Biodegradable Magnesium Stent Treatment of Saccular Aneurysms in a Rat Model - Introduction of the Surgical Technique

Aneurysm healing following treatment with biodegradable embolization materials: assessment in a rat sidewall aneurysm model

BACKGROUND

Biodegradable materials that dissolve after aneurysm healing are promising techniques in the field of neurointerventional surgery. We investigated the effects of various bioabsorable materials in combination with degradable magnesium alloy stents and evaluated aneurysm healing in a rat aneurysm model.

METHODS

Saccular aneurysms were created by end-to-side anastomosis in the abdominal aorta of Wistar rats. Untreated arterial grafts were immediately transplanted (vital aneurysms) whereas aneurysms with loss of mural cells were chemically decellularized before implantation. All aneurysms were treated with biodegradable magnesium stents. The animals were assigned to vital aneurysms treated with stent alone or decellularized aneurysms treated with stent alone, detachable coil, or long-term or short-term biodegradable thread. Aneurysm healing, rated microscopically and macroscopically at follow-up days 7 and 21, was defined by both neointima formation and absence of aneurysm volume increase over time.

RESULTS

Of 56 animals included, significant increases in aneurysm volume 7 days after surgery were observed in aneurysms with vital and decellularized walls treated with a stent only (P=0.043 each group). Twenty-one days after surgery an increase in aneurysm volume was observed in decellularized aneurysms treated with long- and short-term biodegradable threads (P=0.027 and P=0.028, respectively). Histological changes associated with an increase in aneurysm volume were seen for aneurysm wall inflammation, periadventitial fibrosis, and luminal thrombus.

CONCLUSIONS

An increase in aneurysm volume was associated with an absence of intrasaccular embolization material (early phase) and the breakdown of intrasaccular biodegradable material over time (late phase). Thrombus remnant and aneurysm wall inflammation promote aneurysm volume increase.

Aneurysm healing after endovascular treatment in the Helsinki sidewall aneurysm model: a systematic review

AIMS

Intracranial aneurysms are treated with a variety of endovascular devices including coils, stents, and flow diverters. The mechanisms by which these devices result in aneurysm occlusion and subsequent healing have been the subject of significant research using various animal models. The murine Helsinki aneurysm model is a sidewall aneurysm created by the end-to-side anastomosis of a donor aortic graft onto the abdominal aorta of a recipient animal. The aim of this systematic review is to assess the efficacy of different endovascular devices for the treatment of the Helsinki model aneurysm.

METHODS

We performed a systematic review of Pubmed in accordance with PRISMA guidelines, yielding eight studies detailing the results of endovascular treatment of this preclinical aneurysm model. Studies were included if they provided rates of complete aneurysm occlusion after treatment.

RESULTS

In these studies, aneurysms were treated with coiling (n=81, 7 studies), stenting (n=67, 3 studies), stent-coiling (n=13, 1 study), and flow diversion (n=49, 2 studies). The results of each individual study are discussed with the goal of providing a measure of the relative efficacy of different endovascular devices for the treatment of this particular model aneurysm. We also pay special attention to insights into the mechanisms underlying aneurysm healing after different forms of endovascular therapy.

CONCLUSION

The data presented here may be useful to investigators attempting to demonstrate superiority of novel endovascular devices relative to previous device iterations using this preclinical model.

Bioresorbable flow diverters for the treatment of intracranial aneurysms: review of current literature and future directions

The use of flow diverters is a rapidly growing endovascular approach for the treatment of intracranial aneurysms. All FDA-approved flow diverters are composed of nitinol or cobalt-chromium, which will remain in the patient for the duration of their life. Bioresorbable flow diverters have been proposed by several independent investigators as the next generation of flow diverting devices. These devices aim to serve their transient function of occluding and healing the aneurysm prior to being safely resorbed by the body, eliminating complications associated with the permanent presence of conventional flow diverters. Theoretical advantages of bioresorbable flow diverters include (1) reduction in device-induced thrombosis; (2) reduction in chronic inflammation and device-induced stenosis; (3) reduction in side branch occlusion; (4) restoration of physiological vasomotor function; (5) reduction in imaging artifacts; and (6) use in pediatric applications. Advances made in the similar bioresorbable coronary stenting field highlight some of these advantages and demonstrate the feasibility and safety of bioresorbable endovascular devices in the clinic. The current work aims to review the progress of bioresorbable flow diverters, identify opportunities for further investigation, and ultimately stimulate the advancement of this technology.

Parent artery-initiated and stent-mediated neointima formation in a rat saccular side wall model

Background

Unlike clipping that forms an immediate barrier of blood flow into intracranial aneurysms, endovascular treatments rely on thrombus organization and neointima formation. Therefore, a continuous endothelial cell layer is crucial to prevent blood flow in the former aneurysm. This study investigates the origin of endothelial cells in the neointima of endovascular treated aneurysms, specifically whether cells from the parent artery play a role in neointima formation.

Methods

In male rats, decellularized and vital side wall aneurysms were treated by coil (n=16) or stent embolization (n=15). The cell tracer CM-Dil dye was injected into the clamped aorta before aneurysm suture to mark initial endothelial cells in the parent artery and enable tracking of their proliferation during follow-up. Aneurysms were analyzed for growth, thrombus formation, and recurrence. Histological evaluation followed with cell counts for specific regions-of-interest.

Results

During follow-up, none of the 31 aneurysms ruptured. Macroscopic residual perfusion was observed in 12/16 rats after coiling and in 1/15 after stenting. Amounts of CM-Dil +cells in coiled versus stented decellularized aneurysms significantly decreased in the thrombus on day 7 (p=0.01) and neointima on day 21 (p=0.04). For vital aneurysms, the number of CM-Dil +cells in the neointima on day 21 showed no significant difference.

Conclusions

Healing patterns were worse in coil-treated than stent-treated aneurysms. Cell migration forming a neointima seemed mainly dependent on the adjacent vessel in decellularized aneurysms, but appeared buoyed by recruitment from aneurysm wall cells in vital aneurysms. Therefore, a cell-rich parent artery might be crucial.

Current status and outlook of biodegradable metals in neuroscience and their potential applications as cerebral vascular stent materials

Over the past two decades, biodegradable metals (BMs) have emerged as promising materials to fabricate temporary biomedical devices, with the purpose of avoiding potential side effects of permanent implants. In this review, we first surveyed the current status of BMs in neuroscience, and briefly summarized the representative stents for treating vascular stenosis. Then, inspired by the convincing clinical evidence on the in vivo safety of Mg alloys as cardiovascular stents, we analyzed the possibility of producing biodegradable cerebrovascular Mg alloy stents for treating ischemic stroke. For these novel applications, some key factors should also be considered in designing BM brain stents, including the anatomic features of the cerebral vasculature, hemodynamic influences, neuro-cytocompatibility and selection of alloying elements. This work may provide insights into the future design and fabrication of BM neurological devices, especially for brain stents.

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The current status of the application of biodegradable metals (BM) in neuroscience was presented.

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We analyzed the possibility of producing biodegradable cerebrovascular Mg alloy stents for ischemic stroke treatment.

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Key factors in designing BM brain stents were discussed.

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This work may provide insights into the future design and fabrication of BM neurological devices, especially for brain stents.

Preliminary study on modelling, fabrication by photo-chemical etching and in vivo testing of biodegradable magnesium AZ31 stents

Magnesium metal (Mg) is a promising material for stent applications due to its biocompatibility and ability to be resorbed by the body. Manufacturing of stents by laser cutting has become an industry standard. Our alternative approach uses photo-chemical etching to transfer a pattern of the stent onto a Mg sheet. In this study, we present three stages of creating and validating a stent prototype, which includes design and simulation using finite element analysis (FEA), followed by fabrication based on AZ31 alloy and, finally, in vivo testing in peripheral arteries of domestic pigs. Due to the preliminary character of this study, only six stents were implanted in two domestic farm pigs weighing 25–28 kg and they were evaluated after 28 days, with an interim follow-up on day 14. The left and right superficial femoral, the left iliac, and the right renal artery were selected for this study. The diameters of the stented artery segments were evaluated at the time of implantation, on day 14 and then, finally, on day 28, by quantitative vessel analysis (QVA) using fluoroscopic imaging. Optical Coherence Tomography (OCT) imaging displayed some malposition, breaks, stacking, and protrusion into the lumen at the proximal, distal, and mid-sections of the stented arteries. The stents degraded with time, but simultaneously became embedded in the intima. After 28 days, the animals were euthanized, and explanted vessels were fixed for micro-CT imaging and histology studies. Micro-CT imaging revealed stent morphological and volumetric changes due to the in-body degradation. An in vivo corrosion rate of 0.75 mm/year was obtained by the CT evaluation. The histology suggested no-life threatening effects, although moderate injury, inflammation, and endothelialization scores were observed.

Aneurysm wall cellularity affects healing after coil embolization: assessment in a rat saccular aneurysm model

Background and purpose

Despite significant technical advances, recanalization rates after endovascular therapy of ruptured intracranial aneurysms (IAs) remain a clinical challenge. A histopathological hallmark of ruptured human IA walls is mural cell loss. Mural smooth muscle cells (SMCs) are known to promote intraluminal healing in thrombosed experimental aneurysms. In this rat model we assess the natural history and healing process after coil embolization in SMC-rich and decellularized aneurysms.

Methods

Saccular aneurysms were created by end-to-side anastomosis of an arterial graft from the descending thoracic aorta of a syngeneic donor rat to the infrarenal abdominal aorta of recipient male Wistar rats. Untreated arterial grafts were immediately transplanted, whereas aneurysms with loss of mural cells were chemically decellularized before implantation. Aneurysms underwent coil implantation during aneurysm anastomosis. Animals were randomly assigned either to the non-decellularized or decellularized group and underwent macroscopic and histological analyses on days 3, 7, 21, or 90 post-coil implantation.

Results

A total of 55 rats underwent macroscopic and histologic analysis. After coil embolization, aneurysms with SMC-rich walls showed a linear course of thrombosis and neointima formation whereas decellularized aneurysms showed marked inflammatory wall degeneration with increased recanalization rates 21 days (p=0.002) and 90 days (p=0.037) later. The SMCs showed the ability to actively migrate into the intra-aneurysmal thrombus and participate in thrombus organization.

Conclusions

Coil embolization of aneurysms with highly degenerated walls is prone to further wall degeneration, increased inflammation, and recanalization compared with aneurysms with vital SMC-rich walls.

Testing bioresorbable stent feasibility in a rat aneurysm model

Background

Advances in stent-assisted coiling have incrementally expanded endovascular treatment options for complex cerebral aneurysms. After successful coil consolidation and aneurysm occlusion, endovascular scaffolds are no longer needed. Thus, bioresorbable stents that disappear after aneurysm healing could avoid future risks of in-stent thrombosis and the need for lifelong antiplatelet therapy.

Objective

To assess the applicability and compatibility of a bioresorbable magnesium- alloy stent (brMAS) for assisted coiling.

Methods

Saccular sidewall aneurysms were created in 84 male Wistar rats and treated with brMAS alone, brMAS + aspirin, or brMAS + coils + aspirin. Control groups included no treatment (natural course), solely aspirin treatment, or conventional cobalt–chromium stent + coils + aspirin treatment. After 1 and 4 weeks, aneurysm specimens were harvested and macroscopically, histologically, and molecularly examined for healing, parent artery perfusion status, and inflammatory reactions. Stent degradation was monitored for up to 6 months with micro-computed and optical coherence tomography.

Results

Aneurysms treated with brMAS showed advanced healing, neointima formation, and subsequent stent degradation. Additional administration of aspirin sustained aneurysm healing while reducing stent-induced intraluminal and periadventitial inflammatory responses. No negative interaction was detected between platinum coils and brMAS. Progressive brMAS degradation was confirmed.

Conclusions

brMAS induced appropriate healing in this sidewall aneurysm model. The concept of using bioresorbable materials to promote complete aneurysm healing and subsequent stent degradation seems promising. These results should encourage further device refinements and clinical evaluation of this treatment strategy for cerebrovascular aneurysms.