these devices and additional
stent-grafts will extend the range of patients who can be treated.
The status of the iliofemoral arteries is a major consideration with the
currently approved devices because the delivery systems are large (27-French
outer diameter for the Ancure sheath and 21-French for the AneuRx catheter).
Major local vascular injury and inability to insert the device can occur in
patients with diseased or heavily calcified iliac vessels
[34]. Devices with
small-profile delivery systems are in clinical trials.
Given these factors, what is the likelihood that a patient presenting with
an unruptured 5-cm-diameter abdominal aortic aneurysm can be treated with a
stent-graft? In the initial Endovascular Technologies (subsequently renamed
Ancure) tube stent-graft trial, fewer than 12% of screened patients were
treated successfully [35]. The
subsequent availability of a bifurcated stent-graft more than doubled the
number of patients who could be treated
[36]. An estimated 60% of
patients with abdominal aortic aneurysm are eligible for endovascular repair
with current devices on the basis of anatomic criteria, most of whom will
receive a bifurcated device if treated
[37].
Preprocedural Imaging
Preprocedural imaging of potential abdominal aortic aneurysm stent-graft
candidates at our institution consists of both CT and catheter angiography.
Unenhanced abdominal—pelvic helical CT to assess vascular calcification
is followed by thin-section (2-3 mm) helical CT angiography from the celiac
artery to at least the iliac artery bifurcations, but preferably to the groins
(Fig. 1). Diameter measurements
(outer wall-to-outer wall) are obtained from the contrast-enhanced axial
sections by either measuring the narrowest dimension, when the vessel appears
to be imaged on a bias, or using workstations to create true axial sections.
The vessel lumen, particularly at the anticipated attachment sites, is
inspected for thrombus, calcification, and atherosclerotic disease.
Angiography using a graduated pigtail catheter that has markers over a 20- to
25-cm distance is preferred for length measurements
(Fig. 6). Anteroposterior and
lateral views of the aorta, oblique views of the proximal aortic neck, if
necessary, and anteroposterior and bilateral oblique views of the pelvis are
obtained.
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Fig. 6. —Digital subtraction angiogram in 77-year-old man using graduated
pigtail catheter (arrow) shows multiple renal arteries (severe
stenosis in upper right accessory artery) with approximately 4 cm of normal
aorta between lowest renal arteries and aneurysm. Note slight angle between
long axis of normal aorta and aneurysm.
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Reporting standards relevant to stent-graft repair have been proposed for
describing abdominal aortic aneurysm
[38]. These standards are
limited in that they were conceived early in the clinical experience with
stent-grafts. Information that is important when reporting CT or angiographic
findings in a patient undergoing evaluation for abdominal aortic aneurysm
stent-graft is listed in the Appendix.
Ancillary Procedures
Successful endovascular repair of abdominal aortic aneurysm remains
technically challenging despite continued improvement in device design. To
maximize initial success of the procedure, additional interventions may be
required before or during stent-graft placement
[39]. Occlusion of branch
vessels arising from the proximal neck, aneurysm sac, or the iliac arteries is
the most common preprocedural intervention
[31,
39]; this is done to create
attachment sites or prevent retrograde perfusion of the aneurysm after
placement of the stent-graft. For example, contralateral common iliac artery
occlusion is required when inserting an aortounilateral iliac artery
stent-graft [40] (Fig.
5A,5B,5C).
In a patient with a common iliac artery aneurysm, embolization of the internal
iliac artery is necessary before stent-graft extension into the external iliac
artery [41] (Fig.
7A,7B).
These embolization procedures are not, however, without consequence because
new onset of buttock claudication after coil occlusion of the internal iliac
artery before stent-graft placement has been described in approximately 20% of
patients [31].
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Fig. 7A. —73-year-old man with abdominal aortic aneurysm and right common
iliac artery aneurysm. Conventional angiogram shows abdominal aortic aneurysm
and right common iliac aneurysm (arrow). Bifurcated stent-graft will
be placed, but it must extend into external iliac artery on right to effect
adequate seal.
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Fig. 7B. —73-year-old man with abdominal aortic aneurysm and right common
iliac artery aneurysm. Angiogram obtained after insertion of bifurcated
stent-graft. Coils were placed in right internal iliac artery (straight
arrow) before insertion of stent-graft to prevent retrograde flow into
common iliac artery aneurysm. Stent-graft extends into external iliac artery
on right (curved arrow). Note patent left internal iliac artery.
(Reprinted with permission from
[101])
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Catheter-based interventions such as angioplasty or stent placement may be
required during insertion of a stent-graft. These ancillary procedures are
more common when delivery systems are large
[42,43,44].
Angioplasty to dilate conduit artery stenoses should be performed at the time
of the stent-graft procedure, not before. The threshold should be low for
stent placement to repair a traumatic dissection caused by a large delivery
system or to buttress the iliac limb of an unsupported stent-graft
[45,
46]. In patients with severe
iliac artery occlusive disease and abdominal aortic aneurysm, a
retroperitoneal surgical approach to the distal aorta or common iliac artery
can be used to provide access for stent-graft delivery
[47]. Aortounilateral iliac
artery stent-grafts require a surgical femoral-to-femoral bypass graft to
perfuse the pelvis and limb contralateral to the stent-graft
[13,
40,
48] (Fig.
5B,5C).
Procedural Requirements
Successful endovascular repair of abdominal aortic aneurysm is a
multidisciplinary effort, with contributions from several specialties. The
degree of participation from each discipline will vary from one institution to
another. The following procedural components are essential: excellent imaging
with a 12-inch or greater (
30-cm) image intensifier, digital subtraction
angiography, and the capability to perform aortography (preferably with a
power injector) in multiple obliquities; a procedure room that can support
open aortic surgery; access to the full range of interventional radiology
tools; and a dedicated team who is familiar with the operation of the imaging
equipment and the stent-graft delivery system.
The anesthetic requirements for endovascular repair of abdominal aortic
aneurysm are much less than those for open repair. Initial early concern
regarding the potential need for emergent open surgical repair resulted in the
use of general anesthesia in many centers
[17,
49]. As experience with the
procedure has grown and device delivery improved, most procedures are now
performed with epidural anesthesia
[50]. Stent-graft repair of
abdominal aortic aneurysm using local anesthetic and conscious sedation has
been described [51].
Outcomes
Successful insertion of an aortic stent-graft is possible in more than 95%
of procedures when patients are carefully selected and an appropriate device
is used [17,
52,
53]. The most common cause of
a failed procedure is the inability to insert a device through diseased or
tortuous iliac arteries. This occurs more often early in an operator's
experience and with devices with large profiles
[17,
34,
42,
54]. Misplacement of the
stent-graft, failure to deploy, or acute and irreversible occlusion are
unusual but may require urgent open surgical repair
[55]. Termed a "surgical
conversion" of the procedure, it is associated with a higher morbidity
than conventional surgery
[56].
When compared with open surgery, there are definite early benefits to
stent-graft repair of abdominal aortic aneurysm. The procedure entails less
hemodynamic stress than surgery, which is an important consideration in
elderly patients [57,
58]. Blood loss is lower with
stent-graft repair compared with open surgery (average, 400 versus 1200 ml),
and the use of the intensive care unit is reduced to the exception rather than
the rule [17,
34]. Patients are able to
ambulate the next day, and hospital stays are reduced from 7-10 days to 2-3
days.
The 30-day mortality rate in large stent-graft series ranges from 0.7% in
low-risk populations to 15.7% in high-risk patients
[52,
59,
60]. These statistics compare
favorably with those associated with surgical repair of abdominal aortic
aneurysm [61]. Death during a
stent-graft procedure is rare. Acute intraprocedural rupture of abdominal
aortic aneurysm during stent-graft placement with successful outcome has been
reported [62,
63]. Multiorgan system
failure, myocardial infarction, bowel infarction, stroke, pulmonary embolism,
and peripheral arterial embolism have all been described after stent-graft
procedures, but most early complications are minor and consist of injuries to
access arteries or issues related to groin incisions
[34,
48,
64,
65].
Complete exclusion of the aneurysm sac is the goal of stent-graft placement
and the definition of early clinical success
[38]. Persistent opacification
of the aneurysm sac after insertion of a stent-graft is termed an
"endoleak" and is classified by cause and time of occurrence
[66]. This classification of
leaks is described in Table 2
and examples are shown in Figures
8 and
9A,9B.
The reported rates of immediate aneurysm exclusion range from 66% to 87%
[53,
67,68,69].
Most early endoleaks are currently types II and IV, because type I leaks can
be minimized by careful patient selection and preprocedural measurements
[70] (Fig.
10A,10B).
Tube stent-grafts may have a higher rate of type I leak, particularly at the
distal attachment site, than bifurcated stent-grafts
[69]. Large attachment leaks
that result in continued pressurization of the aneurysm sac indicate a failed
procedure and may leave the patient at risk for subsequent abdominal aortic
aneurysm rupture
[71,72,73].
The early management of small endoleaks that cannot be eliminated
intraproceduraly is usually expectant because more than 50% will resolve
spontaneously [16,
17,
52,
53,
74,
75]. Although many early
endoleaks disappear within 6 months, reappearance of leaks and delayed
appearance of new leaks can occur at any time
[52,
76,
77].
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Fig. 8. —Angiogram of 83-year-old man with type I endoleak shows large distal
attachment endoleak (straight arrow) after placement of tube
stent-graft. Note lumbar arteries (curved arrows) providing outflow
for endoleak.
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Fig. 9A. —71-year-old man with type II endoleak. Axial contrast-enhanced CT
scan after placement of bifurcated stent-graft shows opacified inferior
mesenteric artery (open arrow) and contrast material in sac
(solid arrows) outside of stent-graft limbs flowing toward pair of
lumbar arteries.
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Fig. 9B. —71-year-old man with type II endoleak. Late image obtained from
digital subtraction angiogram after superior mesenteric artery injection
confirms retrograde flow from inferior mesenteric artery into aneurysm sac
(arrow) as source of inflow for type II endoleak.
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Stabilization of aneurysm size or even shrinkage and avoidance of secondary
endovascular procedures or open surgery are considered measures of long-term
success [38] (Fig.
11A,11B,11C).
The presence of an endoleak is correlated with less reduction in the diameter
of the sac and even slight enlargement
[68,
70,
78,
79]. After endovascular
repair, an increase in the aneurysm sac diameter of less then 0.5 cm is
considered acceptable [38]. A
greater degree of enlargement should prompt treatment of the endoleak using
endovascular means or surgical conversion
[73,
75,
77]. The currently reported
rate of long-term surgical conversion due to persistent endoleak and aneurysm
expansion is low, having been required in only 1.6% of 669 patients treated
with the early versions of the current Ancure device
[56]. Conversion due to
aneurysm expansion with no visible leak is even less frequent, but both
indications may become more common as long-term follow-up is accumulated.
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Fig. 11A. —Shrinking abdominal aortic aneurysm in 74-year-old man after
treatment with bifurcated stent-graft. Pretreatment axial contrast-enhanced CT
scan shows abdominal aortic aneurysm (arrows).
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Fig. 11B. —Shrinking abdominal aortic aneurysm in 74-year-old man after
treatment with bifurcated stent-graft. Axial contrast-enhanced CT scan
obtained shortly after placement of bifurcated stent-graft shows no evidence
of endoleak. Abdominal aortic aneurysm (arrows) is unchanged.
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Fig. 11C. —Shrinking abdominal aortic aneurysm in 74-year-old man after
treatment with bifurcated stent-graft. Axial contrast-enhanced CT scan
obtained 12 months later shows marked reduction in diameter of abdominal
aortic aneurysm (arrows).
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Delayed rupture of abdominal aortic aneurysm after endovascular repair is
rare but does occur. In the same series of 669 patients described previously,
the cumulative rate was 0.4% over 4.5 years, including a patient with an
aneurysm that had shrunk considerably in diameter
[56] (Fig.
12A,12B,12C).
The time to rupture after stent-graft implantation ranged from 19 to 31
months, but only one of the three patients died. The presence of a persistent
endoleak is clearly a major contributing factor to delayed rupture. In an
interesting corollary from surgery, persistent sac perfusion was found in 44
of 1218 patients who had undergone surgical exclusion and bypass of abdominal
aortic aneurysm sacs [80]. In
the patients with persistant sac perfusion, 63% had sac enlargement and 16%
had aneurysm rupture [80].
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Fig. 12A. —Delayed rupture of abdominal aortic aneurysm after treatment with
stent-graft in 83-year-old man. Unenhanced axial CT scan shows abdominal
aortic aneurysm 2 years after treatment with tube stent-graft. No endoleak was
present on contrast-enhanced study (not shown).
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Fig. 12B. —Delayed rupture of abdominal aortic aneurysm after treatment with
stent-graft in 83-year-old man. Unenhanced axial CT scan obtained at same
level as A 3 years after treatment. Note decrease in size of abdominal
aortic aneurysm. No endoleak was present on contrast-enhanced study (not
shown).
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Fig. 12C. —Delayed rupture of abdominal aortic aneurysm after treatment with
stent-graft in 83-year-old man. Axial unenhanced CT scan obtained
approximately 6 weeks after B shows reexpansion and rupture
(arrow) of abdominal aortic aneurysm. Detachment of distal attachment
site was found at surgery.
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In addition to aneurysm enlargement or rupture, other late complications of
stent-grafts include limb thrombosis, infection, disconnection of modular
components, and distortion and fracture of the stent-graft during aneurysm
shrinkage [56,
68,
81,
82]. The latter complications
were the least expected (Fig.
13A,13B).
The etiology of the changes in stent-graft morphology with decrease in sac
size is not known, but this phenomenon indicates that much remains to be
learned about the dynamics of abdominal aortic aneurysm and these devices.
Limb occlusion appears to be the most common clinical manifestation of
stent-graft distortion and is not restricted to one type of device
[56,
70].
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Fig. 13A. —Change in graft morphology with decrease in aneurysm size in
74-year-old man. Axial contrast-enhanced CT scan obtained shortly after
placement of bifurcated stent-graft shows no evidence of endoleak. Note
orientation of two limbs of stent-graft.
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Fig. 13B. —Change in graft morphology with decrease in aneurysm size in
74-year-old man. Axial contrast-enhanced CT scan obtained at same level as
A 12 months after treatment shows aneurysm has decreased substantially
in diameter (straight arrows). Note almost 90° rotation in
orientation and slight separation of limbs (curved arrow).
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There is great interest and potential benefit in predicting which patients
will have endoleaks after stent-graft placement. Severe neck angulation and
calcification may contribute to proximal type I endoleaks
[30,
83] (Fig.
10A,10B).
Although initially thought not to be important, the number and pattern of
patent branch vessels from the aneurysm sac do correlate with the presence of
early endoleaks [84,
85]. The greater the number of
patent vessels, the inferior mesenteric artery in particular, the higher the
incidence of early type II endoleak
[85]. On the basis of this
observation, one might intuit that the quantity and distribution of mural
thrombus in the abdominal aortic aneurysm sac before insertion of the
stent-graft would influence outcome. This does appear to be the case:
abdominal aortic aneurysms with preexisting thick posterior or circumferential
thrombus have significantly fewer endoleaks and a greater reduction in
diameter than sacs with little or no preprocedure mural thrombus
[78,
79].
Imaging Follow-Up of Patients with Stent-Grafts
The follow-up of patients with stent-grafts requires evaluation of
abdominal aortic aneurysm sac size and perfusion, stent-graft patency, changes
in diameter of the vessels at the sites of endograft attachment, changes in
stent-graft morphology, and detection of new aneurysms. The imaging
requirements are quite different from those after surgical repair of abdominal
aortic aneurysm, in which imaging is limited in scope and frequency. The
single most useful imaging test that allows rapid assessment of patients with
stent-grafts is contrast-enhanced helical CT
[86].
Contrast-enhanced CT scans are sensitive for the detection of endoleaks,
although some authors have promoted color-flow Doppler sonography
[10,
65,
87,
88]. Visualization of contrast
material in branch vessels without sac opacification, as opposed to actual sac
opacification by contrast material, may not represent an endoleak, although
this possibility remains debatable
[84]. However, the crucial
issue after stent-graft placement is correlation of sac opacification from any
cause with pressure within the aneurysm sac because continued pressurization
results in continued risk of abdominal aortic aneurysm rupture
[89]. This relationship may
prove difficult to determine because there is no easy way to obtain sac
pressures in a noninvasive manner. Some investigators have suggested that lack
of visualization of contrast material within the aneurysm sac does not
guarantee lack of pressurization
[24,
73,
90].
The lack of knowledge of long-term outcomes and the low but real incidence
of delayed abdominal aortic aneurysm rupture after endovascular repair
indicate that the duration of follow-up required for patients with
stent-grafts is open-ended. Essentially, all patients with stent-grafts should
be followed up for life with contrast-enhanced CT (Fig.
14A,14B).
In addition, abdominal radiographs are invaluable tools for assessment of the
integrity of the metallic components of the stent-graft. Until larger studies
of imaging after stent-graft placement become available, the imaging protocol
in Table 3 is recommended.
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Fig. 14A. —72-year-old man with stent-graft that requires life-long follow-up.
Axial contrast-enhanced CT scan obtained 1 year after insertion of bifurcated
stent-graft shows no evidence of endoleak. Diameter of abdominal aortic
aneurysm had decreased compared with that seen on pretreatment CT scan (not
shown).
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Fig. 14B. —72-year-old man with stent-graft that requires life-long follow-up.
Axial contrast-enhanced CT scan obtained during subsequent hospitalization for
septic knee joint. Patient complained of abdominal pain. Acute expansion,
perianeurysmal inflammatory changes, and rupture (arrow) of abdominal
aortic aneurysm are present, without opacification of sac. At surgery, pus was
found in sac, but no evidence of endoleak. Organism in sac was same as that in
joint.
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Future Trends
Endovascular repair of abdominal aortic aneurysm is a new evolving
procedure that is undergoing rapid clinical implementation. As is often the
case with new technologies, there is more that we do not know than we do. It
is entirely possible that this procedure will undergo major modification as
long-term outcomes become known. Even at this early stage, it is evident that
a good stent-graft by itself is not enough to make this procedure successful.
Persistent perfusion of the aneurysm sac from retrograde flow in branch
vessels is a major long-term concern. An additional intervention may be
required, such as sac ablation with a thrombogenic substance, to ensure
complete depressurization of the aneurysm. Perhaps combined endovascular
deployment of the stent-graft with retroperitoneal endoscopic ligation of
branch vessels will be required
[91].
Patients whose aneurysms begin just below the renal arteries are difficult
or impossible to accommodate with current stent-grafts. Experience with the
placement of uncovered metal extensions of the stent-graft over the renal
artery ostia to obtain a secure proximal seal has been favorable, with no
increased incidence of renal dysfunction when compared with infrarenal designs
[92,93,94].
The development of stent-grafts with side-arms to accommodate critical aortic
branches will further increase the applicability of this technology
[95,
96].
Despite the immense clinical and industrial enthusiasm that stent-grafts
have generated, careful evaluation of the outcomes in terms of cost and
quality of life for the patient is necessary. Stent-graft repair of abdominal
aortic aneurysm requires less utilization of hospital resources, but the
device is 10-20 times more expensive than a surgical graft, and use of costly
imaging studies for follow-up is more extensive. Careful analysis and modeling
of current costs suggest that stent-graft repair is cost-effective provided
that few conversions or reinterventions are necessary
[97,
98]. Patient interest in the
procedure is recognized as a powerful incentive driving the application of
this technology [99].
Certainly all current cost estimates and models are limited in that
percutaneous outpatient endovascular repair is inevitable, as are adjunctive
procedures that will reduce the incidence of type II endoleaks. Although
current results with stent-grafts do not justify modification of traditional
size thresholds for repair in patients with abdominal aortic aneurysm, this
too may change in the future
[21].
In the simplest of terms, a successful stent-graft procedure results in
freedom from aneurysm rupture with long-term device patency. Although early
results are promising, the future role of this technology in the treatment of
abdominal aortic aneurysm is not certain. Imaging has maintained and will
continue to maintain a central role in the preprocedural evaluation of
candidates, deployment of stent-grafts, and follow-up of patients with these
devices.
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Top
Introduction
Abdominal Aortic Aneurysms
