Aortic Neck Dilation after EndovascularAbdominal Aortic Aneurysm Repair: ShouldOversizing Be Blamed?

Sergio M. Sampaio, MD,1 Jean M. Panneton, MD,1 Geza Mozes, MD, PhD,1

James C. Andrews, MD,2 Audra A. Noel, MD,1 Manju Kalra, MB, BS,1

Thomas C. Bower, MD,1 Kenneth J. Cherry, MD,1 Timothy M. Sullivan, MD,1 and

Peter Gloviczki, MD,1 Rochester, Minnesota

Long-term durability after endovascular abdominal aortic aneurysm repair (EVAR) is dependentupon the maintenance of an effective seal between the endograft and the proximal landing zone.Continuous neck dilation might lead to the loss of such a seal. This study aims at evaluating theincidence, risk factors, and clinical consequences of post-EVAR aneurysm neck dilation inpatients treated with two types of endografts: AneuRx� and Ancure�. We reviewed data con-cerning all consecutive patients submitted to primary EVAR using the AneuRx and Ancuredevices. Preoperative neck anatomic characteristics (diameter, calcification, and thrombus load)were evaluated, and device oversize percentage was calculated. Postoperative same-level neckdiameter was measured on all postoperative computed tomographic (CT) scans. Probabilities ofneck dilation (‡10% and ‡15%) relative to preoperative diameter and first postoperative diam-eter were estimated with the Kaplan-Meier method and compared between patients using bothtypes of endograft. The impact of anatomic characteristics on neck dilation incidence wasevaluated using Cox proportional hazards models. Mean neck dilation was compared betweenpatients with and without device migration and proximal type I endoleak. Both groups had similarprobabilities of dilating >10% relative to preoperative diameter and to first postoperative diam-eter. Proximal necks in AneuRx-treated patients had higher probabilities of dilating ‡15% relativeto preoperative diameter than Ancure-treated patients (45.5% vs. 18.7% at 1.5 years,p = 0.025), but the probability of such dilation relative to the first postoperative diameter was notdifferent between the two groups (12.4% vs. 9.1% at 1.5 years, p = 0.832). None of the pre-operative neck characteristics was associated with neck dilation risk. Device oversize per-centage was correlated with the percentage of neck dilation at first postoperative CT scanrelative to preoperative diameter in both the AneuRx (correlation coefficient = 0.469, p < 0.0001)and the Ancure (correlation coefficient = 0.464, p < 0.011) groups, but it was not correlated withthe percentage of neck dilation at 1 or 1.5 years relative to first postoperative CT scan in eithergroup. Patients with and without caudad device migration (‡5 mm) had similar percentages ofneck dilation at 1.5 years relative to preoperative diameter, but migrators had higher meanpercentages of dilation at 1.5 years relative to first postoperative neck diameter (11.4% vs. 5.6,p = 0.012). Two phenomena may be differentiated: an immediate postimplant dilation, stronglycorrelated with the percentage of oversize and more likely to reach values ‡15% with an AneuRxdevice than with an Ancure graft, and a subsequent dilation, relative to the first postoperativelymeasured diameter, equally probable with either type of device, not correlated with the per-centage of oversizing but associated with caudad device migration. Our study does not supportany adverse role for the degree of oversize.

1Division of Vascular Surgery, Mayo Clinic, Rochester, MN, USA.2Department of Radiology, Mayo Clinic, Rochester, MN, USA.

Presented at the Twenty-sixth Meeting of the Canadian Society forVascular Surgery, October 22-23, 2004, Quebec City, Quebec, Canada.

J. M. Panneton is currently at Eastern Virginia Medical School andVascular and Transplant Specialists, Norfolk, VA.

Correspondence to: Jean M. Panneton, MD, Vascular and TransplantSpecialists, 250 West Brambleton Avenue, Suite 101, Norfolk, VA,23510, USA, E-mail: jpanneton@vascularandtransplant.com

Ann Vasc Surg 2006; 20: 338-345DOI: 10.1007/s10016-006-9067-2� Annals of Vascular Surgery Inc.Published online: May 19, 2006

338

INTRODUCTION

Since its introduction in 1991,1 endovascular aortic

abdominal aortic aneurysm repair (EVAR) main

outcome analysis has shifted from feasibility and

short-term results to long-term durability. Lasting

effectiveness is dependent upon the maintenance

of a water-tight seal between the device and the

landing zones. One of the key points in keeping

such sealing is the proximal landing zone, or

aneurysm neck: close apposition around the device

is essential in preventing proximal type I endoleaks

and device migration. Aortic neck dilation has been

associated with adverse events such as device

migration and proximal seal failure.2 Reports on

the incidence, risk factors, and consequences of

aortic neck dilation, however, are remarkably

scarce. This study aims at investigating these issues

in two cohorts of patients: one treated with the

AneuRx� (Medtronic, Santa Rosa, CA) and an-

other with the Ancure� (Guidant, Menlo Park, CA)

device.

METHODS

We abstracted information relative to all consecu-

tive patients undergoing primary EVAR at our

institution between December 15, 1999, and May

7, 2003, using the following inclusion criteria:

� Use of either the AneuRx or the Ancure device

� Primary EVAR

� Availability of a preoperative computed tomo-

graphic (CT) scan

� Availability of at least two postoperative CT

scans, performed on two different occasions

Anastomotic aneurysms were excluded. Postopera-

tive surveillance consisted of a visit during the first

month, every 6 months during the first 2 postoper-

ative years, and thereafter yearly. Imaging modali-

ties included a contrast-enhanced CT scan,

ultrasound, and plain abdominal radiographs at each

follow-up visit. CT scans were obtained using GE

Lightspeed� scanners (General Electric, Milwaukee,

WI). Noncontrast images, 5 mm thick with 5 mm

spacing, were obtained through the abdomen and

pelvis. Contrast-enhanced images were then ob-

tained with a single breath-hold during the injection

of 120 mL of non-ionic contrast at 4 mL/sec. Helical

acquisition was performed with reconstruction of

the images into 2.5 mm slices every 2.5 mm.

All CT scan measurements were performed using

electronic calipers, available on the digital image

archive software, allowing for a 300% magnifica-

tion.

Mid-neck level was defined as the CT slice at

mid distance between the more caudad renal

artery and the craniad extremity of the aneurysm

sac. The distance in the CT table position be-

tween this level and the last slice showing the

most caudad main renal artery was recorded and

used as the reference to determine the level for

postoperative neck diameter measurements in

order to ensure that all subsequent measurements

were performed at the same aortic segment.

Adventitia to adventitia distances were evalu-

ated,3 using the minor axis of the cross-sectional

ellipse. Neck dilation was evaluated as a per-

centage of dilation relative to the baseline diam-

eter, according to the formula [(neck diameter at

any postoperative time point) – (baseline diameter)]/

baseline diameter. Two different thresholds for neck

dilation were set (10% and 15%), and their

probability was estimated relative to two different

baselines: preoperative diameter and first postop-

eratively measured diameter.

Mean percentages of neck dilation were calcu-

lated at first postoperative CT scan (relative to

preoperative diameter) and at 12 and 18 months

post-EVAR (relative to the two above-mentioned

baselines).

Proportion of proximal endograft oversizing was

calculated using the following formula: (endograft

diameter - mid-neck diameter)/mid-neck diameter.

Calcification and thrombus presence were

quantified at the mid-neck level (defined above),

aiming at characterizing the planned landing seg-

ment. Calcification was measured as a percentage

of calcified neck perimeter. Thrombus presence was

quantified in three different ways: thrombus max-

imal thickness, percentage of thrombus-lined neck

wall, and percentage of cross-sectional area occu-

pied by thrombus.

Reverse-tapered necks were defined as the ones

in which the difference between the most caudad

and the most cephalad neck diameters was ‡3 mm.

Aneurysm maximal diameter was measured

using adventitia to adventitia distances along the

ellipse minor axis.3 The percentage of the cross-

sectional area occupied by thrombus was also

evaluated at the maximal diameter level.

Endograft position was evaluated, measuring the

distance in the CT table positions between the last

slice showing the most distal renal artery and the

first slice showing evidence of endograft. Endograft

migration was defined as an increase ‡5 mm in this

distance, assuming as baseline the distance mea-

sured in the first available postoperative CT scan.

The 5 mm threshold was chosen according to the

Lifeline Registry guidelines.4

Vol. 20, No. 3, 2006 Aortic neck dilation after EVAR 339

All postoperative CT scans were also reviewed

for the presence of proximal type I endoleaks.

Whenever characterization of the type of endoleak

was not possible based on CT scan, angiography

reports were reviewed.

Statistical Analysis

The probabilities of neck dilation ‡10% and ‡15%

were estimated using the Kaplan-Meier method,

and Wilcoxon�s survival test was used to compare

the two devices. Associations between probability

of neck dilation and preoperative anatomic neck

characteristics were investigated with Cox�s pro-

portional hazards models.

Impact of neck dilation on adverse events was

tested comparing means of percent neck dilation

(at first postoperative CT scan, at 12 and 18 months

post-EVAR) between migrators and nonmigrators

and between patients with and without proximal

type I endoleak. Whenever normality assumptions

were not met, Wilcoxon�s rank sum tests were

used; otherwise, two sample t-tests were per-

formed. p < 0.05 was considered statistically sig-

nificant.

RESULTS

From a total of 144 patients, 112 (77.8%) were

submitted to EVAR using the AneuRx device and

32 (22.2%) using the Ancure endograft. Median

available follow-up was lower in the AneuRx-

treated patients: 257 vs. 629 days.

Neck Dilation

Fifty-four necks in the AneuRx group and 19 necks

in the Ancure group dilated ‡10% relative to the

preoperative diameter. The probability of neck

dilation ‡10% relative to preoperative diameter

was not different between patients treated with the

AneuRx [91.9% at 2 years, standard error

(SE) = 7.2] and Ancure (80% at 2 years, SE = 9.4)

devices, p = 0.53. The median time to neck dilation

‡10% was 290 days in the AneuRx group and 365

days in the Ancure group (Fig. 1).

When we defined neck dilation as an increase

‡15% relative to preoperative neck diameter, such

an event occurred in 30 AneuRx-treated patients

and in seven Ancure-treated patients. AneuRx

patients had a higher probability of such dilation

(45.5%, SE = 8.1 at 1.5 years) than Ancure pa-

tients (18.7%, SE = 8.6 at 1.5 years), p = 0.025.

Median time to neck dilation ‡15% was 585 days

in the AneuRx group and 1,066 days in the Ancure

group (Fig. 2).

Taking the first postoperatively measured neck

diameter as baseline, subsequent neck dilation

‡10% occurred in 21 patients from the AneuRx

group and eight patients from the Ancure group.

Probabilities of these events were not different

among the AneuRx (36.1% at 1.5 years, SE = 9.1)

and Ancure (23.5% at 1.5 years, SE = 9.26) groups,

p = 0.366 (Fig. 3). Using the same baseline, neck

Fig. 1. Freedom from early neck dilation (‡10% relative

to preoperative diameter) after EVAR. Dark line, Ancure;

light line, AneuRx; p = 0.532, Wilcoxon�s test. SE <10%

for the entire curves� length.

Fig. 2. Freedom from early neck dilation (‡15% relative

to preoperative diameter) after EVAR. Dark line, Ancure;

light line, AneuRx; p = 0.025, Wilcoxon�s test. SE <10%

for the entire curves� length.

340 Sampaio et al. Annals of Vascular Surgery

dilation ‡15% was observed in eight AneuRx-

treated patients and two Ancure-treated patients,

both groups having similar probabilities of such an

event (at 1.5 years, AneuRx 12.37%, SE = 5.4;

Ancure 9.1%, SE = 6.2), p = 0.832 (Fig. 4).

Neck Dilation Magnitude

Mean neck dilation at the first postoperative CT

scan was similar between AneuRx [5.7%, standard

error of the mean (SEM) = 0.7] and Ancure (4.5%,

SEM = 1.2) patients, p = 0.226.

At 1 year, AneuRx-treated necks had dilated on

average 5% (SEM = 0.8) and Ancure-treated ones

4.7% (SEM = 1.3) relative to their first postopera-

tively measured diameter, p = 0.863.

At 1.5 years, mean neck dilation was still not

different between the AneuRx (6.5%, SEM = 1) and

the Ancure (6.5%, SEM = 1.3) groups, p = 0.973.

Neck Dilation Determinants

The impact of preoperative anatomic neck charac-

teristics was investigated in the cohort as a whole

and separately for each endograft group. None of

the neck characteristics was significantly associated

with the probability of neck dilation (using either

the 10% or the 15% threshold) relative to the

preoperative neck diameter or the first postopera-

tive diameter (associations tested using Cox pro-

portional hazards). Anatomic characteristic

frequencies for each of the endograft groups are

summarized in Table I.

Percentage of oversizing was correlated with

percentage of neck dilation at first postoperatively

measured neck diameter in both the AneuRx (cor-

relation coefficient = 0.469, p < 0.0001) and the

Ancure (correlation coefficient = 0.464, p = 0.011)

groups. Oversizing, however, was not correlated

with the percentage of neck dilation at 1 or 1.5 years

(Table II) relative to first postoperative diameter, for

either the AneuRx or the Ancure group.

Impact of Neck Dilation

Patients with and without postoperative proximal

type I endoleak (which occurred in six patients)

had similar mean percentages of neck dilation at

the first postoperative CT scan and at 1 and 1.5

years relative to first postoperative CT scan.

Patients with and without caudad device

migration (‡5 mm) had similar mean percentages

of early neck dilation relative to preoperative

diameter at first postoperative CT scan.

Migrators, however, had higher mean percent-

ages of late neck dilation at 1.5 years (11.4%,

SEM = 1.9) compared with nonmigrators (5.6%,

SEM = 0.8%), p = 0.012 (Fig. 5).

These results are summarized on Table III.

DISCUSSION

Ancure and AneuRx devices were chosen by their

characteristics: they were at the same risk of

migration regarding their absence of suprarenal

Fig. 3. Freedom from late neck dilation (‡10% relative

to first postoperative diameter) after EVAR. Dark line,

Ancure; light line, AneuRx; p = 0.366, Wilcoxon�s test. SE

‡10% for the dotted portion of the curve.

Fig. 4. Freedom from late neck dilation (‡15% relative

to first postoperative diameter) after EVAR. Dark line,

Ancure; light line, AneuRx; p = 0.832, Wilcoxon�s test. SE

‡10% for the dotted portion of the curves.

Vol. 20, No. 3, 2006 Aortic neck dilation after EVAR 341

fixation, but their respective theoretical impact on

dilation should be different due to their different

expansion modes. Neck dilation after EVAR ap-

pears to be an almost universal phenomenon. In

fact, almost every patient treated with any kind of

device the neck was dilated by 10% relative to the

preoperative diameter. This high incidence of neck

dilation is in agreement with the findings of

Prinssen et al., who found that all patients had

increased their neck area after Ancure device

implantation.5 Cao et al. estimate a 15% rate of

neck dilation 2 years after AneuRx implantation,

relative to first postoperative diameter,2 but they

had defined neck dilation as a diameter increase ‡3

mm. We believe there might be some advantages in

defining neck dilation according to a relative per-

centage and not an absolute threshold. In fact, 3

mm represents a 10% increase (a trivial finding in

our study) in a 30 mm neck but a 16.6% dilation (a

much more rare event) in an 18 mm diameter

neck. Both situations surely have different signifi-

cances, and their occurrence should not be treated

as a similar event. May et al.,6 on the other hand,

refer to freedom from neck dilation ‡3 mm of

94.4% 7 years after EVAR relative to first postop-

erative diameter, but their sample consisted of only

28 patients, with various devices.

In this study, we opted for serial measurements

at a level defined by native anatomic criteria (dis-

tance to more caudad main renal artery), rather

than using the endograft position as reference level.

We believe this option to be advantageous since

device migration, a phenomenon reported to have

an incidence that may reach 66.7% at 4 years,7

Table I. Baseline characteristics in AneuRx- and Ancure-treated subjects

Variable

Mean value amongAncure subjects(n = 32)

Mean valueamong AneuRxsubjects (n = 112) p

Neck diameter (mm) 24.4 (0.46) 25.4 (0.28) 0.104a

Percentage of calcium-lined neck perimeter (%) 13.7 (2.75) 9.4 (1.02) 0.235b

Percentage of thrombus-occupied

cross-sectional neck area (%)

33.5 (1.51) 32.1 (1.22) 0.531a

Aneurysm maximal diameter (mm) 52.9 (1.27) 55.9 (1.03) 0.142a

Proximal oversizing (%) 5.2 (2.07) 7.9 (1.08) 0.239c

at-test assuming equal variances.bWilcoxon�s rank sum test.ct-test assuming unequal variances.

Table II. Oversizing impact on neck dilation

Ancure AneuRx

Baseline % Neck dilation atCorrelation coefficientwith % of oversize pa

Correlation coefficientwith % of oversize pa

Preoperative CT scan First postoperative CT scan 0.464 0.011 0.469 <0.0001

First postoperative CT scan 1 Year 0.213 0.103 )0.205 0.340

1.5 Years 0.246 0.174 0.274 0.343

aCorrelation p values.

Fig. 5. Mean proportion of neck dilation at 1.5 years

relative to first postoperative diameter among migrators

and nonmigrators. Whiskers represent SEM. p = 0.012,

Wilcoxon�s rank sum test.

342 Sampaio et al. Annals of Vascular Surgery

would imply measuring neck diameters at levels

progressively more distal when using the endograft

position as reference level. Since a significant pro-

portion of necks have a reverse-tapered configu-

ration (reaching proportions as high as 24.2%,8

32.6% in this study�s series), this situation could

easily result in an overestimation of neck dilation

incidence. AneuRx devices increased the probabil-

ity of neck dilation ‡15% relative to preoperative

diameter but not of subsequent dilation. This

finding may result from the existence of two sep-

arate phenomena: a relatively early dilation,

resulting from the aortic neck adaptation to the

immediate radial force of the endograft presence,

and a subsequent dilation, independent of the de-

vice�s expanding forces. Interestingly, in none of

the groups did the occurrence of neck dilation

reach a plateau. As late as 900 days after EVAR,

there were still patients reaching 10% increases in

their neck diameter relative to their first postoper-

ative diameter. This suggests it will not be possible

to define a safe line in postoperative follow-up time

beyond which neck dilation should no longer be

expected.

Cao et al.2 defined the risk factors of aortic neck

dilation as large neck diameter, large aneurysm

diameter, and circumferentially thrombus-lined

neck. We believe their findings might be the result

of defining neck dilation according to an absolute

threshold (3 mm). In fact, it should be expected

that in such a frequent event as neck dilation after

EVAR is proving to be, higher absolute dilations

(but not proportional) would occur preferentially

in wider necks. Therefore, in spite of similar per-

centages of dilation between wide and narrow

necks, one would still be able to document

a higher frequency of dilation beyond a certain

absolute threshold in wider necks. To this fact,

one could add that wider necks are also correlated

with larger aneurysm diameters (correlation

coefficient = 0.174, p = 0.037 in our series), which

could possibly have contributed to the previously

mentioned associations. In our series, none of the

preoperative anatomic characteristics was associ-

ated with the probability of neck dilation (above

10% or 15% relative to preoperative or first

postoperative diameters). This lack of association

concurs with the findings of Matsumura and

Chaikof.9

Oversizing is intuitively associated with neck

dilation, and this association has been reported

several times.7,10 Such reports, however, were

based on dilations in relation to the preoperative

diameter as baseline. Cao et al.,2 evaluating neck

dilation relative to the first postoperative diameter,

failed to find any association with oversizing. We

combined both types of analysis in this study and

confirmed the role of oversizing in early dilation

relative to preoperative diameter, but subsequent

dilation was independent from device oversizing.

Dilation relative to preoperative diameter is more

likely to reach percentages ‡15% among patients

treated with AneuRx devices than among subjects

treated with Ancure grafts; however, such a dif-

ference was not present for dilation relative to first

postoperatively measured diameter, as one would

expect if continued neck dilation resulted from

radial force applied against the aortic wall.

One could hypothesize that later neck dilation,

occurring relative to a postoperative baseline, re-

sults mainly from the continuing degeneration of

the aortic neck segment and not from the

expanding forces of the device. Aortic neck dilation

after aortic aneurysm open repair,11 as well as

similar degrees of neck dilation after EVAR using

supra and infrarenal fixation devices,2 seem to

support the role of aortic wall degeneration as a

primary factor in post-EVAR aortic neck dilation.

Caution needs to be exerted when evaluating

the impact of aortic neck dilation on clinical ad-

verse events, such as proximal type I endoleak and

caudad device migration. In fact, these risk factors

Table III. Comparative mean neck dilation percentages between patients with and without proximal type

I endoleak and with and without caudad device migration

Proximal type I endoleak Migration

Baseline % Neck dilation at na Yes No pb Yes No pb

Preoperative CT scan First postoperative CT scan 144 1.4% 5.6% 0.117 3.7% 5.5% 0.499

1 Year 81 7% 11.2% 0.327 10.1% 11.1% 0.958

1.5 Years 48 16.5% 12.9% 0.336 18.2% 12.3% 0.072

First postoperative CT scan 1 Year 81 4.4% 4.9% 0.992 6% 4.7% 0.304

1.5 Years 48 9.4% 6.3% 0.349 11.4% 5.6% 0.012

aAvailable patients for analysis.bWilcoxon�s rank sum test.

Vol. 20, No. 3, 2006 Aortic neck dilation after EVAR 343

and outcomes are both time-dependent variables,

and by testing for their association in a nonsyn-

chronous cohort,2 one might simply select patients

with longer follow-up times, documenting spurious

associations. For this reason, we compared the

dilation percentages at fixed time points between

migrators and nonmigrators and between patients

with and without proximal type I endoleak. The

only significant difference was found at 18 months,

when migrators had higher mean dilation com-

pared with nonmigrators. Theoretically, one would

expect device migration to be a precursor to prox-

imal type I endoleak.12 This study may have ana-

lyzed sufficient follow-up time to detect differences

associated with migration but not endoleaks. The

method we used to assess distances is not devoid of

caveats either but allows for the direct interpreta-

tion of data from routinely used surveillance

imaging modalities. It has been used before.7,10,13

We quantified migration by reviewing axial cuts of

CT scans. Almost certainly, this methodology led to

an underestimation of the true length of migration

on heavily angulated necks.

Interestingly, a significant difference between

migrators and nonmigrators was found in the

percentage of dilation relative to first postopera-

tive CT scan, suggesting that clinically important

dilation occurs after the initial aortic adaptation to

the endograft. Conners et al.7 established an

association between dilation relative to preopera-

tive diameter and oversize degree, but they failed

to associate oversize and migration. Such an

association has never been documented, and we

hypothesize that it might in fact not exist. Of

course, more oversized necks have dilated more

by 1 or 1.5 years after EVAR, but they had al-

ready dilated more by the first postoperatively

measured diameter. Our study suggests a scenario

where all (or almost all) endograft-treated aortic

necks do adjust to the device presence, resulting

in an obvious and consistently reported associa-

tion between dilation and oversizing; some necks

keep dilating beyond such initial adaptation,

independent of the degree of oversizing, subse-

quently increasing the risk of migration. Alterna-

tively, these data may simply reflect the dilation

of proximal necks exposed to systemic pressures

after endograft slippage. Theoretically, grafts mi-

grate because of low friction forces between them

and the aortic walls. Oversizing could dilate the

aortic neck to a point of equilibrium between

graft expansion and aortic recoil, but why should

it contribute to continuing expansion beyond

such point, allowing for low friction? Less over-

sizing may simply mean that the imbalance is

achieved at the cost of a smaller degree of

expansion.

One of the obvious limitations of this study is its

small sample size. Emphasis should therefore be

put on the positive results and caution exerted

when interpreting the absence of differences across

groups.

CONCLUSIONS

Aneurysm neck dilation is an almost universal

event after EVAR. Two types of phenomena may be

differentiated: an immediate postimplant dilation,

strongly correlated with the percentage of oversiz-

ing and more likely to reach values ‡15% after

usage of an AneuRx device than after an Ancure

graft placement, and a subsequent dilation, relative

to the first postoperatively measured diameter,

equally probable after using either type of device,

not correlated with the percentage of oversizing but

associated with caudad device migration. Our study

does not support any adverse role for the degree of

oversizing.

S. M. S. is the recipient of an Edward S. Rogers Clinical

Research Fellowship in Vascular Surgery.

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