Macroporous Polyester–Covered Stent in an Experimental Abdominal Aortic Aneurysm Model

390 J ENDOVASC THER2001;8:390–400

Q 2001 by the INTERNATIONAL SOCIETY OF ENDOVASCULAR SPECIALISTS

lEXPERIMENTAL INVESTIGATION l

Macroporous Polyester–Covered Stent in anExperimental Abdominal Aortic AneurysmModel

Philippe Soula, MD; Bertrand Janne d’Othee, MD*;Philippe Otal, MD*; Chauki Amin, MD†; Joe El Khoury, MD*;Marie-Bernadette Delisle, MD†; Alain Cerene, MD;Francis Joffre, MD*; and Herve Rousseau, MD*

Departments of Cardiovascular Surgery, *Radiology, and †Pathology, CentreHospitalier Universitaire, Hopital de Rangueil, Toulouse, France

l l

Purpose: To validate a recently described animal model of abdominal aortic aneurysm(AAA) and to assess a new macroporous polyester–covered stent for endovascular AAAexclusion.Methods: Twenty adult sheep had AAAs surgically created by replacing a segment of theinfrarenal aorta with an autologous jugular venous graft. Three months later, survivinganimals underwent percutaneous implantation of macroporous polyester–covered nitinolstents; 3 animals with untreated AAAs served as controls. Follow-up surveillance includedspiral computed tomography at 1 month and digital subtraction angiography at 3 and 6months. Endografted animals were sacrificed at 1, 3, and 6 months after implantation;specimens from all animals were examined grossly and microscopically.Results: Seven (35%) animals died within 24 hours of causes related to the technique; 1animal developed paraplegia and was sacrificed on day 1. Three (25%) animals died ofspontaneous aneurysm rupture at ,10 days, and 6 received the stent-graft at 3 months.The macroporous cover did not prevent continued perfusion of the sac early after stent-graft deployment, but all aneurysms were excluded on the 1-month CT.Conclusions: Spontaneous AAA rupture occurred earlier and was not as frequent as pre-viously described for this model. Implantation of the covered stent was feasible, but an-eurysm exclusion was not immediate.

J Endovasc Ther 2001;8:390–400

Key words: aneurysm, sheep model, Memotherm stent, polyester, aneurysm exclusionl l

Despite growing knowledge in both the clini-cal and experimental fields, several questionsremain unresolved about the endovascular

The authors disclose a grant provided by C.R. Bard.

Address for correspondence and reprints: Professor HerveRousseau, Department of Radiology, C.H.U. Rangueil, 1 av-enue Jean Poulhes, 31403 Toulouse Cedex 4, France. Fax:33-5-61-32-28-51; E-mail: [email protected]

treatment of abdominal aortic aneurysms(AAA), notably the long-term consequencesof stent-grafts and the management of peri-graft leaks. These doubts have prompted thecompletion of experimental trials and feasi-bility studies before new devices are used inclinical practice.

The purpose of this study was twofold: (1)to validate a previously described1 animalmodel that creates an aneurysm by replacing

J ENDOVASC THER2001;8:390–400

COVERED STENT IN EXPERIMENTAL AAASOULA ET AL.

391

Figure 1lOperative view of a venous segmentalAAA.

a segment of the infrarenal aorta with an au-tologous internal jugular vein graft and (2) toassess a new macroporous polyester–cov-ered stent for endovascular AAA repair.

METHODS

We created AAAs in 20 male adult sheep (agerange 1–2 years) that were of the same breedand weighed between 35 and 45 kg. The ani-mals were maintained on a regular diet andwere cared for according to regulations of theEuropean Community on animal research.2

Three animals were assigned to the controlgroup, and the remainder were to receive anexperimental covered stent 3 months afterAAA creation. These stent-grafted animalswere to be sacrificed after implantation peri-ods of 1, 3, or 6 months (4, 6, or 9 monthsafter AAA creation).

Aneurysm Creation

After a 24-hour fast and intravenous seda-tion with acepromazine acetate (1 mL/10 kg),the animals were intubated and placed undergeneral anesthesia using a mixture of thio-barbital (125 mg/kg), bromochlorofluoroetha-ne (2%), and oxygen (FiO2 5 60%).

Each animal was placed supine, and afterlocal shaving and disinfection, a 5- to 7-F in-troducer was placed percutaneously in thefemoral artery. An aortogram was obtained.After turning the animal on its right side, ster-ile preparation and draping were repeated inthe cervical and abdominal regions. Througha left lateral cervicotomy, the ipsilateral inter-nal jugular vein was dissected and its branch-es were ligated. A 40- to 50-mm-long segmentof this vein was removed and stored in hep-arinized saline.

A left retroperitoneal approach to the aortawas made through a lateral skin incision. Af-ter dissection that spared the lumbar arteriesand systemic heparinization (50 U/kg), theaorta was clamped, transected, and a 1-mmcircular segment of the aortic wall was re-moved for analysis. A medullary protectionsolution containing 12,500 units of heparinand 7.5 g of N-methyl-thiourea (NMTU) per 1liter of cold (48C) chlorinated saline was in-fused intra-arterially to avoid paraplegia sec-

ondary to prolonged aortic clamping. (NMTUprotects against the effects of cellular ische-mia by inhibiting free oxygen radicals, whichreduces reperfusion injury. Its liposolubilityand low molecular weight allow easy penetra-tion into the central nervous system.) The so-lution was infused via the sheath into the dis-tal aorta (i.e., downstream of the distal aorticclamp) at a flow rate of 100 mL/min through-out aortic clamping (total volume 2L). As fa-miliarity with the technique was gained, theaortic clamping time was shortened, obviat-ing the need for medullary protection in thelast 6 sheep.

The harvested venous segment was anas-tomosed end-to-end with double rows of run-ning 6–0 polypropylene sutures (Fig. 1). Afterunclamping, hemostasis was checked. Bothproximal and distal aortic suture levels weremarked with either surgical clips or a radi-opaque cerclage. The retroperitoneum wasclosed in layers without drainage. Aortogra-phy was performed in the operating room atthe end of the procedure (Fig. 2). Periopera-

392 COVERED STENT IN EXPERIMENTAL AAASOULA ET AL.


Figure 2lDigital subtraction arteriogram of a sur-gically created AAA. Note the upper and lower me-tallic cerclage.

tive antibiotic therapy was administered intra-venously using 1.5 g cefuroxime. Extubationwas completed within 2 hours, and the ani-mals were monitored under close supervisionfor 24 to 48 hours in an isolated room.

Covered Stent Design and Implantation

The covered stent was made from a 12- 380-mm self-expanding metallic stent (Mem-otherm, Angiomed-Bard, Karlsruhe, Germa-ny) covered on the outside by a 60-mm-longknitted macroporous polyester tube, whichleft a 1-cm portion of the stent uncovered ateach end. The polyester material, which wassutured to the stent, was made of 2 differentdiameter threads (‘‘micromesh’’), which ren-dered the material macroporous (exact poros-ity undisclosed by the manufacturer owing toa pending patent). The covered stent wascompressed on a carrier catheter, which wasinserted into a 70-cm-long, 10-F sheath (deliv-ery system).

Stent-graft implantation was performed ina digital angiography suite (DRS, GeneralElectrics Medical Systems) with the animal ly-ing supine and anesthetized as describedabove. A graduated ruler was placed on theangiographic table. Percutaneous femoral ar-tery access was obtained, and an 11-F intro-ducer was inserted over a 180-cm-long, 0.035-inch superstiff Amplatz guidewire (BostonScientific/Medi-tech, Natick, MA, USA). A cal-ibrated pigtail catheter was passed through a6-F introducer in the contralateral femoral ar-tery for contrast injection. No heparin wasgiven during the procedure. The stent-graftwas positioned across the aneurysm with theaid of radiopaque markers on the carrier cath-eter and the external sheath. The device wasdeployed by trigger retrieval of the externalsheath while maintaining the carrier cathe-ter’s position. A 750-mg bolus of cefamandolewas administered intravenously at the end ofthe procedure.

One covered stent was implanted in eachanimal except the first one, in which 2 deviceswere deployed in overlapping fashion in anattempt to immediately exclude the aneu-rysm. After deployment of the covered stent,digital subtraction angiography (DSA) of theaorta was performed using pump injection of20 mL of ioxaglate (320 mg iodine/mL) at aflow rate of 10 mL/s. Both anteroposterior andlateral views were obtained at an acquisitionrate of 4 images per second.

Surveillance Imaging

Animals were examined with spiral com-puted tomography (CT) under anesthesia (in-tramuscular injections of acepromazine ace-tate [1 mL/10 kg] and ketamine [500 mg]) at 1month and with angiography at 3 and 6months, depending on the sacrifice date. Ev-ery sacrificed animal underwent DSA beforeeuthanasia. Spiral CT scanning (SomatomPlus 4 CT scanner, Siemens AG, Forchheim,Germany) was performed with 5-mm colli-mation, pitch 5 1, and a reconstruction incre-ment of 2.5 mm. Images were acquired beforeand after intravenous injection of ioxitalamatemeglumine (30 g iodine/100 mL) at a dose of2.5 mL/kg. Postcontrast data were also pro-cessed to obtain 3-dimensional views in max-



393

Figure 3lLevels of aortic sampling for pathologicexamination: A and H, aortic wall only; B and G,junction zones (stent only); C and F, intermediatezones (nitinol stent and polyester covering); D andE, upper and lower aneurysm zones (15 mm fromsurgical anastomoses).

imum intensity projections and multiplanarreconstructions.

No pressure measurements in the aneu-rysm sac were taken in this experiment. Tomonitor aneurysm sac behavior, dimensionsof the aneurysm sac and the aortic blood flowcolumn were recorded at baseline and seriallycompared during follow-up using a commondistance ratio, R, to consistently relate thesemeasurements over time, across individualanimals, and between imaging modalities. Rwas the ratio of the maximal diameter of theaneurysm sac in the transverse horizontalplane to the diameter of the blood flow col-umn in the aorta at the same level. In additionto being dimensionless and independent ofmagnification errors, this ratio was believedto reduce intermodality variability in mea-surement and provide a meaningful compar-ison among angiography, CT, and pathology.

Pathological Analysis

Specimens from all animals were retrievedand examined. Animals surviving to the endof their scheduled implantation period under-went angiography under anesthesia (intra-muscular injection of ketamine [500 mg]). Af-ter intravenous injection of 1,000 units ofheparin, the animal was sacrificed by an in-travenous injection of 2 mL of T-61 (Hoechst,Paris, France), a muscular paralytic agent con-taining a local and general anesthetic agentthat produced immediate death. The aortawas then removed en bloc from the suprare-nal portion to the aortoiliac bifurcation,washed with saline, and immersed in a 10%neutral buffered formalin solution.

After 24 to 48 hours of fixation, the aorticspecimen was opened longitudinally. The en-doluminal surface of the aorta was examinedvisually before 1-mm samples were taken at8 levels, as shown in Figure 3. The metallicstruts were removed with a tweezers. Afterdehydration with alcohol and acetone, thespecimens were embedded in paraffin andsectioned into 5-mm slices for staining withhematoxylin-eosin, Masson trichrome (forcollagen fibers), and Verhoeff’s ferric iodinat-ed hematoxylin (for elastic fibers). Microscop-ic examination was performed using a lightmicroscope.

For surface analysis, a quantitative histo-morphometric study was performed on thebasis of the digitalized images obtained frommagnified views of the specimens. Thicknessassessments of the aortic intima, neointima,and media (Fig. 4) were made using a dedi-cated software package (NIH Image, Macin-tosh, Apple Computer, Inc., Cupertino, CA,USA). A mean value was averaged from 3measures obtained at each level studied.

In each animal, an average of 4 samplesfrom inside and outside the aneurysm zonewere prepared for scanning electron micros-copy. After fixation in formaldehyde and os-mium tetroxide, samples were dehydrated ina graded alcohol series (50–100%) and sput-ter-coated with a mixture of gold and plati-num (50-nm layer).

Definitions and Statistical Analysis

According to the guidelines of the Societyfor Vascular Surgery/ International Society for



Figure 4lTissue section from an aneurysm zone stained with Masson trichrome (34), illus-trating the quantitative computer-assisted method of assessing the thickness of the neointima(NI) and media (M). The hole (B) was the site of a metallic strut, which had been removed.

Cardiovascular Surgery and the Society forCardiovascular Interventional Radiology,3

stent-grafts were assessed for safety and ef-ficacy based on the following criteria: (1) com-plete aneurysm exclusion, (2) absence of ex-pansion and rupture of the excludedaneurysm, (3) patency of the covered stent lu-men, and (4) no device migration, infection,perforation, or hematoma.

Results are expressed as mean 6 SD.Group comparisons of continuous data wereanalyzed with nonparametric paired and non-paired Student t tests using a 5 0.05 (p ,0.05).

RESULTS

Experimental model

The aneurysms were created in the 20 sheepduring a mean 21-minute clamp time (range15–30) in native aortas measuring 7.2 6 0.4mm (range 6.4–8.4) in diameter. The meanlength of the surgically created aneurysm was32.8 6 7.5 mm (range 22.6–53) and the meaninternal diameter was 11.8 6 1.7 mm (range8.7 6 14.4). The average baseline R ratio was1.58 6 0.23 (range 1.20–1.89).

Seven (35%) animals died within 24 hours,3 of suspected heparin overdose (retroperi-toneal hemorrhage without intimal or anas-tomotic rupture) and 1 owing to hypoxemiarelated to a mucous plug in the endotrachealtube. The cause of death in the other 3 ani-mals remained unclear but might have beenrelated to the rapid infusion of the cold med-ullary protection solution. Another (5%) ani-mal presented with postoperative paraplegiaand an aneurysmal thrombosis; it was sacri-ficed at day 1.

Three animals died of aneurysm rupture at6, 8, and 9 days (mean 7.7 6 1.5). In the 9surviving sheep, the mean R ratio increasedbetween preimplantation (1.57 6 0.28) andfollow-up at 3 months (1.63 6 0.43), but thedifference was not significant (t 5 0.771, p ,0.46). The mean R ratios did not differ be-tween the AAA control group and the 6 ani-mals in which the stent-grafts were implantedat 3 months. The 3 sheep in the AAA controlgroup were sacrificed at 3, 6, and 9 monthsafter AAA creation.

Over a follow-up period that extended to280 days (mean 85.6 6 75.0), no death, localor general complication, or secondary device



395

Figure 5lA contrast-enhanced spiral CT scan 1month after stent-grafting showing the metallicstruts surrounding the patent lumen of the stent-graft with complete aneurysm exclusion; there isno evidence of a perigraft leak.

Figure 6lDigital subtraction arteriogram 3months after stent-grafting in the animal shown inFigure 2. Note the regular, patent aortic lumen andabsence of endoleak.

migration was observed in the 6 stent-graftedanimals. Immediate arteriographic controlsshowed both the absence of complete aneu-rysmal exclusion and the presence of slowflow in the aneurysm sac in all 6 cases. How-ever, the aneurysm sacs were always exclud-ed on the 1-month CT (Fig. 5) and on laterangiograms (Fig. 6). The lumen of the devicewas patent on all imaging studies. In 1 ani-mal, the arteriogram at 3 months showedsmall irregularities along the covered portion,but these completely disappeared at 6months.

In the 6 stent-grafted sheep (Table), theslight decrease in the mean R ratio observedbetween preimplantation and on-table post-implantation measurements was not signifi-cant (p , 0.16). On the other hand, there wasa statistically significant reduction in mean Rratio between preimplantation and measure-ments at both 1 month (p , 0.034) and theend of the study (p , 0.034).

Histopathological Study

On gross examination, all aortic specimensexcept those from the 8 early autopsies weresurrounded by highly adherent retroperito-neal tissue. The 3 ruptured aneurysms eachhad a 1- to 2-cm-long tear in the venous wall

at some distance from both anastomoses.The 3 control aneurysms demonstrated a reg-ular, thin aortic wall up- and downstreamfrom the AAA. There was a slight parietalthickening at anastomotic sites, and the an-eurysm wall was thick, regular, and fully cov-ered with neointima (including the valvularremnants). No thrombus or fibrin depositswere present in the aneurysm lumen at nec-ropsy.

In the 6 stent-grafted animals, the nonstent-



l l

TABLE

Follow-up in 6 Animals Receiving Stent-Grafts 3Months After AAA Creation

Time Interval R Ratio

Before implantationPostimplantation (completion

angiography)At 1-month follow-up CTAt sacrifice

1.78 6 0.46

1.51 6 0.161.34 6 0.16*1.33 6 0.15*

l l* Significant difference from baseline (t 5 2.87, p ,

0.034) by Student t test.

Figure 7lPhotograph of an explanted specimen.The metallic stent is covered by neointima, whichappears thicker at junction zones (upper portion)than at the aneurysm level (lower portion). The an-eurysm sac is collapsed and closely adherent tothe stent-graft.

ed portions of the aorta were normal. A renalartery in 1 animal and 1 lumbar artery in 2animals were covered by stent struts and re-mained fully patent. The aneurysm sac ad-hered closely to the stent, and exclusion wasconfirmed in each case.

Junction zones (i.e., segments crossed bythe bare portion of the stent) were covered bya thick, regular neointima without narrowing(Fig. 7), except in 1 case with 20% stenosis ofthe lower inferior junction. At the level of thejunction zones, 3 different patterns were ob-served: (a) absence of neointima, (b) multifo-cal, small, central neointimal islets, or (c) fullneoendothelialization of the stent. There wasno clear relation between the pattern ob-served and the duration of device implanta-tion. The nonendothelialized areas were ei-ther fibrinous and regular or fibrinocruoric,irregular, and mixed with small cells. Fibri-nocruoric deposits were found in 5 of 6 ani-mals in both nonendothelialized and endothe-lialized zones.

Under light microscopy, the nonstentedportions of the aorta appeared normal in thestent-graft group. Junction zones were char-acterized by increased fibrosis. The intimawas thick and fibrocellular; the media wasthin, fibrous, and deviated by the metallicstruts but without rupture of the elastic lami-nae.

In the aneurysm sac, large variations werenoted depending on the level studied regard-less of the follow-up duration. No inflamma-tion was seen. Endothelialization was focalonly. The neointima was of a highly variablethickness and contained some smooth mus-cle cells. The metallic endoprosthesis was

surrounded by a loose connective tissue(myofibroblasts, collagen fibers, a few elasticfibers), neovessels, and giant cells.

The polyester covering appeared undulatedon axial slices and sometimes very superfi-cial, even slightly protruding into the lumenlike an ulcer. A loose connective tissue wasobserved between the polyester fibrils. Theaneurysm sac was collapsed and containedexceptionally scarce thrombi. The venouswall structure was difficult to identify owingto a thin and fully fibrous venous media anda thick and highly vascularized adventitia. Nocleavage plan was found.

A quantitative surface analysis was made atthe level of zones A, B, G, and H (Fig. 3) ofthe 6 implanted animals and on the aneurys-mal neointima and the normal upper and low-er aortic portions of the 3 controls. No statis-tically significant difference in the mean



397

Figure 8lA series of scanning electron photomicrographs from an explanted specimen. Notethe folded appearance of the polyester fabric, which pushes the thin nonendothelialized neo-intima toward the lumen in the upper left scan (3100). The upper right image (3200) is anendoluminal view at the level of the stent-graft, showing good endothelialization. A fewthreads of polyester are protruding in the lumen. The lower left image (3100) shows thepolyester well integrated in the wall around a metallic strut that had been removed. In thelower right scan (3500), polyester filaments covered by blood cells protrude into the lumen.

intimomedial thickness was found betweenthe upper and lower unstented aortic seg-ments (A and H) or between stent-graftedsheep and the controls (paired series). Therewas also no difference in intimomedial thick-ness between the upper and lower junctionzones (B and G) in stented animals (paired se-ries). The 3 control animals had a highly var-iable neointimal thickness at the level of theaneurysm sac, which appeared unrelated toAAA duration.

Scanning electron microscopy confirmedthe light microscopy data by visualizing en-dothelialized areas and integration of the me-tallic structure and the polyester in the aorticwall at the different levels (Fig. 8).

DISCUSSION

Several animal models of aneurysmal ab-dominal aorta are available,4–13 but none of

them can currently be considered as the idealreference standard. To be an accurate model,the aorta must be of approximately the samesize as in a human being to study the devicesunder comparable situations. However,weight and cost are significant factors influ-encing the choice of the animal. Experimentalstudies on healthy arteries are useful only tostudy the technical (implantation) and biolog-ical (biocompatibility) characteristics of theendoprostheses. Aortic dilation by a ballooncatheter results in a limited increase in lumi-nal diameter only. Neither healthy nor dilatedaortic models evolve toward arterial rupture;however, elastase-based models8,9 seem tobetter match the actual pathophysiology ofnatural human AAAs. They are, however,technically difficult to create (especially inlarge animals), and their results are unpre-dictable.



Several materials have been used to re-place an aortic segment by a more easily ex-pandable component. Polyester patches or in-terposition grafts5,11,12 do not mimic the trueevolution of AAAs. Heterologous (e.g., bovinejugular vein in a pig4) and autologous (e.g.,fascia of rectus abdominis muscles13) mate-rials do not result in aortic rupture, except forthe jejunum patch,7 which invariably leads tospontaneous perforation within a few days(i.e., too early). However, Bougdhene et al.1

have described a model that seems very at-tractive because it uses autologous material,looks quite similar to human pathophysiolo-gy, and is associated with a significant risk ofrupture under arterial pressure. Their experi-mental AAA model, which used a left internaljugular vein segment anastomosed end-to-end to the divided infrarenal abdominal aortaof sheep, resulted in rupture in all animalswithin ,4 months. We used the same modelin our study owing to its progressive naturaldilatation; however, the rate of spontaneousaneurysm rupture at 3 months was only 25%in our series, and the R ratio did not show asignificant increase in aneurysm size over the3 months following AAA creation.

There were other limitations with this ex-perimental model, notably, the absence ofbackflow supply from collaterals into the an-eurysm sac. Retrograde flow from lumbar orinferior mesenteric arteries is now well rec-ognized as a factor in endoleaks and perhapsalso late ruptures. Moreover, this model canhave a high rate of paraplegia, but we en-countered this problem in only 1 of our ani-mals. We believe this was due to our im-proved technique after we became familiarwith the procedure, which reduced operativetime and avoided aortic thrombosis and sub-sequent spinal ischemia in our last 6 animals.However, there may be other means of avoid-ing paraplegia in these models. Beygui et al.10

described an interesting maneuver to preventspinal ischemia during AAA creation in anovine model using an internal shunt and aninternal iliac-to-external iliac transposition.

Owing to these deficiencies in current mod-els, we are continuing to develop a new ex-perimental AAA based on Bougdhene’s workbut altering the approach to spare the collat-eral aortic branches. Design of this model in-

cludes surgical placement of a longitudinalautologous venous patch on the anterior wallof the abdominal aorta. In this model, the su-ture plane is coronal and not transverse, andthe initial diameter of the AAA is higher. Wehope that this model will allow a more reli-able rupture rate with better reproducibility.

Stent-grafts are now available in a widechoice of materials for both the metallic frameand the textile covering. Synthetic fabrics cur-rently used include thin woven polyester,polytetrafluoroethylene, and polyurethane.11

They are advantageous because of their de-cades-long use and well known physicalproperties. However, the rigidity and size ofdevices made with these materials makesthem poorly suited to percutaneous delivery.

Another concept of aneurysm exclusionuses macroporous endoprostheses. Even baremetallic stents have been proposed14; theirclose mesh allows delivery via smaller intro-ducer systems on the one hand, and, in theory,preserved patency of the visceral arteries ad-jacent to the implantation zone on the otherhand. Among covered stents, a tantalum–Da-cron coknit balloon-expandable device gavegood results in both in vivo and clinical use.12

The device we used combined the high flex-ibility, maneuverability, and shape-memoryadvantages of nitinol with a low profile intro-ducer system made possible by the macro-porous polyester. The fact that this device isconstructed at the time of use and not com-mercially manufactured is a definite asset, asseveral cost-effectiveness studies haveshown that the financial benefit of reducedhospital stays achieved with endovasculartreatment is countered to a significant degreeby the cost of manufactured stent-grafts.15–17

The weak point of this covered stent designis obviously the covering, which does not im-mediately exclude the aneurysm, so postim-plantation assessment in the operating roomwould not be possible. Although leaks maystill be treated by balloon dilation, stent-graftextension, or even embolization in secondaryprocedures, many teams now choose to treatprimary endoleaks before the procedure iscompleted.

Typically, major leaks lead to aortic ruptureand necessitate aggressive management. Inminor leaks, the consequences on sac pres-



399

sure are not well known, and therapeuticmanagement is subject to discussion. More-over, no clear-cut limit exists to delineatethese two degrees of severity. The experi-mental study of Marty et al.18 is interestingwith respect to this problem. Complete exclu-sion significantly lowers the pressure in theaneurysm sac. In animals where a preruptureof the endoprosthesis can be shown, the im-portance of the leak is highly variable (mainlydepending on individual factors, particularlythe coagulation system), but whatever the im-portance of the leak, the pressure in the sacis always significantly elevated. This confirmsthat any leak should be closely followed andsometimes aggressively treated.

Furthermore, aneurysmal expansion, andeven aortic rupture, have also been observedin the absence of any demonstrable leak.19,20

Explanations for this phenomenon include (1)the supposed ineffectiveness of imagingmethods to show some leaks and (2) the per-sistent transmission of aortic pressure intothe sac despite delayed thrombosis of anyleak. For instance, the use of coils may resultin thrombosis of the sac and aneurysmal ex-clusion without significant reduction of intra-sac pressure. Although we did not measurepressure in the sac, the endograft we testeddid eventually exclude the aneurysm despitethe macroporous fabric. Furthermore, the de-crease in aneurysm diameter was constant inall animals, which argues in favor of an effec-tive exclusion from the blood flow.

In conclusion, this study showed the limitsof a previously described experimental AAAmodel using autologous venous segmentalgrafting of the infrarenal aorta. Although themodel has some similarities with humanpathophysiology, timing and reproducibilityof aneurysm rupture can be improved. Im-plantation of the macroporous covered stentwas feasible and easy, but some modificationmay be necessary to shorten the delay in ob-taining aneurysm exclusion.

Acknowledgments: The authors thank Marie-TheresePieraggi, MD, for providing the scanning electron micros-copy and Marie-Laure Vidal for her kind help in the prep-aration of the manuscript.

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Macroporous Polyester–Covered Stent in an Experimental Abdominal Aortic Aneurysm Model

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