aneurysm repair using injectable elastomer
Bosman, W.M.P.F.
Citation
Bosman, W. M. P. F. (2011, September 1). Customized aortic repair : an alternative approach to aortic aneurysm repair using injectable elastomer.
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chAPter 5
The proximal fixation strength of modern EVAR-grafts in a short aneurysm neck An in-vitro study
Eur J Vasc Endovasc Surg; 2010; Volume 39; Issue 2; 187-192 W.M.P.F. Bosman, T.J. van der Steenhoven, D.R. Suárez,
J.W. Hinnen, E.R. Valstar, J.F. Hamming
ABstrAct
objectives: To measure the strength of the proximal fixation of endografts in short and long necks.
design: Three types of endografts were compared: Gore Excluder®, Vascutek Ana- conda® and Medtronic Endurant®.
materials & methods: The proximal part of the stent grafts was inserted in a bovine artery and the graft was then attached to a tensile testing machine. The force to obtain dislodgement (DF) from the aorta was recorded for each graft at proximal seal lengths of 10 and 15 mm.
results: The median DF (IQR) for the Excluder, the Anaconda and the Endurant with a seal length of 15 mm was: 11.8 (10.5–12.0)N, 20.8 (18.0–30.1)N and 10.7 (10.4–11.3) N. With the shorter proximal seal of 10 mm DF was respectively: 6.0 (4.5–6.6)N, 17.0 (11.2–36.6)N and 6.4 (6.1–12.0)N.
conclusions: The proximal fixation of the Anaconda is superior to the Excluder and the Endurant at short necks of 10 and 15mm in an experimental set-up. There is a statistically significant decrease of proximal fixation for the Excluder stent graft, when decreasing the length of the proximal neck from 15 to 10 mm.
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introduction
Endovascular aneurysm repair (EVAR) has become a well-established treatment of abdominal aortic aneurysms. Mortality after EVAR is significantly reduced compared to open repair.1, 2 Despite these successes, EVAR has several drawbacks. Complications and re-interventions caused mainly by endoleaks, endotension, stent-graft migration and device failure are problems of major concern.3
One of the most important complications affecting the long-term success of EVAR is stent-graft migration, which can cause type I endoleak and even aneurysm rupture.1, 2, 4–9 Lifelong follow-up is therefore necessary. Fixation is dependent on friction and other mechanical forces between the surfaces of the graft and the aortic neck (Fig. 5.1).10, 11 Therefore EVAR has anatomical restrictions. Manufacturers of commercially available EVAR-grafts state that an infrarenal aneurysm neck of at least 15mm is needed to ensure a strong proximal seal, and potential angulation of the aneurysm is a (relative) contra-indication for endovascular treatment. Despite these manufacturers instructions for use, EVAR-grafts are inserted in necks shorter than 15 mm.12, 13
Fig. 5.1 A schematic presentation of the forces and friction exerted on the endoprothesis. The red arrows show the forces exerted by the downward bloodflow, the blue dotted arrows show the upward forces due to friction keeping the graft in place. The white arrow shows the result of the forces exerted on the graft, which may dislodge the graft if the force is high enough.
During the last few years, new and improved endovascular stent-grafts have become available. It is believed that these grafts have a superior proximal fixation and it is stated that they can be fixated in shorter and more angulated necks.14, 15
Aim of this in-vitro study was to measure the strength of the proximal fixation of recently available endografts and the potential influence of short aneurysm neck length on the fixation of these grafts.
mAteriAls And methods
set-up
We obtained 5 fresh bovine aortas from the abattoir. The abdominal portion of the aorta was retained and all non-vascular tissue was removed. Three to four samples with a length of 45mm were selected with a mean diameter of 19.5 mm (+/- 1.0mm). The samples of bovine artery were fixated in an experimental set-up (Fig. 5.2). Side-branches of the aorta were ligated with the same 4.0 prolene stitches [Ethicon, Somerville; USA].
Fig. 5.2 Schematic drawing of experimental set-up: A. Sample of bovine artery (45mm long). B Distal fixation plug on which the bovine artery is attached. C. Stent-graft inserted in the bovine neck of the aneurysm. D. Rigid rod with anchor through the endograft, connecting it to the tensile testing machine. E.
Plugs for fixation on tensile testing machine.
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stent grafts
Three types of commercial endografts of different manufacturers were compared in this experimental study (Fig. 5.3): An Excluder® “AAA Endoprothesis” [Gore, Flagstaff, Arizona, USA], an Anaconda® “AAA Endovascular Graft” [Vascutek, Inchinnan, Scotland], and an Endurant® “AAA Stent Graft” [Medtronic, Minneapolis, MN, USA]. The diameter of all grafts used was 23mm. All manufacturers supplied the grafts after acceptance of the study protocol.
As can be seen in Fig. 5.3 the three endografts-bodies used in this study differ in their design. The Gore Excluder is an ePTFE graft with a nitinol stent frame. The stents are connected with each other over the entire length of the graft, providing columnar strength. The proximal portion of the main body has a scalloped end that is entirely covered with seven paired flexible nitinol anchors for fixation. The Vascutek Anaconda has a woven polyester graft body without any kind of stent-frame. The graft is equipped with a dual proximal ring stent design with 4 pairs of rigid nitinol hooks. The Medtronic Endurant is made of multi-filament polyester and has wire-formed M-shape body stents, without vertical linkage, permitting flexibility. The proximal side has five suprarenal nitinol anchoring pins designed to improve migration resistance.
method of excluding the aneurysm
The proximal part of a stent graft was inserted in the bovine artery, using a custom made sheath (inner Ø 30 mm), as each graft was used several times. The neck length was measured from the start of the aortic segment to the lowest covered part of the proximal site of the stent graft. With the Endurant, this meant that that the suprarenal fixation stent were inserted further down in the aortic neck. After insertion the grafts were fixated by inflating a 30 mm Ø Reliant endovascular balloon [Medtronic, Minne- apolis, MN, USA]. Using 23mm Ø stents in arteries of 19.5mm Ø, a mean 13% oversizing was applied.
Fig. 5.3 The endografts used in this study: A. Gore Excluder, B. Vascutek Anaconda and C.
Medtronic Endurant.
measurements
After insertion in the artery, the grafts were attached to a rigid hook, which was con- nected to a tensile testing machine [Lloyd LR5K, Ametek, Paoli, USA]. Longitudinal, uniaxial traction was applied to the stent graft and quantified using a ZFA 250N loadcell [Scaime, Annemasse, France]. The retrieved data was digitized and stored using a volt- meter [Voltcraft, Oldenzaal, The Netherlands] and a personal computer. Traction was gradually increased, as the traction bar moved upward at 1mm/s. The dislodgement force (DF) was the force needed to dislodge the stent graft from the aorta. The DF was noted on visual inspection and was confirmed by analysis of the force/displacement graph. The DF of each graft was measured at 2 different lengths of proximal seal: 10 and 15 mm. The test was repeated 5 times for each type graft at each length of proximal seal. The same stent and same sample of artery were used, if there was no extensive damage or deformation on visual inspection. The intimal layer was considered damaged if there was a macroscopically disruption of the tissue. At least 2 segments were used per graft to prevent differences in measurements due to different aorta characteristics.
statistics
Non-parametric Mann-Whitney Tests were used for the comparison of the DF between each of the grafts for each seal length [SPSS 16.0, SPSS Inc, Chicago, Ill, USA]. The influ- ence of proximal seal length on DF was analyzed and compared with the Mann-Whitney Test for each graft as well to see if a longer proximal seal leads to a stronger fixation.
Fig. 5.4 Forces needed to dislodge the different kinds of stent-grafts from the aorta. Non- parametric Mann-Whitney tests were done to compare the pull-out forces between the grafts. The symbols *, #, and + were applied if there was a significant difference (p<0.05) between the grafts.
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results
The median dislodgement force (IQR) for the Excluder, the Anaconda and the Endurant with a seal length of 15 mm was respectively: 11.8 (10.5–12.0) N, 20.8 (18.0–30.1) N and 10.7 (10.4–11.3) N (Fig. 5.4). The DF of the Anaconda was significantly higher than the Excluder (p=0.009) and the Endurant (p=0.009). There was no significant difference between the Excluder and the Endurant (p=0.251).
With the shorter proximal seal of 10 mm the DF was respectively: 6.0 (4.5–6.6)N, 17.0 (11.2–36.6) N and 6.4 (6.1–12.0) N (Fig. 5.4). The Anaconda had a significant higher DF than the Excluder (p=0.009) and the Endurant (p=0.028). The Excluder did not differ from the Endurant (p=0.251).
The larger proximal seal had a beneficiary effect with the Excluder. The DF was sig- nificantly higher with the 15mm proximal seal (p=0.009) (Fig. 5.5). The fixation was not altered significantly with a longer neck with the Anaconda (p=0.754) or the Endurant (p=0.117).
Per graft we used the following number of aortic segments: Excluder (n=2), Anaconda (n=4) and Endurant (n=2).
discussion
The long-term success of EVAR depends on secure stent-graft fixation. Complications such as stent-graft migration, endoleaks and graft kinking have been reported by many investigators.9, 12, 16–20 Earlier research has shown the importance of proximal neck length on forces needed for dislocation.10, 21–26 This study is the first to present the dislodge- Fig. 5.5 Influence of proximal seal length on dislodgement force of the different grafts. Non- parametric Mann-Whitney tests were done to compare the pull-out forces between the seal-length with each graft. The symbol * was applied if there is a significant difference (p<0.05) between the seal-lengths.
ment forces of the latest generation of commercially available endografts. Furthermore, it is the first in-vitro study to investigate the proximal fixation of these grafts in short aneurysm necks. While these grafts are already inserted in short necks in-vivo without knowledge of this fixation strength.
The Anaconda had the most rigid proximal fixation of the 3 types of endografts. With the minimally required proximal seal length of 15 mm and with an off-label seal length of 10 mm the Anaconda had a stronger fixation then the Excluder and the Endurant (Fig. 5.4).
At a neck length of 10mm, the Anaconda device had a large IQR. This was due to the fact that 2 out of the 5 measurements were higher then 35+ N. At these high forces, the graft was still in-situ, but deformed and would be not functional in-vivo. However we decided to include these measurements as the scope of our study was to measure the force needed for graft dislodgement, and the proximal fixation was still in place at that moment.
The number of aortic segments needed for each graft was comparable (Table 5.1). For the Anaconda we had to use 4 segments, while we used 2 segments for the Excluder and the Endurant. This was due to the macroscopical damage to the intimal layer (Fig.
5.6) of 3 segments with the runs with the Anaconda and to 1 segment with the Endurant.
Mathematical models have shown that an endoluminal graft needs to withstand pulsatile drag forces of 3.8–6N in an aneurysm with a friendly anatomy and drag forces up to 14 N in case of angulated, more hostile anatomy.27, 28 In case of a straight and simple anatomy the proximal fixation of the Excluder and the Endurant would be suf- ficient, while this might not be the case with an angulated anatomy, as the theoretically calculated drag force would exceed the proximal fixation (Fig. 5.4). This might lead to graft migration and endoleakage. It should be noted clearly that this is an assumption, comparing our in-vitro results with the previously calculated drag forces. Clinical studies should be done to investigate if this also occurs in-vivo.
The differences in graft design might explain the differences in proximal fixation strength (Fig. 5.3). The hooks of the Excluder are more bendable and less sharp than the fixation hooks of the Anaconda and the Endurant. Research by Malina et al. has shown
grAFt dislodgement runs
Aortic segments used
mAcroscoPic dAmAged Aortic segments
excluder 10 2 0
Anaconda 10 4 3
endurant 10 2 1
table 5.1. Number of dislodgement runs and aortic segments used per graft. The last row shows the number of aortic segments, which were damaged during the experiments. The intimal layer was considered damaged if there was macroscopically disruption of the tissue.
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that the use of rigid hooks leads to a strong proximal fixation.25 Another explanation of the higher dislodgement force of the Anaconda is the presence of the stiff dual ring in the proximal segment of the graft. The oversized stiff rings strive to be perfectly round in an artery with a smaller diameter, leading to a snakemouth configuration with much radial force. This leads to a high amount of friction between the graft and the artery, providing a strong “passive” fixation. The Excluder and Endurant have nitinol struts in an expanding M-configuration, providing friction between the artery and the grafts as well. However the wire of these struts is less rigid, leading to radial compressibility and less friction and poorer fixation. These differences in design may explain the stronger fixation of the Anaconda in this study with short neck aneurysms. The presence of the struts over the whole body of the graft, may benefit the Excluder and the Endurant in case of a longer aneurysm neck as it will lead to more graft-artery surface interaction.
Although the Anaconda shows the best proximal fixation in this study, the graft is certainly not flawless. The rigid hooks led to strong proximal fixation but also produced extensive damage of the intima layer of the arteries, when applying large forces on the hooks (Table 5.1 & Fig. 5.6). This damage also occurred once with the Endurant, but was less evident. With some of the modern graft designs, the distal iliac seal is thought to
Fig. 5.6 Effect of the endovascular hooks of the Anaconda on the intima of the vessel after several dislodgement runs.
provide additional fixation to the grafts.29 Murphy et al. showed in their experimental in-vivo set-up that maximum iliac fixation contributes to the fixation strength.30 It is possible that in-vivo larger forces are needed to dislocate a graft with a stented body, as the stent-graft would have a stronger fixation due to the iliac seal and columnar strength of the graft-body. The Anaconda will not benefit from this iliac fixation, as it lacks struts on the graft body.
This study has some limitations. The aortic specimens were harvested from healthy young animals. There was no sign of calcification or thrombus. This might have lead to higher dislocation forces in comparison to the in-vivo situation.20 However the use of healthy animal artery is an accepted method for examining proximal fixation.21, 23, 26, 30
In this experimental set-up, we only used uni-axial distraction forces, as we were not in state to include rotational distraction forces, due to technical limitations. In-vivo experiments have shown that rotational forces influence graft behavior.31
Furthermore we only tested the effect of the proximal seal of each graft. As stated above, the distal iliac seal is thought to provide extra fixation to the grafts in-vivo.29, 30 Another potential difference with the in-vivo situation is that the specimens were straight segments of an artery and there was no angulation or tortuosity. Earlier studies have shown that (severe) angulation of the proximal neck may lead to post-operative stent graft migration.32, 33
It is difficult to state in which way these results are comparable to the in-vivo situation as in-vitro the forces were applied uni-axial and were increased in constant manner. In- vivo the applied forces are not only pulsatile but also more dynamic due to anatomical differences.28 However, the use of uni-axial tension to record pullout forces and the use of animal arterial material are standard methods to test proximal fixation strength of EVAR grafts.10, 21–26, 30, 34 Although there are small differences in the methodology, the set-ups of all these studies are very comparable. The results of our study correspond with results in similar peer-reviewed studies. Malina et al. showed that grafts with rigid hooks had a dislocation force of 22.5N, while the grafts with weaker hooks dislodged at 7.8N.25 The importance of hooks was underlined by the experiments of Andrews et al. and Lambert et al. as they reported dislodgement forces of 2–4 N in stents without hooks or other anchorage on the outside of the graft.22, 26
In experiments with grafts from an earlier generation, Resch et al., showed dislodge- ment forces of 4.5–24N.10 High pull-out forces were recorded as they worked with a proximal seal of 5cm. Using an more recent developed graft, namely the Cook Zenith, Zhou et al. reported dislodgment forces of 8.1–16.8N.23 Veerapen et al. recorded a com- parable dislodgement force of 6.5N for the Excluder.24 The most recent data is available from Kratzberg et al., who measured a median pull-out force of 6.25N with 11–20%
oversized Excluder grafts.21 It should be noted, however, that the top-site of the Exclud-
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ers was equipped with 7 pairs of custom-made nitinol hooks. It is remarkable that the grafts did not benefit from the additional rigid hooks.
Our results with the Excluder compare well with the earlier published data.21, 24 As the launch of the grafts was quite recent, no other in-vitro proximal fixation data is published from the Endurant and the Anaconda.
clinical relevance
Earlier research has shown proximal fixation to be important for preventing migra- tion.9 Migration may lead to endoleak and treatment failure. Preventing migration will enhance the clinical success of endovascular aneurysm repair and will diminish the need for secondary interventions. This study is the first to present the proximal fixation strength of recently available stent-grafts such as the Endurant and the Anaconda.
conclusions
This study has shown that the proximal fixation of the Anaconda is superior to the Excluder and the Endurant at short necks of 10 and 15mm in an experimental in-vitro set-up. Rigid, sharp hooks enhance proximal fixation but also lead to extensive damage of the intima layer of an artery, when high forces are applied. Further research should be done to develop new methods to enhance proximal fixation of endovascular grafts without the disadvantage of extensive damage to the artery after extensive loading.
AcknoWledgements
We would like to thank the following companies for supplying us with stent grafts thereby making this study possible: Gore, Flagstaff, USA; Vascutek, Inchinnan, Scotland;
and Medtronic, Minneapolis, USA. Furthermore we would like to acknowledge the help of M. Boonekamp and his colleagues of the Department of Fine Mechanics, Leiden University Medical Center, for their assistance in designing and building the models.
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