Filament inadequate wall apposition of the different ends of flow diverters in the abdominal aorta of rabbits

Abstract

Background

Flow diverters (FDs) with flared ends (FEs) or straight ends (SEs) are used either alone or when overlapped to treat complex intracranial aneurysms. We evaluated filament inadequate wall apposition (IWA) of the FEs and SEs of FDs in vivo.

Methods

A total of 24 FDs with FEs and SEs were placed in an overlapping manner in the abdominal aortae of six rabbits (two sets per rabbit). Digital subtraction angiography was performed immediately after stent insertion and three months later. The anatomical and histopathological aortic features at FEs and SEs were evaluated.

Results

Angiography revealed no significant difference in terms of changes in arterial diameter between the FE and SE groups (p = 0.877). Gross anatomical evaluation revealed IWA of the different ends of FDs but no thrombi or bleeding, showing that the metallic filaments were not in touch with the vessel wall and nor had they penetrated the vessel wall. The filaments’ IWA rates of FEs and SEs were 8.33% and 8.85%, respectively. The IWA rate at overlapping ends was lower than that at non-overlapping ends. The maximum neo-intimal thickness at FEs was greater than that at SEs (149.4 ± 48.9 and 98.6 ± 26.6 µm, respectively; p < 0.001).

Conclusions

Both the FEs and SEs of FDs can exhibit IMA. IWA events are reduced at the overlapped regions. On pathological evaluation, FEs increased neo-intimal thicknesses more than SEs did, but hyperplasia was minimal on angiography.

Introduction

The use of endovascular stents to treat wide-necked intracranial aneurysms significantly increases the cure rate and reduces recurrence. Recently, clinical applications of various types of flow diverters (FDs) have been reported.^1–4 When using FDs, intracranial aneurysm treatment focuses on vascular remodelling rather than intra-aneurysm embolisation. Inadequate wall apposition (IWA) of FDs around the neck of the aneurysm has been associated with delayed or incomplete aneurysm occlusion.^5,6 IWA occurring at the end of the FD increased the risk of thrombosis in the parent artery or a progressive stenotic deformation.^7,8 Most cases of IWA exhibit metallic filaments not in touch with the vessel wall (NTVW), and filaments penetrating the vessel wall (PVW) is another rare phenomenon in clinic. During peripheral vascular interventions, the end of the stent filament PVW is uncommon but can potentially cause severe complications.^9–12

Flared ends (FEs) and straight ends (SEs) are the common shapes of the end of the intracranial stent. In the clinical treatment of complex intracranial aneurysms, different types of stents are sometimes used in an overlapping manner. Few in vivo studies have examined vessel and filament morphologies at the ends of FDs. Here, we explored angiographic and pathological changes in animals after placement of FDs with ends of various shapes.

Methods

Experimental materials

A total of 24 FDs were braided using 22 nickel-titanium alloy filaments (40 µm in diameter) and two parallel platinum radio-opaque filaments (80 µm in diameter). The FDs had two open ends; the main body was 4 mm in diameter and 12 mm in length. One end was a FE that extended outward at an angle of 45°, and the other end was a SE. The diameter and length of the FE were 6.5 and 1.5 mm, respectively (Figure 1). All FDs were cramped into sheaths and sterilised with ethylene oxide. The stents were placed into arteries using four telescoping schemes: (a) FEs at both tips, (b) SEs at both tips, (c) an FE at one tip and an SE at the other (the FE was pressed into the SE to form an overlap) and (d) an SE at one tip and an FE at the other (the SE was pressed into the FE to form an overlap; Figure 2). These placement methods featured six combinations (FE in a non-overlapping location, SE in a non-overlapping location, an FE out of the second stent, an SE out of the second stent, an FE in the first stent and an SE in the first stent); these arrangements are common at the ends of overlapping FDs. The metal coverage at the overlapping zones can reach to 36%.

Figure 1.

A photograph of a flow diverter (FD) used in our experiment (one ruler division = 1 mm).

Figure 2.

FD insertion patterns.

Animal experiments

Six adult New Zealand white rabbits (2.0–2.5 kg; three males and three females) were used. The experiment protocol was approved and accepted by the Animal Care and Use Committee at our institute. All animals were given aspirin (10 mg/kg) and clopidogrel (10 mg/kg), as previously described,⁵ for three days before and 10 days after the procedure; this covered the acute thrombotic period. Anaesthesia involved intramuscular injection of ketamine hydrochloride (30 mg/kg) and xylazine (5 mg/kg). Using sterile techniques, a 5-Fr sheath was introduced into the right femoral artery and heparin (100 IU/kg) given intravenously. The head of a compatible micro-catheter was advanced distal to the left renal artery ostium, and digital subtraction angiography (DSA) of the abdominal aorta was then performed. The first FD was released in the abdominal aorta, and a second FD was then overlapped and inserted distal to the heart. The head of the micro-catheter was then retracted about 2 cm from the distal end of the FD. A further two FDs were overlapped and inserted using the same method. All devices were released with one attempt by pushing the wire and withdrawing the micro-catheter. The FD morphology and blood flow were re-examined via DSA. The femoral artery was ligated, and the animals were woken from anaesthesia and housed in separate cages. DSA was again performed via a left trans-femoral access route three months later. The animals were sacrificed with an overdose of pentobarbital, and the hearts were injected with 100 mL 4% (v/v) paraformaldehyde to drive the blood in artery. Specimens were collected and fixed in 10% (v/v) neutral formalin for 24 hours. The arterial walls were cut longitudinally, and the relationships between the filaments and the arterial walls at the ends of FDs were observed under a stereomicroscope. Specimens at the ends of FDs were cut into pieces approximately 3 mm wide and 6 mm long, and the metal filaments were carefully removed under a stereomicroscope. The specimens were embedded in paraffin and cut into sections 5 µm thick for hematoxylin and eosin and Weigert staining.

Data analysis

The blood-vessel diameters at FEs and SEs were measured via DSA after FD insertion and again three months later. The numbers of IWA events at the filament ends, including NTVW and PVW, were counted under the stereomicroscope. On Weigert-stained slices, the maximum distance between the edge of the neo-intimal lumen and the internal elastic layer of the blood vessel at the FD end was the measure of end maximum neo-intimal thickness.

Statistical analysis

Data analyses were performed with the aid of SPSS Statistics for Windows v13.0 (SPSS, Inc., Chicago, IL). Frequency data were compared using chi-square tests. Measurements were compared via analysis of variance. Statistical significance was defined as a p-value of <0.05.

Results

DSA findings

Angiography revealed no evident stenosis, stent displacement or vasospasm at any end of an FD. All arterial lumina were smooth (Figure 3). The average arterial diameters at stent FEs immediately after stent insertion and three months later were 3.35±0.23 (95% confidence interval (CI) 3.26–3.45) and 3.12 ± 0.23 (95% CI 3.03–3.21 mm), respectively. The corresponding arterial diameters at SEs were 3.34 ± 0.23 (95% CI 3.24–3.43) and 3.12 ± 0.21 (95% CI 3.03–3.21 mm), respectively. Analysis of the variance of repeated measures revealed no significant difference in terms of changes to the arterial diameters between FEs and SEs (F = 0.024, p = 0.877). Also, the various end combinations did not differ in terms of changes to the arterial diameters (Table 1; F = 0.561, p = 0.729).

Figure 3.

An angiograph obtained after FD insertion (a) and at the three-month follow-up (b). No significant stent displacement or deformation is evident. The overlapping radio-opaque filaments (black arrows) reflect the overlap regions of the stents.

Table 1.

Blood-vessel diameters at various stent end locations measured via digital subtraction angiography at different time points.

Time point	FE in non-overlapping (n = 12)	SE in non-overlapping (n = 12)	FE out stent (n = 6)	SE out stent (n = 6)	FE in stent (n = 6)	SE in stent (n = 6)
After stent insertion	3.28 ± 0.23	3.29 ± 0.27	3.45 ± 0.27	3.43 ± 0.21	3.42 ± 0.18	3.33 ± 0.18
Three-month follow-up	3.12 ± 0.26	3.08 ± 0.22	3.07 ± 0.27	3.17 ± 0.20	3.18 ± 0.15	3.15 ± 0.16

FE: flared end; SE: straight end.

IWA event evaluation using a stereomicroscope

All 24 FDs (48 ends and thus 1152 end filaments) were carefully inspected. A total of 99 filaments from 44 ends exhibited IWA events, including NTVW and PVW (Figure 4). Gross anatomical observations revealed no bleeding or hemosiderin sedimentation around the filaments. The overall filament IWA event rate was 8.59% (99/1152), including 2.34% (27/1152) PVW events and 6.25% (72/1152) NTVW events. The NTVW rate was significantly higher than that of PVW (χ²= 21.37, p < 0.001). The IWA event rates of FE and SE filaments were 8.33% (48/576) and 8.85% (51/576), respectively, and thus not significantly different (χ²= 0.099, p = 0.834). The PVW rates at FEs and SEs were 3.47% (20/576) and 1.22% (7/576), respectively, and thus significantly different (χ²= 6.409, p = 0.018). The NTVW rates at FEs and SEs were 4.86% (28/576) and 7.64% (44/576), respectively, and thus not significantly different (χ²= 3.793, p = 0.067). However, significant differences in the IWA event rates were evident among the different subgroups defined above (χ²= 35.13, p < 0.001). The IWA event rate of stent ends at overlapping regions was lower than that of non-overlapping regions (Table 2).

Figure 4.

The effect of FD end shape on gross arterial wall gross anatomy (one ruler division = 1 mm). Some filaments were (a) penetrating the vessel wall (PVW; black arrows) or (b) not in touch with the vessel wall (NTVW; white arrows).

Table 2.

Numbers and rates of IWA events (PVW and NTVW) triggered by FE and SE filaments.

Event	FE in non-overlapping (n = 288)	SE in non-overlapping (n = 288)	FE out stent (n = 144)	SE out stent (n = 144)	FE in stent (n = 144)	SE in stent (n = 144)
PVW	17	6	2	1	1	0
NTVW	18	34	1	0	9	10
Total rate (%)	12.15	13.89	2.08	0.69	6.94	6.94

IWA: inadequate wall apposition; PVW: penetrating the vessel wall; NTVW: not in touch with the vessel wall.

Section evaluation under a microscope

Measurements on pathological sections revealed that the maximum neo-intimal thicknesses at FEs and SEs were 149.4 ± 48.9 (95% CI 128.8–170.1) and 98.6 ± 26.6 (95% CI 87.3–109.8 µm), respectively, and thus significantly different (F = 19.991, p < 0.001). Significant differences were also evident among the different subgroups (Table 3; F = 5.732, p < 0.001). The neo-intimae were composed principally of fusiform smooth-muscle cells and collagenous fibres (Figure 5).

Table 3.

Maximum neo-intimal thicknesses at the ends of various shapes.

	FE in non-overlapping (n = 12)	SE in non-overlapping (n = 12)	FE out stent (n = 6)	SE out stent (n = 6)	FE in stent (n = 6)	SE in stent (n = 6)
Maximum neo-intima thickness (µm)	151.7 ± 44.0	86.1 ± 22.1	128.7 ± 20.1	125.7 ± 26.0	165.7 ± 74.1	96.3 ± 15.9
95% CI	123.7–179.6	72.1–100.2	107.6–149.8	98.4–152.9	87.9–243.5	79.6–113.0

CI: confidence interval.

Figure 5.

The neo-intimae of FD ends as seen in pathological sections. The round circles are the residual traces of removed filaments. (a) One filament lies outside the blood vessel wall (*) (hematoxylin and eosin staining). (b) Neo-intimal morphology at the stent overlap positions (Weigert staining). (c) Some filaments caused NTVW (black arrows); the neointimae featured principally smooth-muscle cells and collagenous fibres (hematoxylin and eosin staining). (d) The elastic layers of blood vessels and the neo-intimae; the NTVWs were identically located (black arrow, Weigert staining).

Discussion

Filament IWA at the ends of FDs often occurs in the curved regions of the artery. In this study, we selected a straight region of the abdominal artery to implant FDs, and also observed filament IWA, including NTVWs and PVW.

Stent NTVWs are increasingly detected using new imaging tools and methods in patients who have undergone vascular interventions. Stent NTVWs have been extensively studied by coronary interventionists employing intravascular ultrasound or optical coherence tomography. One meta-analysis estimated the prevalence of late NTVW to be 16%.¹³ Neuro-interventional professionals often evaluate stent shape and in-stent hyperplasia using VasoCT.^14–16 Filament NTVW of FDs can also be detected via new-generation intravascular optical coherence tomography in intracranial aneurysms treated via stent-assisted coiling, although routine follow-up angiography may not reveal aneurysm recurrence.¹⁷ Filament NTVW can occur at either the ends or the mid-regions of neurovascular stents, especially when arteries bend.^6,7,16,18 We observed both NTVW and PVW at the ends of FDs in vivo. The results show that PVW is not rare, even if the device release is simple in the straight abdominal aorta of a rabbit in one attempt. In the peripheral vascular intervention field, the rate of filter filament penetration ranges from 9% to 24% in the inferior vena cava; filaments may even exit the vena cava to enter the aorta.¹² Tadakoshi et al.⁹ found that stent filament penetration triggered aortic dissection during treatment of a distal-arch thoracic aortic aneurysm.

We speculate that two factors trigger PVW. The first is the persistent force imparted by a self-expanding stent. Sequeira et al.¹¹ reported that stent FEs penetrated the walls of arteriovenous fistulae, and considered that that a low (but nonetheless excessive) chronic outward force imparted by a self-expanding stent damaged the vessel. When the FD diameter is nominal, the radial FD force is maximal in the centre. If an FD is placed in an artery with a vessel diameter smaller than the nominal value, the radial force drops dramatically.⁷ The ends of an FD placed in an artery exert lengthwise expansion or compression forces in addition to a radial force. A persistent expansion or compression force may trigger device-end PVW. The second factor is the small filament diameter at the end of an FD. The filament diameters used in conventional stents employed to treat intracranial aneurysms exceed 80 µm. FD filaments are thinner than those of Neuroform™, Enterprise™ or LEO™ stents. The filament diameter of FDs used to reconstruct vessel blood flow is 25–50 µm.^2–4 As with a needle, an excessively thin metal filament may trigger PVW when the filament adheres inappropriately to a vessel wall. Although radiography or gross anatomical evaluation revealed neither haemadostenosis nor arterial wall bleeding, a PVW event might risk haemorrhage where a FD end slides into an aneurysm or locates in a weak arterial wall.

Designers of stent FEs seek to enhance stent opening, facilitate stent luminal adherence and prevent stent displacement.^19–21 An FE can enhance filament adherence in regions where the luminal diameter changes markedly.²⁰ Park et al.²² reported that compared to SE stents, FE stents exhibited less stent displacement (2.4% vs. 15.4%) and reduced the complication risk in those with diseases of the digestive system. Previous studies on patients with intracranial vascular diseases found that FDs may become displaced or shortened to various extents.²³ In the present study, radiography did not reveal any stent displacement. Filament PVW may prevent stent displacement or deformation just like an anchor.

We also found no significant difference in the IWA event rates attributable to FEs and SEs. However, subgroup analysis showed that the IWA event rate associated with non-overlapping regions was significantly higher than that of overlapping regions. We think the filaments at the end pressed by another device reduced the rate of NTVW, while the filaments at the end in another device reduced the chance of PVW. To correct the IWA after FDs implanted in patients, manipulation of the FD is needed, including use of wires, catheters, balloons or additional self-expanding stents/flow diverting devices.^7,24

Analysis of pathological sections showed that the maximum neo-intimal thicknesses at FEs were significantly greater than those at SEs, perhaps reflecting FE angulation per se. We observed that in vitro (data not shown), filaments did not attain the tubal wall at the junction of the FE and the stent body, attributable to flare angulation. Although luminal reduction caused by stent FEs reduced blood flow within trans-jugular, intrahepatic portosystemic shunts,²⁵ we recorded only minimal neo-intimal thickening relative to that of the overall arterial lumen. Conventional angiography cannot detect this fine distinction. DSA revealed no significant difference in the luminal diameters of FE and SE stents.

Our work has certain limitations. First, all stents featured 24 braided filaments, unlike commercial FDs which feature ≥48 filaments, although the metal coverage at the overlapping FD zones was 36% in the present study. The rate of IWA events may fall as braiding increases. Second, all animals were sacrificed at the same time, and stent filament morphologies were not well revealed via radiography. It was extremely difficult to define the time at which a stent PVW occurred. Third, we only evaluated one flare angle (45°); this may not be ideal. In future, it would be useful to explore whether increasing the number of filaments and optimisation of the stent flare angle reduces the rate of IWA events.

Conclusions

We found that following FD insertion, some filaments at the device ends triggered IWA events, including NTVW and PVW. The IWA event rate did not differ between FEs and SEs. Device end-related IWA events were fewer at the overlapped regions. FEs triggered pathological increases in neo-intimal thickness, but such increases were minimal on angiography.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was funded by the National Nature Science Foundation of China (Grant No. 81301020).

Conflict of interest

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

ORCID iD

Kuizhong Wang https://orcid.org/0000-0003-2341-0136