Over the past 20 years the availability of aortic stent-grafts has allowed a major
step change in the management of abdominal aortic aneurysms (AAA). Prior to this open
surgical resection, first described by Charles Dubost in 1950, was the mainstay of
AAA management(1). For elective cases open surgical repair has now largely been superseded
by the deployment of a covered stent (stent-graft) through minimal surgical access
in the common femoral arteries. Over the past few years there have been significant
developments in stent-graft technology and an improved understanding of how best to
utilise stent-grafts when treating aortic disease. Despite this, their role in the
management of a patient with an AAA is still, at times, debatable. This article aims
to discuss the modern day utilisation of aortic stent-grafts in patients with AAA.
Introduction
Based on data from the Health and Social Care Information Centre there were 63.9 million
people registered with a general practitioner (GP) in the United Kingdom in 2007(2).
For those aged between 65 and 80 years the incidence of an AAA is 7.6% in men(4) and
lower (4.2%) for women(5). For an average General Practice of 6,300 patients(3) this
would suggest that around 44 patients will have an AAA at any one time. If left untreated,
Wilmink and Quick(6), estimate that a third of these aneurysms would eventually rupture.
Rupture is usually lethal with overall mortality rates of up to 90% common(7). Currently
around 7,000 people in England and Wales die each year as a result of ruptured AAA(8).
Endovascular aortic aneurysm repair (EVAR) is a treatment option directed at removing
the risk of rupture in patients with known AAA. Over recent years EVAR has developed
into the most common method for the elective management of AAA(9) and large randomised
controlled trials (RCTs) are evaluating its efficacy in those with rupture(10, 11).
Potential advantages of EVAR over traditional open repair include reduced time under
general anaesthesia, elimination of the pain and trauma associated with major abdominal
surgery, reduced length of stay in the hospital and intensive care unit (ICU), and
reduced blood loss(12).
Like open surgical repair, decisions regarding whether to use EVAR for the management
of an AAA are still partly based on the maximum aneurysm diameter. The natural course
of an AAA is continued expansion of between 2-3mm per year and a rupture risk which
is exponentially related to AAA diameter(4, 13). Treatment recommendations are based
on data from the UK Small Aneurysm Trial(14) and advise the referral of patients with
large AAA (>5.5 cm) to a vascular specialist for consideration of surgery (15). Patients
with smaller aneurysms, either found incidentally or in screening programmes, are
now followed up with surveillance ultrasound (US) examinations generally run through
Vascular Surgery units.
Technique
EVAR involves the internal lining of the aorta using a stent-graft. A stent-graft
comprises a metallic (stainless steel or nitinol) skeleton covered with an impermeable
(polytetrafluoroethylene or polyster) fabric and is implanted using fluoroscopic guidance
through the femoral arteries. Sealing of the device against the aortic wall is achieved
above (proximal) and below (distal) the aneurysm and this thereby excludes the aneurysm
from the systemic circulation and aims to prevent subsequent rupture (Fig 1). Sealing
of the stent-graft unlike a surgically sutured anastomosis is achieved by the radial
force of the stent-graft on the aortic wall. Three configurations of stent-graft are
currently available: tube, bifurcated and aorto-uni-iliac (AUI) (Fig 2). Bifurcated
systems are used in the majority (90%) of cases(16-18) having the added advantage
of being more stable within the aorta and avoiding the risks of supplying blood to
the lower limbs via a single common iliac artery (CIA).
Fig 1
Diagram illustrating the components of a bifurcated modular aortic stent-graft used
to treat infrarenal abdominal aortic aneurysms
Fig 2
Configurations of aortic stent-graft
EVAR was first undertaken by a Ukranian surgeon Nicholas Volodos in 1987(19); however,
it was a later publication by Juan Carlos Parodi in 1991(20) that was responsible
for the widespread introduction of EVAR across the globe. In the UK, EVAR is typically
performed by an endovascular team that includes a vascular surgeon and an interventional
radiologist. Procedures may be performed under general, regional or local anaesthesia.
The stent-graft is delivered into the aorta within a long flexible sheath of between
16F to 24F (8mm) in diameter which allows the stent-graft to be remotely positioned
within the abdominal aorta. Surgical exposure and control of the common femoral arteries
is the most common means of accessing the aorta. Totally percutaneous EVAR is now
available with haemostasis achieved using a percutaneous suturing device. This technique
requires careful patient selection with excessive vessel calcification and obesity
being associated with technical failures and increased access site complications(21).
For modular devices, composed of two or more parts, the main component (main body)
is inserted first. The undeployed device is then positioned so that the fabric at
the proximal margin of the device is immediately below the most caudal renal artery.
Once deployment has started, a check angiogram is undertaken to confirm the deployment
position and the patency of the renal arteries. The main body is then released and
the device extended so that both distal ends are within the CIAs (Fig 1).
CIA aneurysms are present in around 43% of patients with intact AAA(22). For these
patients a decision is made on whether to extend one or both stent-graft limbs into
the external iliac artery (EIA). If this is the case then it may be necessary to embolise
the internal iliac artery (IIA) in order to prevent backfilling of the aneurysm. Occlusion
of an IIA may cause significant buttock claudication and iliac branched devices (Fig
3) are now available which provide an option for preserving blood flow to the IIA(23).
Whichever strategy is employed, every effort is made to preserve at least one IIA
since bilateral IIA occlusion is associated with a risk of neurological and ischaemic
complications.
Fig 3
Iliac branched devices (IBD) are an option for treating an isolated CIA aneurysm or
an aorto-iliac aneurysm avoiding the need for deliberate occlusion of the IIA. Image
A, a CT maximum intensity projection (MIP) demonstrating an isolated CIA aneurysm.
Image B shows an in vitro deployed IBD. Image C, post-procedural angiogram of an isolated
CIA aneurysm (same case as image A) successfully treated with an IBD
Patient Selection
The majority of AAA are first diagnosed on ultrasound (US). If EVAR is considered
then a further assessment of aneurysm morphology is necessary. For the majority of
patients this will involve a thin-slice contrast-enhanced CT scan of the abdomen and
pelvis. The CT data are then displayed as a series of virtual 3D aortic models. From
this the diameter, length and quality of the aorta immediately below the renal arteries
(aortic neck) can be assessed together with the suitability of the distal landing
zones (CIA) and femoral artery access vessels (Fig 4). One of the fundamental aims
of the EVAR procedure is to cover the entire aorto-iliac segment from below the renal
arteries to the CIA bifurcation. Vessel length measurements are therefore very important
and stent-grafts are planned to be of adequate length whilst allowing sufficient overlap
of the modular components. Suitability for EVAR is also determined by the relevant
stent-graft manufacturer's eligibility criteria set down in their ‘ Instructions for
Use'. One of the strictest criteria surrounds the aortic neck which, for the majority
of devices, must be parallel for 15 mm below the lowest renal artery, free from thrombus
and excessive angulation. Off-label use of stent-grafts, by treating patients who
do not fulfil such anatomical inclusion criteria, is associated with poorer outcomes
but is known to be common practice(24).
Fig 4
Criteria used to assess anatomical suitability for EVAR
It is also important that any preoperative imaging data are scrutinised for the presence
of non-aortic pathology. Incidental pathology has been reported in over 75% of cases,
with around 20% considered as clinically significant e.g. malignancy(25). In these
situations unexpected findings may affect the decision to treat the aneurysm or alter
the timing of repair.
Complications
Complications following EVAR can occur early or late and are a distinct limitation.
Large RCTs have documented a 20-30% higher complication rate for EVAR when compared
to open surgery(26, 27). This has led to some opposition to the widespread use of
EVAR. Renal impairment, graft-related endoleak, device occlusion, migration, distal
embolisation and femoral access site complications all may be encountered(28).
A decline in renal function is often seen after EVAR and usually reverts back to preoperative
levels. Permanent renal damage may result from deliberate or unintentional coverage
of a renal artery by the stent-graft fabric, toxic effects of the iodinated contrast
media and cholesterol emboli. Graft occlusion may occur and is generally the result
of poor blood flow due to either graft kinking or poor outflow(29). Infection of an
aortic stent-graft and embolisation to the lower limb from arterial debris dislodged
during implantation are both rare. Vascular access complications can occur and include
haematoma, infection and seroma.
Persistent blood flow outside the stent-graft and within the aneurysm is defined by
WHITE et al. (30) as an endoleak (Fig 5). The most serious endoleaks are types I and
III which are associated with aneurysm enlargement and rupture. Secondary intervention
to correct these endoleaks is almost always necessary. Whilst rupture has been reported
with type II endoleaks these are considered to have a more benign course and conservative
management is recommended unless there is evidence of continuing aneurysm enlargement.
Fig 5
Endoleak classification system
Stent-grafts are subjected to distraction forces from the relentless force of pulsatile
blood flow. These distraction forces act longitudinally and challenge the fixation
of the graft and the modular overlap zones. Failure of fixation will lead to migration
or disconnection of overlapping components with late type I or type III endoleaks
and the risk of aortic rupture. Graft limb distortion with subsequent thrombosis can
also arise secondary to device migration. Metallic component fractures and fabric
tears have also generated additional durability concerns. Component fractures may
lead to diminished stent strength and loss of radial force which can result in graft
migration. The jagged ends of metal fractures can also result in fabric tears and
subsequent endoleaks.
Secondary Intervention And Surveillance
The complication rate associated with EVAR has been described by some as its Achilles
heel. The EUROSTAR registry reported EVAR outcomes for 2846 patients and highlighted
reintervention rates at 1, 2, 3 and 4 years of 6, 9, 12 and 14 per cent(31). These
rates are similar to those reported in the main UK and European RCTs (32). There is
pathological evidence that the aneurysm wall atrophies with the depressurisation that
results from an initially successful EVAR. If there is late repressurisation of the
aneurysm due to endoleak then rupture may occur(Fig 6). Rupture should be an urgent
consideration in any post-EVAR patient presenting with acute abdominal pain with or
without collapse. In the UK EVAR trials a total of 27 post-EVAR ruptures have been
reported(33). Eighty-two per cent of these ruptures occurred greater than 30-days
following implantation and the majority of these (63%) were in patients with previously
reported complications or signs of failed EVAR. In a review of the literature Schlösser
et al.,(34) also reported on post-EVAR ruptures (n=270). In this series 35 patients
had no prior abnormalities found during follow-up, 39/101 ruptures showed no change
in aneurysm diameter during follow-up and 26/101 showed evidence of aneurysm regression.
Research has identified types I and II (with sac enlargement) endoleaks, migration
and graft kinking as risk factors for aortic rupture(33). With the possibility of
rupture all EVAR patients are recommended to undergo regular follow-up imaging surveillance,
especially for patients where secondary intervention would be considered.
Fig 6
Post-EVAR CT image of a patient with a ruptured AAA. The aortic stent-graft (yellow
arrow) can be seen within the aneurysm and there is extensive haemorrhage surrounding
the aorta (white arrows)
Current EVAR surveillance protocols are mostly based on costly and time-consuming
imaging procedures and aim to detect adverse events such as graft migration, endoleaks
or aneurysm enlargement. These imaging procedures are either associated with serial
radiation exposure or may be potentially harmful due to the use of iodine- or gadolinium-based
contrast agents. CT is considered by many as the gold standard for imaging surveillance
and is typically performed at 1, 6 and 12 months and then annually thereafter. With
the economic and logistical drawbacks of repeat CT a demand has arisen for an alternative
examination. Combined ultrasound and plain abdominal radiography (AXR) has emerged
as an alternative with studies confirming their efficacy(35, 36).
Device Modifications
The main anatomical feature that limits the role of current devices is the quality
and length of normal aorta below the renal arteries (aortic neck). For standard stent-grafts
the fabric cannot be placed above the renal arteries without occluding them. If the
aortic neck is small or non-existent then renal blood flow may be preserved by manufacturing
holes or fenestrations in the fabric and sealing the aneurysm above the renal arteries.
Fenestrated stent-grafts (Fig 7) have been available for several years and are now
in widespread use. The locations of the fenestrations must be carefully planned using
preoperative CT data and are manufactured to be at the correct height and correct
position on the circumference of the graft.
Fig 7
An in vitro deployed fenestrated stent-graft (Image A). Image B – a magnified view
demonstrating a pre-planned fenestration within the proximal component. Image C –
a preoperative CT scan of patient with a short and angulated aortic neck that is unsuitable
for standard EVAR. Image D - postoperative CT scan (same patient as Image C) showing
a deployed fenestrated stent-graft with exclusion of the AAA and patent visceral arteries
Fenestrated stent-grafts are not well suited if the fabric of the stent-graft cannot
abut the aortic wall at the level of the visceral arteries. This is common in thoraco-abdominal
aortic aneurysms (TAA) and for these situations custom made stent-grafts with side
branch protrusions can be used(37). The idea is that the stent-graft side branches
will provide a bridge to reduce the distance from the main body to the visceral artery
orifice (Fig 8). Covered stents are inserted through the branches and help achieve
a better seal due to a longer overlap zone.
Fig 8
Image A demonstrates an AAA where apposition of the stent-graft against the aortic
wall across the visceral arteries would be unlikely. For this situation a branched
aortic stent-graft (Image B) was successfully utilised (Image C)
The utility of aortic stent-grafts has also extended to aneurysm disease above the
diaphragm (TEVAR). Even with a lack of RCT evidence stent-grafts are now the preferred
treatment option for aneurysms involving the thoracic aorta. The less invasive nature
of TEVAR offers the potential for lower mortality and post-operative morbidity. Anatomical
suitability is still a concern; many thoracic aneurysms lie close to or involve the
origins of the great vessels arising from the aortic arch. Advances in fenestrated
and branched stent-grafts have expanded the application of EVAR for these more complex
aortic pathologies and those involving the thoracoabdominal aorta and aortic arch(38).
Combined surgical debranching and stent-graft implantation procedures exist(39) and
the implantation site can also be lengthened, whilst preserving side-branch blood
flow, using a branched stent-graft(40).
In the early days of EVAR treatment decisions were relatively straightforward. With
the introduction of these newer technologies there is an increased range of endovascular
options for patients. Appropriate consideration of all of these newer techniques must
be given when considering a patient for any intervention.
The combination of metal and currently available fabrics has resulted in cases of
fabric degeneration with resulting endoleaks. Alternative devices which do not rely
on a fabric mounted on a metallic frame are being considered. Such devices may allow
the treatment of an AAA by modulating haemodynamic flow(41). In other areas research
has focused on treating the aneurysm by the injection of polymers and elastomers.
The Endologix Nellix system (Endologix, Irvine, CA) uses two polymer PTFE endobags
that are inserted and seek to freeze the aneurysm and prevent any further morphological
changes. Other researchers have attempted to fill the aneurysm with polymers whilst
temporarily occluding the aortic lumen(42, 43). All of these technologies are currently
in the early phases of development and are not part of routine care.
Current Evidence
The most important sources of evidence concerning EVAR are the UK EVAR trials (EVAR
1 and EVAR 2). In the EVAR 1 trial(44) 30-day mortality data showed a significant
advantage in favour of EVAR (1.7%) when compared with open surgery (4.7%). All-cause
mortality was similar for both procedures by 4-years (28%), however a 3% difference
in aneurysm-related mortality, in favour of EVAR remained (4% versus 7%)(17). Post-operative
complications were more frequent for EVAR (41%) but had a negligible difference on
health-related quality of life (HRQL), being similar for both procedures. Over the
long-term the benefit of a lower aneurysm-related mortality for EVAR was eventually
lost due to a higher incidence of fatal ruptures in the EVAR group(26). New complications
were still occurring up to 8 years following treatment.
EVAR 2 was designed to investigate the role of EVAR in patients deemed unfit for open
surgery(45). The 30-day operative mortality for the EVAR group was 7.3%, higher than
in the EVAR 1 trial. This trial did not demonstrate a survival benefit for EVAR in
patients unfit for open repair, EVAR was costly and did not improve HRQL. With long-term
follow-up there is a benefit from EVAR in terms of a lower aneurysm-related mortality
(3.6 vs 7.3 deaths per 100 person-years)(46). No differences in total mortality rates
have been demonstrated and the differences in aneurysm-related mortality are limited
since this cohort has a limited life expectancy with few surviving >8 years.
Similar results to EVAR 1 have been published by the Dutch Randomised Endovascular
Aneurysm Management (DREAM) trial group(47). 30-day mortality rates of 1.2% (EVAR)
and 4.6% (open repair) are consistent with the UK trial as is the persistent reduction
in aneurysm-related deaths in the EVAR group. Again, no overall survival difference
was observed during the mid and long-terms(27). By six years, higher incidences of
complications and reinterventions were still being reported in the EVAR group.
The more recent US OVER (Veteran Affairs Open vs. Endovascular Repair) trial demonstrated
a lower 30-day mortality for EVAR (0.5%) when compared with open repair (3.0%)(48).
This survival advantage, unlike in other studies, was maintained by 2-years. The more
recent French ACE trial(49) reported no significant differences in 30-day mortality
between open surgery and EVAR (0.6% vs 1.3%). This continued at one and three years
but there were higher reintervention rates for EVAR (24% versus 14%). The French study
opposes the findings of the three previous RCTs which all favoured EVAR in terms of
short-term mortality. Even the large case-matched Medicare analysis (45,660 patients)
showed a reduction of post-operative mortality favouring EVAR (1.2% vs. 4.8%)(50).
Differences between the ACE trial results and the remaining EVAR mortality data can
be attributed to numerous factors which may include study design, experience of the
centres and the overall national standards of care.
Three multi-centre trials are currently recruiting or have recently completed evaluating
open repair and EVAR in patients with ruptured aneurysms. The Amsterdam Acute Aneurysm
(AJAX) trial started in 2004(51) and initially failed to show any differences between
techniques for ruptured AAA. Recruitment was extended twice and the study finally
completed in 2010 and showed no difference in mortality between the two techniques(52)
. The Paris-based Endovasculaire vs Chirurgie dans les Anevrysmes Rompus (ECAR) trial
started in 2008 and aims to recruit 190 patients(53). The larger and more recent UK
Immediate Management of the Patient with Rupture: Open versus Endovascular Repair
(IMPROVE) trial started in 2009 and aims to recruit 600 patients(10).
One of the main criteria for AAA treatment is maximum aneurysm diameter. Data from
the UK Small Aneurysm Trial(14) reported a 5.8% 30-day operative mortality rate for
AAA between 4.0-5.5 cm and concluded that there is no long-term survival advantage
from early surgery. With the lower 30-day mortality for EVAR questions have arisen
whether this treatment criterion should now be revised. The European CAESAR (Comparison
of Surveillance versus Aortic Endografting for Small Aneurysm Repair) trial failed
to demonstrate any advantage of early EVAR over traditional surveillance(54). 30-day
mortality rates for EVAR were low (0.6%) with overall aneurysm-related mortality and
rupture rates low and comparable between the groups. The study did, however, raise
concern that up to 60% of surveillance patients will require treatment within 3 years
and that 16% of these patients may lose anatomical suitability for EVAR during this
time. Similarly, the US PIVOTAL (Positive Impact of Endovascular Options for Treating
Aneurysms Early) trial showed that early all-cause mortality for both EVAR and surveillance
groups were equal (4.1%)(55).
Conclusion
Over the past three decades the number of patients treatable by EVAR has grown. What
started as a series of devices constructed in the operating theatre has evolved into
mass produced ‘off-the-shelf’ systems which can treat a range of patients. Not only
has anatomical eligibility increased but other vascular diseases are now being treated
using a stent-graft. The endovascular treatment of complex type B dissections, traumatic
aortic transections and aorto-enteric fistulae is possible. Pushing the boundaries
of both patient and disease selection does, however, bring additional uncertainties.
Studies published over recent years have shown that ‘off-label’ device use brings
poorer outcomes and post-EVAR rupture will occur in a limited number of patients.
Nevertheless, we have seen huge developments in EVAR technologies and their applicability.
Devices are now deployable on smaller delivery systems, are repositionable within
the aorta and can conform to more challenging anatomy. Durability issues still exist
and appropriate diligence is needed during follow-up. With this in mind, manufacturers
are now starting to explore alternatives to a metallic skeleton covered by an impermeable
fabric. Open surgery dominated for over forty years but we are currently living in
the endovascular era and we anticipate further improvements in the safety and applicability
of endovascular therapy for aortic aneurysms.
The authors have no conflicts of interest.