Experimental Testing of a New Generation of Flow Diverters in Sidewall Aneurysms in Rabbits

Materials and Methods

The FRED device is a self-expanding nickel-titanium, compliant closed-cell design compatible with a 0.027-inch Headway microcatheter (MicroVention). The FRED system has a unique dual-layer design composed of a low-porosity inner flow-diverter mesh with 48 braided nitinol strands and a high-porosity outer stent with 16 nitinol struts. This dual-layer coverage is designed to put the working layer (inner part) mainly at the neck, particularly at the inflow zone of the aneurysm and to create a flow-diversion effect. The outer part, which determines the total length, serves as a protection for the inner stent. The outer stent is 3 mm longer than the inner flow-diverter mesh at each end so that the proximal and distal parts of the FRED can be used to cover the adjacent perforating arteries or small branches of the parent vessel with the advantage of high porosity. The FRED has 4 radiopaque markers on each end of the outer stent and 2 interwoven helical marker strands that attach to the inner and outer stents and run the entire length of the inner stent. It can be resheathed up to 85% deployment of its total length (Fig 1).

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Fig 1.

FRED flow-diverter system demonstrating a dual-layer design with the working section marked with a radiopaque helix to provide fluoroscopic visibility. Distal and proximal markers are evenly dispersed on its outer layer ends.

All animal procedures were performed at ISIS Services, San Carlos, California. Following approval from the Institutional Animal Care and Use Committee of ISIS, sidewall aneurysms were created in the right common carotid artery (RCCA) in 22 New Zealand white rabbits. Details of the creation procedure have been published previously.⁴ Briefly, anesthesia was induced by intravenous administration of xylazine (1 mg/kg) and ketamine (10 mg/kg) and was maintained with isoflurane (2%–3%) in oxygen (2 L/min). Using a sterile technique, we made an 8-cm paramedian incision in the neck. The RCCA and right external jugular vein were exposed and dissected. After permanent ligation of the proximal and distal sides of the right external jugular vein, a 1-cm vein pouch was harvested between the 2 sides (right external jugular vein proximal and distal). A 5-mm arteriotomy of the RCCA was made, and ligation was performed to temporarily occlude the proximal and distal sides of the arteriotomy. Heparin (150 U/kg) was injected intravenously through the ear vein before anastomosis. An end-to-side anastomosis between the vein pouch and the RCCA by using an 8–0 Prolene suture (Ethicon, Cincinnati, Ohio) was performed.

Three weeks after creation, patency of all the aneurysms and parent arteries was confirmed by DSA before FRED deployment. The detailed procedure has been published before.^5,6 Briefly, a 5F sheath was advanced on 1 side of the femoral artery via cutdown, followed by a 5F Chaperon guiding catheter (MicroVention). A Headway 27 microcatheter (MicroVention) was advanced into the distal end of parent artery (RCCA) over a Traxcess 14EX microguidewire (MicroVention) through the guide catheter. The first FRED (3.5 mm outer stent diameter × 17/11 mm) was deployed across the aneurysm neck within the RCCA. The second device (4.0 mm outer stent diameter × 18/12 mm) was deployed within the abdominal aorta crossing multiple lumbar arteries. DSA was performed through the guide catheter immediately after deployment. Aspirin (10 mg/kg) and clopidogrel (10 mg/kg) were given daily 2 days before implantation and were continued until 90 days after treatment.

Follow-up DSA was performed at 1 month (n = 5), 3 months (n = 5), 6 months (n = 4), and 12 months (n = 8) after treatment. Animals were sacrificed with a lethal injection of pentobarbital at each time point after DSA follow-up. The RCCA, including the aneurysm sac and abdominal aorta with lumbar arteries, was immediately fixed in 10% neutral buffered formalin.

Degrees of intra-aneurysmal flow disruption immediately after device deployment and before sacrifice were graded on a 3-point scale based on DSA images, including grade I (complete flow cessation, no flow within the aneurysm), grade II (near-complete flow, <10% residual flow), and grade III (incomplete occlusion, ≥10% residual flow).⁷ The degree of aneurysm occlusion at follow-up was compared with the immediate postembolization images by using another 3-point scale, including stable aneurysm occlusion, progressive aneurysm occlusion, and aneurysm recanalization. Patency of parent artery and lumbar artery branches was also evaluated.

Carotid aneurysm samples were dehydrated in a graded series of ethanol and embedded in methyl methacrylate plastic. After polymerization of the plastic, 2- to 3-mm transverse sections were sawed from the proximal and distal ends of the FREDs and from the area of the aneurysm neck. The sections were ground to a thickness of 42–50 μm by using Linear Grinding technology (Exakt Technologies, Oklahoma City, Oklahoma), polished, and stained with toluidine blue and basic fuchsin. All sections containing the aneurysm sac and aneurysm neck were evaluated by light microscopy to score histologic changes on the basis of specific parameters. The tissues within the aneurysm were categorized as unorganized or organized connective tissue. The degree of endothelialized neointima across the neck was also assessed.

DSA and a scanning electron microscope were used to evaluate the patency of the lumbar arterial branch ostia on the aortic luminal surface. Before processing, the samples were opened longitudinally to expose the luminal surface and the FRED implants were photographed. The sample was rinsed in 0.1-mol/L sodium phosphate buffer and then postfixed in 1% osmium tetroxide for 30 minutes. The samples were then dehydrated in a graded series of ethanol. After critical point drying, the tissue was mounted and sputter coated with gold; the sample was visualized by using a scanning electron microscope (magnification range X15-X600; Hitachi, Tokyo, Japan). Tissue processing and the histopathology report (including SEM results) were offered by CVpath Institute, Gaithersburg, Maryland.

Histologic sections containing the proximal, mid-, and distal sections were analyzed with a National Institute of Standards and Technology calibrated microscope system (IP Lab software, Rockville, Maryland) under a BX51 light microscope (Olympus, Melville, New York). The average value of each histologic factor at the above 3 locations was calculated. The cross-sectional areas (internal elastic lamina [IEL] and lumen) of each stented section were measured. The percentage of luminal narrowing was calculated with the following formula:

The extent of stenosis was classified as 4 categories: minimal (<20%), mild (20%–35%), moderate (>35% to <50%), and marked (≥50%).

Neointima formation and endothelial coverage was semiquantified and expressed as the percentage of the lumen circumference covered by neointima and endothelium, respectively. Their different extents were categorized as the following: minimal (<25% of the luminal surface, score: 1), mild (25%–50% of the luminal surface, score: 2), moderate (51%–75% of the luminal surface, score: 3), and marked (>75% of the luminal surface, score: 4).