310
`
`J ENDOVASC SURG
`1998;5:31 0-317
`
`+ EXPERIMENTAL ____________________________________ _
`
`Endovascular AAA Exclusion: Will Stents With
`Hooks and Barbs Prevent Stent-Graft
`Migration?
`
`Martin Malina, MD, PhD; Bengt Lindblad, MD, PhD;
`Krasnodar lvancev, MD, PhD*; Mats Lindh, MD*;
`Janne Malina, MOt; and Jan Brunkwall, MD, PhD
`
`Departments of Vascular Surgery, *Radiology, and tPathology, Malmo
`University Hospital, Lund University, Malmo, Sweden
`
`·------------------------------------------------------+
`Purpose: To investigate if stents with hooks and barbs will improve stent-graft fixation in
`the abdominal aorta.
`Methods: Sixteen- to 24-mm-diameter Dacron grafts were deployed inside cadaveric aortas.
`The grafts were anchored by stents as in endovascular abdominal aortic aneurysm repair.
`One hundred thirty-seven stent-graft deployments were carried out with modified self-ex(cid:173)
`panding Z-stents with (A) no hooks and barbs (n = 75), (8) 4 5-mm-long hooks and barbs
`(n = 39), (C) 8 10-mm-long, strengthened hooks and barbs (n = 19), or (D) hooks only (n
`= 4). Increasing longitudinal traction was applied to determine the displacement force
`needed to extract the stent-grafts. The radial force of the stents was measured and correlated
`to the displacement force.
`Results: The median (interquartile range) displacement force needed to extract grafts an(cid:173)
`chored by stent A was 2.5 N (2.0 to 3.4), stent B 7.8 N (7 .4 to 1 0.8), and stent C 22.5 N ( 17.1
`to 27.9), p < 0.001. Both hooks and barbs added anchoring strength. During traction, the
`weaker barbs were distorted or caused intimal tears. The stronger barbs engaged the entire
`aortic wall. The radial force of the stents had no impact on fixation, while aortic calcification
`and graft oversizing had marginal effects.
`Conclusions: Stent barbs and hooks increased the fixation of stent-grafts tenfold, while the
`radial force Of stents had no impact. These data may prove important in future endograft
`development to prevent stent-graft migration after aneurysm exclusion.
`J Endovasc Surg 1998;5:370-317
`
`Key words: endovascular grafting, abdominal aortic aneurysm, migration, Z-stents
`•
`
`$
`
`Migration of stent-grafts used for abdominal
`aortic aneurysm (AAA) exclusion is a recog(cid:173)
`nized complication of aortic endografting. 1- 5
`At our center, one third of the AAA patients
`
`Address for correspondence and reprints: Martin Malina,
`MD, PhD, Department of Vascular Surgery, Malmo Univer(cid:173)
`sity Hospital, S-205 02 Malmo, Sweden. Fax: 46-40-33-73-
`35.
`
`treated with endografts containing Z-stents
`with four weak hooks and barbs presented
`with late migration of the proximal stent. 1
`Stent-graft dislodgment ranged from minor (5
`to 10 mm) to severe migration with complete
`descent of the stent-graft into the aneurysm
`sac, requiring late conversion to open surgery.
`Recently, other reports have appeared of simi(cid:173)
`lar problems with various stent-grafts, includ-
`
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`

`J ENDOVASC SURG
`1998;5:31 0-317
`
`ANCHORING STENT-GRAFTS
`MALINA ET AL.
`
`311
`
`ing the Chuter bifurcated graft,2 the Corvita en(cid:173)
`dovascular graft (Dereume JP et al. 24th
`Annual Symposium on Current Critical Prob(cid:173)
`lems, New Horizons and Techniques in Vascu(cid:173)
`lar and Endovascular Surgery, New York, New
`York, USA, November 20-23, 1997), the Van(cid:173)
`guard system (personal observation), and the
`Malmo system.3.4 Late migrations have also
`been reported when balloon-expanded stents
`were used for graft anchoring. 5
`As most migrations are diagnosed ::::: 1 year
`after stent-graft insertion,4 biological incorpo(cid:173)
`ration of endovascular grafts seems insuffi(cid:173)
`cientto resistthe pulsatile forces present inside
`the aorta. These observations call for improved
`mechanical anchoring of stent-grafts. How(cid:173)
`ever, many new stent-grafts, such as the An(cid:173)
`euRx, Corvita, and Talent devices, are an(cid:173)
`chored by smooth stents without hooks and
`barbs or by small barbs only (Vanguard
`system).
`To our knowledge, only two groups have as(cid:173)
`sessed the force needed to dislodge endovas(cid:173)
`cular stent-grafts from cadaveric aortas. In the
`first study, 6 five different stent types were de(cid:173)
`ployed in healthy porcine aortas, which ob(cid:173)
`viated the assessment of stent function in
`atherosclerotic vessels. In the second experi(cid:173)
`ment? only Gianturco stents were used. The
`displacement force was exerted on both the
`distal and proximal stents of an endovascular
`stent-graft bridging an aneurysm. Therefore,
`no definite conclusions could be made about
`the specific anchoring of the proximal stent.
`Prompted by incidences of stent-graft mi(cid:173)
`gration1 and late proximal endoleaks8 in endo(cid:173)
`vascularly excluded AAAs, we designed this
`study to assess if stent-graft fixation can be
`improved mechanically by adding hooks and
`barbs to the graft-anchoring stents.
`
`METHODS
`
`Dacron grafts (Cooley Verisoft, Meadox
`Medicals, Oakland, NJ, USA) with diameters
`of 16, 20, and 24 mm were sutured to self-ex(cid:173)
`pandable Gianturco Z-stents (Cook Inc., Den(cid:173)
`mark), leaving a few millimeters of the stents
`protruding from the grafts proximally as is
`usually done for endovascular AAA exclusion
`
`(Fig. 1 ). The stents were 2.5 em long and 4.5
`em wide in their uncompressed state and con(cid:173)
`sisted of 10 bends of stainless steel wire.
`Four different modifications ofthe Gianturco
`stents were examined (Fig. 1 ): (A) smooth
`stents without any anchoring appendages; (B)
`stents equipped with 4 weak hooks and barbs
`(5 mm long and 0.25 mm thick) similar to those
`used by several groups4 ·9 in AAA repair; (C)
`stents with 8 stronger hooks and barbs (10 mm
`long and 0.3 mm thick); and (D) 8 reinforced
`hooks but no barbs. All barbs protruded out
`from the graft at an angle of about 30°.
`The aortas of 15 human cadavers were ex(cid:173)
`posed with minimal dissection and left in situ
`so as not to disrupt their attachment to sur(cid:173)
`rounding tissue. The vessels were transected
`about 5 em proximal to the aortic bifurcation,
`their proximal end thus mimicking an aneu(cid:173)
`rysm neck. Stent-grafts were deployed 2.5 em
`into the transected aortas, a distance corre(cid:173)
`sponding to the length of many AAA necks en(cid:173)
`countered in clinical practice (Fig. 2). In each
`case, the entire stent was completely con(cid:173)
`tained within the aorta, allowing all hooks and
`barbs to engage the vessel wall.
`The median age of the cadavers was 66
`years (range 46 to 84); half had been smokers
`and half had a previously diagnosed cardio(cid:173)
`vascular disease. The aortic walls were ma(cid:173)
`croscopically classified into five groups: (I)
`nonatherosclerotic, (II) soft intimal thickening,
`(Ill) calcified plaques engaging part of the aor(cid:173)
`tic circumference, (IV) circumferentially lo(cid:173)
`cated calcified plaques, and (V) completely
`calcified, incompressible aortas.
`The sample size of the different stent types
`was dictated by the experimental setting.
`Stentswithout hooks and barbs that did not dis(cid:173)
`rupt the aortic wall were assessed first. Then,
`in the same aortic segment, stents with weaker
`hooks and barbs were tested. Because these
`stents caused damage to the aortic wall, only a
`limited number of trials could be performed in
`each aortic segment. Finally, the stents with
`reinforced hooks and barbs and hooks only
`were tested until the aortic wall was severely
`damaged. Thereby, stents with extra long and
`thick anchoring appendages were always as(cid:173)
`sessed last, when the aortas offered the least
`anchoring support. In this manner, attachment
`of stents with hooks and barbs was not favored.
`
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`312
`
`ANCHORING STENT-GRAFTS
`MALINA ET AL.
`
`J ENDOVASC SURG
`1998;5:310-317
`
`8 9 10 11
`
`Figure 1 +Three of the four types of Z-stents evaluated in this study: smooth stent (left), stent
`with four weak hooks and barbs (middle), and a stent with eight strengthened hooks and
`barbs (right). The barbs protrude through the grafts. Not pictured is the stent with eight rein(cid:173)
`forced hooks only.
`
`Longitudinal traction then was applied to the
`distal end of the grafts (Fig. 2) and measured by
`a tensiometer. The traction was increased
`gradually by steps of 0.5 N until the stent-grafts
`were dislodged from the aorta, thereby defin(cid:173)
`ing the displacement force. After extraction
`from the aorta, the stents were examined for
`deformation of the appendages and the aortas
`for intimal or transmural injury. The damaged
`portion then was excised and additional mea(cid:173)
`surements were taken more proximally. This
`was repeated until the level of the renal arteries
`was reached. In this fashion, the smooth stents
`were tested on 75 occasions, the design with 4
`hooks and barbs in 39 instances, the model with
`8 hooks and barbs 19 times, and the 8 hooks
`only were tested on 4 occasions.
`The radial force exerted by the stents was
`assessed with a tonometer (Ophtalmo Dyna(cid:173)
`mometre du Dr Bailliart, Guilbert & Routit,
`
`Paris, France) by measuring the force required
`to compress the stents 2 and 4 mm (Fig. 3).
`The measurements were carried out with the
`stents compressed inside grafts that mea(cid:173)
`sured 16, 24, and 36 mm wide. To avoid lateral
`bulging during measurement, the sides of the
`graft were supported.
`Nonparametric data were analyzed as me(cid:173)
`dian and interquartile range (lOR). The Wil(cid:173)
`coxon rank sum test was used for comparing
`two groups; three or more groups were com(cid:173)
`pared using the Kruskai-Wallis test. Differ(cid:173)
`ences were considered significant at p < 0.05.
`
`RESULTS
`
`Stent-grafts were implanted 137 times in(cid:173)
`side the 15 infrarenal aortas. The median
`force needed to extract the stent-grafts with
`smooth Gianturco stents was 2.5 N (lOR 2.0 to
`
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`J ENDOVASC SURG
`1998;5:31 0-317
`
`ANCHORING STENT-GRAFTS
`MALINA ET AL.
`
`313
`
`3.4) (Fig. 4). The corresponding displacement
`force for the Gianturco stents with 4 weak
`hooks and barbs was 7.8 N (lOR 7.4 to 10.8),
`while the stents with 8 reinforced hooks and
`barbs required a median force of 22.5 N (lOR
`to 27.9) (p < 0.001 ). Z-stents with 8
`17.1
`strong hooks but no barbs had a displace(cid:173)
`ment force of 11.8 N (lOR 10.4 to 13.0) com(cid:173)
`pared to 22.5 N for stents with both hooks
`and barbs (p < 0.01 ).
`In every segment of cadaveric aorta, the
`stent-graft fixation was compared with the first
`and last time each type of stent was used. The
`median displacement force was slightly higher
`on the first occasion (p = 0.2). Hence, the fixa(cid:173)
`tion ofthe stents with hooks and barbs was not
`favored in this experimental setting because
`these stents were always assessed last.
`The weaker hooks and barbs engaged the
`thickened media but did not penetrate the en(cid:173)
`tire aortic wall. The dislodgment occurred by
`
`two mechanisms: upward distortion of the
`barbs upon traction until the angulation of the
`barbs was such that they could slide out of the
`aortic wall or by tearing of the intima. Often,
`pieces of atherosclerotic intima could be seen
`on the hooks and barbs of the extracted stents.
`In contrast, the reinforced hooks and barbs en(cid:173)
`gaged the entire aortic wall, frequently perfo(cid:173)
`rating it and emerging outside the vessel wall
`(Fig. 2). Upon powerful traction, these barbs
`would also become deformed or cause a
`transmural tear of the aorta different from the
`limited intimal injury produced by the weaker
`barbs.
`Oversizing the graft diameter increased
`slightly the displacement force but only for
`stents without hooks and barbs (p < 0.01 ).
`Stents with hooks and barbs retained their
`strength of fixation even when the grafts were
`undersized by 1 or 2 mm. Smooth stents had
`marginally stronger fixation in increasingly
`
`Figure 2 + Stent-grafts were deployed 2.5 em into transected cadaveric infra renal aortas (A)
`in situ to avoid disruption of the aortic attachment to surrounding tissue. Longitudinal traction
`then was applied to the distal end of the grafts (G). Upon traction, the enforced barbs (arrows)
`penetrated the aortic wall.
`
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`

`314
`
`ANCHORING STENT-GRAFTS
`MALINA ET AL.
`
`J ENDOVASC SURG
`1998;5:310-317
`
`N
`
`Radial force (N)
`.4
`
`.2
`
`. 1
`
`~ .3
`
`16 mm
`
`24 mm
`
`36 mm
`
`0
`
`2
`
`4
`
`Stent depression
`(mm)
`
`Figure 5 +Radial force of stents contained within
`16-, 24-, and 36-mm grafts. Stents compressed to a
`diameter of 16 mm exerted a radial force almost
`twice that seen when compressed to 36 mm. The
`force required to make a 4-mm depression into the
`stents was twice that of making a 2-mm depression
`regardless oft he degree of compression. Each point
`is based on data from 6 registrations.
`
`Thus, the radial force of stents contained
`within 16-mm grafts was almost twice that of
`stents in 36-mm grafts. Similarly, the force
`needed to compress the stents 4 mm with the
`tonometer was almost twice that needed to
`compress them 2 mm, regardless of the diam(cid:173)
`eter of the graft containing the stent. By com(cid:173)
`paring the fixation of stents in aortas of differ-
`
`Displacement
`force (N)
`
`.. ...__ .. ..--......
`
`1 2
`
`8
`
`4
`
`o~-....-----.-........ ---.--
`1.2 1.4 1.6 1.8 2.0 Aortic
`diameter (em)
`
`for stent-grafts
`force
`Figure 6 +Displacement
`placed in aortas of various diameters and anchored
`by smooth stents (circles) or by stents with 4 hooks
`and barbs (squares). The stents (n = 72) exerted a
`greater radial force in small aortas, yet the displace(cid:173)
`ment force was equal at all aortic diameters and
`therefore unaffected by the radial force of the
`stents.
`
`Figure 3 +The radial force of stents at various states
`of compression was assessed by placing the stents
`inside grafts of different diameters: The force (N)
`needed to make a further 2- and 4-mm depression
`of the stents was measured by a tonometer. To
`avoid lateral bulging during measurements, the
`sides of the stent were supported.
`
`atherosclerotic aortas, while the displacement
`force for stents with hooks and barbs was un(cid:173)
`affected by the aortic calcification.
`The radial force of the Z-stents was greater
`the more the stents were compressed (Fig. 5).
`
`Displacement
`Force
`(N)
`30
`
`20
`
`1 0
`
`z
`0 ~-------------------------------
`Smooth
`
`8 strong
`hooks & barbs
`
`4weak
`hooks & barbs
`
`Figure 4 +The force required to displace 3 different
`Z-stent modifications from cadaveric aortas (n =
`75, 39, and 19, respectively). Medians, interquartile
`ranges (within box), and 1Oth and 90th percentiles
`(horizontal bars) are shown.
`
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`

`J ENDOVASC SURG
`1998;5:310-317
`
`ANCHORING STENT-GRAFTS
`MALINA ET AL.
`
`315
`
`ent diameters, the impact of the stents' radial
`force on the displacement force could be as(cid:173)
`sessed (Fig. 6). The displacement force was
`unaffected by the aortic diameter; therefore,
`the radial force of the stents had no impact on
`the fixation of either Z-stent type. This analysis
`included only stent-grafts with a graft diame(cid:173)
`ter exceeding the aortic diameter by ::::: 2 mm
`(n = 72) to ensure that the stents would exert
`their radial force unimpeded.
`
`DISCUSSION
`Stents originally were designed to prevent
`occlusion of stenotic vessels. Today, however,
`they are widely used to anchor endovascular
`grafts, although little has been done to adapt
`the stents for this novel mission. The first com(cid:173)
`mercial aortic endograft, the EVT device, had
`hooks but no barbs for anchorage. Since then,
`numerous stent-graft systems have been de(cid:173)
`veloped worldwide, many without hooks or
`barbs to affix the proximal stent to the aortic
`wall
`(e.g., AneuRx, Corvita, and Talent).
`Among the second- and third-generation en(cid:173)
`dografts, only the Vanguard model has barbs,
`and they are a mere 2 mm long. Despite this,
`most centers report few cases of stent-graft
`10
`11 This may be due to short fol(cid:173)
`5
`migration. 1-
`•
`•
`low-up or careful selection of patients with fa(cid:173)
`vorable neck anatomy, thereby excluding >
`50% of AAA patients from endovascular re(cid:173)
`pair.
`At our institution, endovascular aneurysm
`repair was introduced in 1993; since then, up
`to 70% ofthe aneurysm repairs have been per(cid:173)
`formed endovascularly. Such liberal patient
`selection has unmasked the necessity for safer
`stent-graft anchoring. One third of our patients
`exhibit stent-graft migration. 1 That may be due
`to dilation or shortness of the aneurysmal
`neck, which often contains mural thrombus
`and calcification. However, migration has oc(cid:173)
`curred despite satisfactory neck anatomy,
`graft sizing, and stent-graft deployment. As
`most cases of migration were identified at :::::
`12 months, the healing of endovascular stent(cid:173)
`grafts seems insufficient to withstand the pul(cid:173)
`satile force within the aorta. Wall-penetrating
`stent hooks and barbs may provide improved
`fixation.
`The present study shows that simple stent
`appendages, such as hooks and barbs, areca-
`
`pable of improving the fixation of endovascu(cid:173)
`lar grafts tenfold in this experimental model.
`Both the hooks and the barbs improved attach(cid:173)
`ment. Short hooks and barbs (5 mm) engaged
`the thickened aortic intima only; the fragility
`of the intima then seemed to limit the force
`required to displace the stents. Therefore, the
`stents with longer and stronger barbs, which
`penetrated the entire aortic wall, provided a
`firmer fixation. However, a stent's radial force
`was of no significant importance for graft at(cid:173)
`tachment. Thus, attempts to improve endo(cid:173)
`vascular graft anchoring by excessive oversiz(cid:173)
`ing of smooth stents might be futile, especially
`because stents may cause pressure ulcers and
`perforations of the vessel walls. 12 Rather than
`being pushed into the aortic wall by the radial
`force of the stent, the hooks and barbs engage
`the aortic wall when the stent-graft is pulled
`distally by the bloodstream. The angle be(cid:173)
`tween the stent and its hooks and barbs is im(cid:173)
`portant for this action.
`The forces that cause endovascular aortic
`grafts
`to migrate
`in vivo are unknown.
`Transmural surgical sutures placed during
`open aneurysm repair can withstand the pul(cid:173)
`satile forces in the aorta. Therefore, long hooks
`and barbs that pierce the aortic wall (Figs. 2
`and 7) may also prove more durable, but there
`is a potential danger of penetration to juxta(cid:173)
`aortic structures, such as the duodenum and
`the renal vein. The long-term risks of these
`hooked and/or barbed stents have not been as(cid:173)
`sessed, but no damage to adjacent organs by
`aortic wall-piercing hooks has been demon(cid:173)
`strated in autopsy studies. 13 The metallic
`hooks and barbs must be strong enough to
`resist breakage or distortion by long-standing,
`repetitive trauma. Placing additional hooks
`and barbs on each stent will increase the vol(cid:173)
`ume of the devices and make them more diffi(cid:173)
`cult to introduce through slim sheaths.
`There are other ways to improve stent-graft
`anchoring without perforating the aortic wall.
`By placing additional, serially attached stents
`into the suprarenal aorta, more hooks and
`barbs can be used. This implies that vital aortic
`branches will be covered by stents. Although
`it has been reported that arterial branches cov(cid:173)
`ered by stents remain patent, 14- 16 various
`stents affect the branches differently. 17 Fully
`stented grafts with greater columnar strength
`
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`

`316
`
`ANCHORING STENT-GRAFTS
`MALINA ET AL.
`
`J ENDOVASC SURG
`1998;5:310-317
`
`Figure 7 +Human specimen ofthe neck of an infra renal aortic aneurysm treated by endovascu(cid:173)
`lar exclusion 6 months previously. The caudally oriented barb of the proximal graft-anchoring
`Z-stent has perforated the aortic wall. No bleeding or injury to adjacent viscera was noted.
`The stent-graft was anchored securely without migration.
`
`may provide support for the proximal stent
`and make the graft "sit" on the aortic bifurca(cid:173)
`tion instead of being suspended from the an(cid:173)
`eurysm neck. However, migration of fully
`stented grafts has also occurred (Dereume JP
`et al. 24th Annual Symposium on Current Criti(cid:173)
`cal Problems, New Horizons and Techniques
`in Vascular and Endovascular Surgery, New
`York, New York, USA, November 20-23,
`1997). The extent to which the displacement
`force may be diverted from the proximal stent
`in fully stented grafts is unknown .
`It may be argued that the transected aortas
`in this experimental setting represent a subop(cid:173)
`timal model for aneurysm necks. However,
`one true aneurysm with a diameter of 8 em
`was included in this study. The displacement
`forces recorded at its neck corresponded
`
`closely to those seen in the nonaneurysmal
`aortas (data not presented).
`Today's challenge for those who develop
`endografts is to design a device that inhibits
`stent migration and the continuous dilatation
`2° Few endoleaks due
`ofthe aneurysm neck. 18
`-
`to neck enlargement have been reported so
`far. 1 However, present data suggest that the
`dilatation rate of the neck is about 1 mm per
`2° Considering the limited oversizing of
`year. 19
`•
`endovascular grafts, dilating necks might be a
`problem in the future.
`In summary,
`this experimental model
`showed that stent-graft fixation can be im(cid:173)
`proved by hooks and barbs. The data seem
`particularly valuable, as several devices with(cid:173)
`out stent appendages are being marketed for
`endovascular aneurysm repair. Excessive ra-
`
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`J ENDOVASC SURG
`1998;5:310-317
`
`ANCHORING STENT-GRAFTS
`MALINA ET AL.
`
`317
`
`dial force by stents does not contribute to the
`strength of proximal attachment, suggesting
`that intentional oversizing should be avoided.
`
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`3.
`
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`IPR2014-00100 Pat. Owner Ex. 2012
`Medtronic v. Marital Deduction Trust
`
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