Comparison of small intestinal submucosa-covered and noncovered nitinol stents in sheep iliac arteries: a pilot study

Assessment of Large Animal Vascular Dimensions for Intra-Aortic Device Research and Development: A Systematic Review

Animal studies are often required to evaluate new cardiovascular medical devices before they reach the market. Moreover, first-generation novel devices including aortic endovascular prostheses and circulatory support devices are often larger than later iterations or tested in a limited range of sizes. One of the challenges in evaluating these devices is finding a model that is both accessible and anatomically similar to humans, as there is a paucity of data on vascular dimensions in large animals. We set out to complete a comprehensive review of available reports on vascular dimensions in swine, ovine, and bovine models, with a particular focus on the descending aorta and ilio-femoral arteries. We searched Embase and MEDLINE databases for reports of descending aorta and peripheral vascular dimension in large animal models. Data from swine, ovine, and bovine models were separated by weight into 3 categories: 40 to 60 kg, 61 to 80 kg, and >80 kg. We also incorporate our computed tomography angiography data from 4 large sheep and 9 calves into this review. Swine, sheep, and calf >80 kg may serve as the best models to maximize aortic diameter resemblance to humans. If device implantation can be achieved in aortas of smaller dimensions, care should be taken to ensure access site suitability such as the common femoral artery in these smaller animals.

Carotid paragangliomas. Alternatives for presurgical endovascular management

Carotid paragangliomas (CP) are rare tumors, representing 0.6% of the head and neck tumors. These tumors have their origin in the carotid body located in the adventitia of the vascular wall of the carotid bifurcation. Among their principal characteristics are hypervascularity, primarily dependent on branches of the external carotid artery, the proximity and possible involvement of the cranial nerves IX, X, XI, XII, and extension to the base of the skull. Complete surgical resection is the first line of management; however, this procedure can be a surgical challenge due to the potential risk of bleeding, intraoperative neurovascular injuries, and prolonged surgical time. Tumor embolization, carotid stenting, and tumor embolization with carotid stenting have been developed as alternative presurgical endovascular techniques that decrease tumor vascularity and/or provide structural vascular support, reducing bleeding and facilitating tumor dissection. Two cases of carotid tumors of the same classification, Shamblin II, are presented, one treated by preoperative embolization and the other managed with a carotid stent; the indications, advantages, and possible complications of each one are discussed. Two cases of Shamblin II carotid tumors are presented, one treated preoperatively with a carotid stent and the other with preoperative embolization. A literature review was carried out, with a search in PubMed that includes case reports, case series, review articles, meta-analyses on CP, presurgical tumor embolization, presurgical carotid stent placement, and surgical treatment of carotid body tumor. Hypervascularity and adhesion to the carotid wall are the leading causes of difficulties in surgical resection of CP. Optimal tumor embolization and/or preoperative carotid stent placement reduce intraoperative bleeding and provide vascular structural support, reducing intraoperative and postoperative complications.

A Review on Modifications of Amniotic Membrane for Biomedical Applications

The amniotic membrane (AM) is the innermost layer of the fetal placenta, which surrounds and protects the fetus. Its unique structure, in addition to its physical and biological properties, makes it a useful substance in many applications related to regenerative medicine. The use of this fantastic substance with a century-old history has produced remarkable results in vivo, in vitro, and even in clinical studies. While the intact or preserved AM is widely used for these purposes, the addition of further modifications to AM can be considered as a relatively new subject in its applications. These modifications are applied to improve AM properties, ease of handling, and durability. Here, we will discuss the cases in which AM has undergone additional modifications besides the required processes for sterilization and preservation. In this article, we have categorized these modifications and discussed their applications and results.

Bioactive Anti-Thrombotic Modification of Decellularized Matrix for Vascular Applications

The decellularized matrix derived from porcine small intestinal submucosa (SIS) is a widely used biomaterial being investigated for numerous applications. Currently, thrombus deposition and neointimal hyperplasia have limited the use of SIS in some vascular applications. To limit these detrimental processes, this work applies bioactive, endothelial-inspired properties to the material. SIS is modified with the endothelial cell membrane protein thrombomodulin and the glycosaminoglycan heparin to facilitate protein C activation and anticoagulant activity, respectively. Modifying SIS with thrombomodulin alone enables robust activated protein C (APC) generation, and thrombomodulin activity is maintained after prolonged exposure to fluid shear and blood plasma. Heparin-modified SIS has a potent anticoagulant activity. When both modifications are applied sequentially, SIS modified first with thrombomodulin then with heparin retains the full activity of each individual modification. Tubular SIS devices are connected to a baboon arteriovenous shunt to quantify thrombus deposition on these materials. After being exposed to flowing whole blood for 60 min, SIS devices modified first with thrombomodulin then with heparin have significantly less platelet accumulation compared to unmodified SIS devices. These studies demonstrate that modifying SIS with thrombomodulin and heparin confers APC generation and anticoagulant activity that results in reduced thrombogenesis.

Crosslinking decreases the hemocompatibility of decellularized, porcine small intestinal submucosa

Decellularized tissues have been widely used as scaffolds for biomedical applications due to their presentation of adhesion peptide sequences and growth factors, which facilitate integration with surrounding tissue. One of the most commonly used decellularized tissues is derived from porcine small intestinal submucosa (SIS). In some applications, SIS is crosslinked to modulate the mechanical properties or degradation rate of the scaffold. Despite the widespread use of SIS, there has been no mechanistic characterization of blood reactions with SIS, or how crosslinking affects these reactions. Therefore, we characterized the effect of SIS and carbodiimide-crosslinked SIS (cSIS) on plasma coagulation, including targeted assessments of the intrinsic and extrinsic coagulation pathways, and thrombus formation using flowing whole blood. SIS inhibited plasma coagulation initiated by recalcification, as well as low concentrations of thrombin or tissue factor. SIS prolonged the activated partial thromboplastin time by 14.3 ± 1.54s, indicating inhibition of the intrinsic coagulation pathway. Carbodiimide crosslinking abrogated all anticoagulant effects of SIS, as did heparinase I and III treatment, suggesting that heparin and heparan sulfate are predominantly responsible for SIS anticoagulant effects. Inhibiting contact activation of the intrinsic pathway prevented cSIS-mediated coagulation. When tubular SIS devices were connected to a nonhuman primate arteriovenous shunt loop, which enables whole blood to flow across devices without the use of anticoagulants, SIS demonstrated remarkably limited platelet accumulation and fibrinogen incorporation, while cSIS initiated significantly higher platelet and fibrinogen accumulation. These results demonstrate that SIS is a thromboresistant material and crosslinking markedly reduces the hemocompatibility of SIS.

Small intestinal submucosa sandwich Zilver stent-grafts for TIPS: experimental pilot study in swine

The aim of this study was the evaluation of preliminary suitability of the piglet small intestinal submucosa (SIS) sandwich Zilver endograft for experimental transjugular intrahepatic portosystemic shunt (TIPS) creation. TIPS was created in six swine. The five surviving animals underwent the follow-up portogram and were sacrificed at four weeks after TIPS creation. Gross and histologic examination was performed in all animals. TIPS creation was successful in all animals. One animal died four days after TIPS creation. In the five surviving animals, one shunt was occluded and four shunts were stenosed on the follow-up portogram. The mean percentage narrowing of the most advanced stenoses was 72% (range, 60%-100%). The most advanced stenosis was located in the parenchymal tract in three shunts and in the hepatic vein portion in one shunt. Gross and histologic examination showed abundant neointimal formation composed mainly of fibroblasts leading to the significant shunt stenoses. The piglet SIS sandwich Zilver stent-graft was found to offer only limited TIPS patency and should not be used for the definitive long-term TIPS study.

Estudo da resposta tissular à endoprótese recoberta de jugular bovina em veia cava inferior de suínos

OBJETIVO: Avaliar a resposta tissular a uma endoprótese, com cobertura biológica heteróloga, implantada em veia cava inferior de suínos. MÉTODO: Desenvolvemos uma endoprótese auto-expansível, revestida com um segmento de jugular bovina, conservada por processo L-hydro e suturada em um stent de aço inoxidável 316L. O dispositivo introdutor utilizado foi a bainha de liberação da endoprótese aórtica Taheri-Leonhardt (Flórida, EUA). Foram implantadas endopróteses em 10 suínos, todas na veia cava infra-renal. Os animais foram submetidos à flebografia peroperatória. À necropsia, após 2 meses, cada endoprótese foi retirada em bloco e analisada macroscopicamente, visando a avaliação da perviedade, aderência aos tecidos vizinhos e incorporação à parede venosa; e, histopatologicamente, visando a resposta histológica ao enxerto. RESULTADOS: Na análise macroscópica, todas as endopróteses encontravam-se pérvias e totalmente incorporadas à parede venosa, porém seis apresentavam trabeculações grosseiras no seu interior e quatro algum grau de fibrose perivascular. Três animais desenvolveram linfocele, uma retroperitoneal e as outras na parede abdominal. No estudo histopatológico, observamos reação inflamatória granulomatosa tipo corpo estranho em todos os casos, sendo predominante na camada média (80%). CONCLUSÃO: O modelo estudado apresentou baixa trombogenicidade, corroborando com a eficácia do meio de conservação e material escolhidos; porém, baixa biocompatibilidade, provavelmente pelo obstáculo imunológico dos xenoenxertos e resposta tissular exagerada do território venoso.

Comparison of the Endothelialization of Small Intestinal Submucosa, Dacron, and Expanded Polytetrafluoroethylene Suspended in the Thoracoabdominal Aorta in Sheep

PURPOSE

This study was undertaken to evaluate and compare endothelialization of small intestinal submucosa (SIS), Dacron, and expanded polytetrafluoroethylene (ePTFE) in high-pressure flow without aortic wall contact and to evaluate the suitability of SIS as a vascular graft material.

MATERIALS AND METHODS

In 12 adult sheep, three types of membrane leaflets of similar thickness (approximately 200 mum) were suspended within large square stents without contact with the thoracoabdominal aortic wall: SIS (n = 12), Dacron (n = 12), and ePTFE (n = 12). Each animal received one leaflet of each material. Aortograms were obtained before and after percutaneous implantation and when the animal was killed at 8 weeks (n = 6) or 18 weeks (n = 6). Cell coverage and remodeling of SIS, Dacron, and ePTFE membranes were assessed by gross and histologic microscopic examinations.

RESULTS

Thirty-five successfully implanted leaflets were evaluated. SIS showed progressive remodeling. Thirty-three leaflets exhibited thickening as a result of neointimal formation and endothelialization, most likely from circulating endothelial cells. Dacron exhibited the greatest and most progressing degree of neointimal formation and endothelialization, followed by SIS and then ePTFE. With SIS and ePTFE, neointimal formation decreased with time, but endothelialization was stable. Uneven neointimal formation and endothelialization on the outer surfaces and distal leaflet positions were seen.

CONCLUSIONS

SIS showed progressive remodeling with moderate and regressive neointimal formation and moderate stable endothelialization. Further study of its durability and incorporation into the aortic wall needs to be performed to evaluate its suitability as a cover for aortic endografts.

Optimal Covering Material for Stent-Grafts Placed in the Portal Vein in a Canine Model

PURPOSE

We evaluated the suitability of Dacron, polytetrafluoroethylene (PTFE), and small intestinal submucosa (SIS) as a covering material for stent-grafts placed in the portal vein as compared with a bare stent.

METHODS

Using 24 beagle dogs, either bare stents or stent-grafts covered with Dacron, PTFE, or SIS were placed in the main trunk of the portal vein in 6 animals each. Portography was performed immediately after stent placement, and at 2, 4, and 12 weeks thereafter. Next, the extracted stents or stent-grafts were examined histopathologically. Neointimal thickness adjacent to the stent wire and at the midportion between the stent wires was compared among the groups. Then, the neointimal thickness at the sub- and supragraft sites was compared between each stent-graft group. Serial changes in the histologic features of the thickened neointima were also investigated.

RESULTS

No significant difference was noted in the mean stenotic ratio of the portal vein diameter between the bare stent and PTFE groups, whereas it was significantly higher in the Dacron and SIS groups compared with the bare stent group. In neither of the studies on neointimal thickness adjacent to the stent wire and at the midportion between the stent wires were any significant differences noted between the neointimal thickness of the bare stent group and the sum of the neointimal thickness of the PTFE group, whereas the sum of the neointimal thickness of the Dacron and SIS groups was significantly greater than that of the bare stent group at both sites. In the comparison of the supragraft neointimal thickness, the SIS group showed significantly greater thickness than the PTFE group, while the difference between the Dacron and PTFE groups was not significant. In the comparison of the subgraft neointimal thickness, the Dacron and SIS groups showed significantly greater thickness than the PTFE group.

CONCLUSION

The present results indicate that of the three covering materials examined here, PTFE is the most suitable material for grafts placed in the portal vein.

A flexible stent with small intestinal submucosa covering for direct intrahepatic portocaval shunt: experimental pilot study in swine

The suitability of the flexible sandwich Zilver stent-graft (SZSG) with a biologically active tissue layer (small intestinal submucosa) for creation of the intravascular ultrasound (IVUS)-guided direct intrahepatic portocaval shunt (DIPS) was explored in six young swine in a search for a flexible system to replace the rigid polytetrafluoroethylene (PTFE) stent originally used by this group with limited success. The portal vein was punctured from the inferior vena cava through the caudate lobe of the liver using IVUS guidance. After balloon dilation of the puncture tract, DIPS was successfully created in all animals with use of an SZSG 9 mm in diameter and 6 cm or 8 cm long. Only one DIPS remained well patent at 14 days when the animal had to be killed because of encephalopathy. DIPS in the other five animals were found to be either severely stenosed (3 animals) or occluded (2 animals) at 4 weeks due to accelerated formation of neointimal hyperplasia (NIH) in the liver parenchymal portion of the shunt and superimposed thrombosis. The lack of high pressure in the portal system contributed to early endograft closure. The flexible stent and the covering fail badly. The reason for this could be due to either component. More work is required to find a reliable flexible system with long-term patency. Exploration of the IVUS-guided direct extrahepatic portocaval shunt is suggested.

Remodeling of suspended small intestinal submucosa venous valve: an experimental study in sheep to assess the host cells' origin

PURPOSE

To investigate the origin of host cells during remodeling of small intestinal submucosa (SIS) square stent-based bicuspid venous valves (VVs).

MATERIALS AND METHODS

Suspended VVs (SVVs) were developed by suspending VVs within bare square stents so the valve elements would not contact the vein wall after deployment. Eight SVVs were placed within the intrahepatic and infrahepatic inferior venae cavae (IVCs) of four adult female sheep. Eight standard VVs were implanted in the external jugular veins of these animals for comparison. At 5 weeks after placement, the devices were examined for stability and patency and the animals were killed. Gross, histologic, and scanning electron microscopic (SEM) examinations were performed.

RESULTS

Follow-up spot radiographs and venography showed no migration of the devices, venous occlusion, or thrombus formation. All SVVs were intact without contact with the IVC wall. Six VVs were competent and two were slightly tilted with some reflux. Histologic study showed remodeling of SVVs and VVs with newly formed collagen fibers; fibroblasts and inflammatory cells were found penetrating the SIS leaflets and endothelial cells on the surface. SIS neovascularization was also present. There was no difference regarding SIS remodeling between SVVs and the free part of VV leaflets. The VV leaflets' bases were thicker compared to their free parts (P <.01). SEM examination showed endothelial cells on both sides of the SVVs and VVs. Endothelialization of the SVV central leaflet surfaces and both surfaces of the VV leaflets was more complete than that of the peripheral surfaces of the SVV leaflets.

CONCLUSION

SIS-based valve remodeling occurs independently of vessel wall contact by recruitment of cells directly from the circulation.