Deployment and healing of an ePTFE encapsulated stent endograft in the canine aorta

Tissue responses to stent grafts with endo-exo-skeleton for saccular abdominal aortic aneurysms in a canine model

OBJECTIVE

We evaluated the effect of close contact between the stent and the graft on the induction of endothelial covering on the stent graft placed over an aneurysm.

MATERIALS AND METHODS

Saccular abdominal aortic aneurysms were made with Dacron patch in eight dogs. The stent graft consisted of an inner stent, a expanded polytetrafluoroethylene graft, and an outer stent. After sacrificing the animals, the aortas with an embedded stent graft were excised. The aortas were inspected grossly and evaluated microscopically.

RESULTS

The animals were sacrificed at two (n = 3), six (n = 3), and eight months (n = 2) after endovascular repair. In two dogs, the aortic lumen was occluded at two months after the placement. On gross inspection of specimens from the other six dogs with a patent aortic lumen, stent grafts placed over the normal aortic wall were covered by glossy white neointima, whereas, stent grafts placed over the aneurysmal aortic wall were covered by brownish neointima. On microscopic inspection, stent grafts placed over the normal aortic wall were covered by thin neointima (0.27 ± 0.05 mm, mean ± standard deviation) with an endothelial layer, and stent grafts placed over the aneurysmal aortic wall were covered by thick neointima (0.62 ± 0.17 mm) without any endothelial lining. Transgraft cell migration at the normal aortic wall was more active than that at the aneurysmal aortic wall.

CONCLUSION

Close contact between the stent and the graft, which was achieved with stent grafts with endo-exo-skeleton, could not enhance endothelial covering on the stent graft placed over the aneurysms.

Tissue responses to endovascular stent grafts for saccular abdominal aortic aneurysms in a canine model

We investigated tissue responses to endoskeleton stent grafts for saccular abdominal aortic aneurysms (AAAs) in canines. Saccular AAAs were made with Dacron patch in 8 dogs, and were excluded by endoskeleton stent grafts composed of nitinol stent and expanded polytetrafluoroethylene graft. Animals were sacrificed at 2 months (Group 1; n = 3) or 6 months (Group 2; n = 5) after the placement, respectively. The aortas embedding stent grafts were excised en bloc for gross inspection and sliced at 5 to 8 mm intervals for histopathologic evaluation. Stent grafts were patent in all except a dog showing a thrombotic occlusion in Group 2. In the 7 dogs with patent lumen, the graft overhanging the saccular aneurysm was covered by thick or thin thrombi with no endothelial layer, and the graft over the aortic wall was completely covered by neointima with an endothelial layer. Transgraft cell migration was less active at an aneurysm than at adjacent normal aorta. In conclusion, endoskeleton stent grafts over saccular aneurysms show no endothelial coverage and poor transgraft cell migration in a canine model.

Evaluation of Three Polytetrafluoroethylene Stent-Grafts in a Model of Neointimal Hyperplasia

PURPOSE

The authors tested three different porosities of expanded polytetrafluoroethylene (ePTFE)-covered stents and bare stents by using an animal model of restenosis.

MATERIALS AND METHODS

Both iliac arteries in 18 female pigs were injured by overdilating 20-mm-long angioplasty balloons. A 40-mm-long bare stent or one of three 44-mm-long ePTFE-covered stents was deployed at the injury site. To determine restenosis, neointimal area measurements were made with intravascular ultrasonography. Histologic analyses were performed at an independent laboratory to determine neointimal attachment.

RESULTS

Neointimal area was greatest at the middle of the bare stent, where balloon injury was centered. When the middle location of the covered stents was evaluated, the neointimal area of both the medium- and high-porosity covered stents was smaller than that of the matched control stents (P = .0018 and P = .0118, respectively). The neointimal area of the low-porosity covered stents was similar to that of the bare stents. Histologic study showed dehiscence of the neointima of the low-porosity covered stents.

CONCLUSIONS

The microstructure of the low-porosity covered stents did not provide a suitable surface for neointimal attachment and did not reduce neointimal growth compared to that with the control stents. The microstructure of the medium- and high-porosity covered stents yielded less neointimal growth than both the control stents and the low-porosity covered stents without evidence of neointimal dehiscence. The authors believe that covered stents made with ePTFE with either medium or high porosity could limit restenosis in humans compared to that with bare stents.

Digital morphometric analysis of the aortic wall in pigs following implantation of dacron-covered stents versus non-covered stents

PURPOSE: To evaluate, by digital morphometry, the intimal thickening after the placement of two different kinds of stents: polyester covered stainless steel stents (Dacron) and non-covered stents implanted in young pigs' infrarenal aortas. METHODS: The experiment was carried out on two separate groups of pigs. Eight polyester-covered stainless steel stents (Dacron) and eight stainless steel stents (30-mm long, 8-mm diameter) were deployed through extraperitoneal approach in the normal infrarenal aorta of 16 normolipemic pigs. To allow the passage of the delivery system, a small arteriotomy was performed (phase I). After four weeks, the aorta with the stent was removed en bloc. (phase II). The values of the hematimetric and lipid analysis did not show any changes that could influence the study. Tissue samples of the fixing sites (proximal and distal) of the stents were taken. Microscope slices were prepared and submitted to Verhoeff's hematoxilin and eosin techniques and sent to morphometric analysis. RESULTS: The intima immediately proximal to the device was thicker in the group of covered stents with marginal significance (p=0.054). The distal intimal layer (p=0.185), proximal medial layer of the proximal portion (p=0.141) and distal portion (p=0.375) did not present statistical difference between the two groups. CONCLUSIONS: Patency was 100% in both groups of these normolipemic pigs. After four weeks the intimal layer immediately proximal to the covered stents was ticker when compared to uncovered stents, but this had a borderline significance. The intimal layer distal to covered stents and the media proximal or distal to the devices had similar morphometric features when covered and uncovered stents where compared.

Does prosthetic covering of nitinol stents alter healing characteristics or hemodynamics?

PURPOSE

To evaluate placement of polyester (Dacron) coverings on nitinol stents implanted in the canine aorta to determine the effect on cross-sectional lumen area, development of intimal hyperplasia, device endothelialization, and flow hemodynamics.

METHODS

Ten polyester-covered and 10 uncovered nitinol stents (60-mm length, 10- or 12-mm diameter) were deployed percutaneously in the normal infrarenal aorta of 20 adult mongrel dogs using random assignment. Angiography, intravascular ultrasound (IVUS), and duplex ultrasound performed at device deployment and before explantation at 6 weeks were used to measure aorta/device diameter and cross-sectional area. Pressure-perfusion-fixed aortic segments were compared for surface endothelialization (CD31 staining) and for thickness of neointimal formation.

RESULTS

All 20 endoluminal devices were accurately positioned in the infrarenal aorta without early or delayed evidence of device thrombosis, significant lumen narrowing, or device deformity. IVUS and duplex scanning identified no anatomical stenosis in either the covered or the bare devices by duplex ultrasound; peak systolic velocity measurements were similar (106+/-25 cm/s in the covered stent versus 96+/-25 cm/s for bare stents, p > 0.05). Mean neointimal thickness was significantly greater (p < 0.005) in the covered (326+/-145 microm) compared with the bare (219+/-62 microm) stents. Intima-to-media height ratios were greater in the covered stents (3.0+/-1.1 compared with 1.1+/-0.2, p < 0.003). Mean surface area endothelialization in the proximal, middle, and distal sections of each device was similar (p > 0.05) in covered (59%, 56%, and 69%) and bare (59%, 65%, and 53%) stents.

CONCLUSIONS

Deployment and balloon dilation of a covered nitinol stent in a nondiseased canine aorta increased neointimal development compared with an uncovered stent, but overall lumen cross-sectional area was preserved. No differences in device patency, intradevice thrombus formation, flow hemodynamics, or luminal endothelialization were demonstrated, despite a thicker intradevice neointima induced by the polyester covering.

Transjugular intrahepatic portosystemic shunts formed with polyethylene terephthalate-covered stents: experimental evaluation in pigs

PURPOSE

To evaluate the safety, efficacy, and tissue response associated with Wallstents covered with polyethylene terephthalate (PETP) compared with those associated with uncovered Wallstents for creation of transjugular intrahepatic portosystemic shunts (TIPS) in a porcine model.

MATERIALS AND METHODS

Thirteen TIPS were created in 13 minipigs: eight with PETP-covered Wallstents, five with standard Wallstents. Shunt venography was performed at 5-8 weeks, and necropsy was performed at 7-8 weeks. Histopathologic, immunohistochemical, and scanning electron microscopic examinations were performed.

RESULTS

Mean shunt stenoses of the control and graft groups were 45% and 53%, respectively. Graft stenoses involved the entire graft-bearing segment, whereas bare stent stenoses were localized within the liver tract. Myofibroblast and extracellular collagen matrix proliferation encompassed both control and graft-covered stents. There was one graft TIPS occlusion. One control TIPS stenosis was due to transstent proliferation of normal porcine hepatic tissue. A small focus of bile staining was seen on the abluminal surface of one TIPS, which was a patent PETP-lined shunt.

CONCLUSION

PETP graft TIPS provided equal, but not superior, patency to that of bare stent TIPS. The pattern of PETP TIPS graft healing differed from that of bare stents but was similar to that reported with other polyester graft vascular implants and consisted of diffuse transmural penetration and paving of the graft surface by extracellular collagen matrix and myofibroblasts.

Polytetrafluoroethylene-encapsulated stent-grafts: use in experimental abdominal aortic aneurysm

PURPOSE

To evaluate expanded polytetrafluoroethylene (ePTFE) encapsulated stents for the treatment of aortic aneurysms with emphasis on the blood and tissue-material interactions.

MATERIALS AND METHODS

Experimental aortic aneurysms were created in dogs by enlarging the aortic lumen with an abdominal fascial patch. Twenty animals underwent endoluminal repair after allowing the surgically created aneurysm to heal for 2 months prior to transluminal aneurysmal exclusion. The device used consisted of an 8-cm-long ePTFE encapsulated stent graft. The animals were killed in groups at 1 week and at 1, 2.25, 6, and 12 months. Specimens were processed for histologic and luminal surface studies.

RESULTS

Before the animals were killed, aortography demonstrated two thrombosed aortae in the 6-month group and two endoleaks in the 12-month group. Endothelialized neointima extended into the proximal and distal portions of the prosthetic lumen, with minimal cell coverage in the center of the graft. The overall percent surface area covered by endothelialized neointima was 22% +/- 6% at 6 months and 18% +/- 10% by 1 year (P = .75). Histologic examination demonstrated minimal tissue penetration into the ePTFE.

CONCLUSION

Transluminal exclusion of abdominal aortic aneurysms by encapsulated stent-graft is easily accomplished. With this device, tissue coverage and penetration of the stent graft is limited and does not tend to increase with time.

Histopathologic evaluation of an expanded polytetrafluoroethylene-nitinol stent endoprosthesis in canine iliofemoral arteries

PURPOSE

The authors assess a new ePTFE-nitinol stent for its long-term patency, healing, and properties of endothelialization.

MATERIALS AND METHODS

Adult greyhounds (n = 18) underwent bilateral iliofemoral placement of an endoprosthesis (Hemobahn) consisting of a nitinol stent lined with an ultrathin expanded polytetrafluoroethylene (ePTFE) material. Histologic and quantitative morphometric analyses were performed on devices explanted at 2 weeks and 1, 3, 6, and 12 months. The source of endothelialization was examined in four additional devices modified by sealing either the proximal and distal ends or the entire graft with poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP), a nonporous laminate to prevent potential transgraft endothelial cell migration.

RESULTS

Device patency assessed with both intravascular ultrasound and histologic study showed minimal arterial stenosis, irrespective of implant duration. The neointima at less than 3 months consisted of fibrin and inflammatory infiltrate; at later time points, it was composed of mostly smooth muscle cells. Flow surfaces were more than 75% endothelialized by 3 months, which was nearly complete by 6 months. Modified endoprostheses entirely enveloped with FEP resulted in endothelialization of only the proximal and distal ends; the middle regions showed nonocclusive thrombi. Conversely, devices wrapped proximally and distally showed nearly complete endothelialization.

CONCLUSIONS

This ePTFE-nitinol endoprosthesis demonstrated long-term patency at up to 1 year after implantation and showed early and nearly complete endothelialization by 6 months. The design promoted rapid endothelialization of flow surfaces, particularly in the midregion of the device possibly by transgraft migration.