Georgi R. Marinov

A comparative study of bovine and porcine pericardium to highlight their potential advantages to manufacture percutaneous cardiovascular implants.

Rationale: Prosthetic heart valves designed to be implanted percutaneously must be loaded within delivery catheters whose diameter can be as low as 18 F (6 mm). This mandatory crimping of the devices may result in deleterious damages to the tissues used for valve manufacturing. As bovine and porcine pericardial tissue are currently given preference because of their excellent availability and traceability, a preliminary comparative study was undertaken to highlight their potential advantages. Materials and methods: Bovine and pericardium patches were compared morphologically (light microscopy, scanning electron microscopy and transmission electron microscopy). The acute thrombogenicity of both materials was measured in term of platelet uptake and observed by scanning electron microscopy, porcine intact and injured arteries being used as controls. The pericardium specimens were also subjected to uniaxial tensile tests to compare their respective mechanical characteristics. Results: Both pericardiums showed a layered architecture of collagen bundles presenting some interstitial cells. They displayed wavy crimps typical of an unloaded collagenous tissue. The collagen bundles were not bound together and the fibrils were parallel with characteristic periodicity patterns of cross striations. The mesothelial cells found in vivo on the serous surface were no longer present due to tissue processing, but the adjacent structure was far more compacted when compared to the fibrous side. The fibrinocollagenous surfaces were found to be more thrombogenic for both bovine and porcine tissues and the serous side of the porcine pericardium retained more platelets when compared to the bovine samples, making the acute thrombogenicity more important in the porcine pericardium. Conclusion: Both bovine and porcine pericardium used in cardiovascular implantology can be selected to manufacture percutaneous heart valves. The selection of one pericardium preferably to the other should deserve additional testing regarding the innocuousness of crimping when loaded in delivery catheters and the long-term durability after percutaneous deployment.

Biocompatibility Studies of the Anaconda Stent-Graft and Observations of Nitinol Corrosion Resistance

Purpose: To validate the deployment, in vivo performance, biostability, and healing capacity of the Anaconda self-expanding endoprosthesis in a canine aortic aneurysm model. Methods: Aneurysms were surgically created in 12 dogs by sewing a woven polyester patch onto the anterior side of the thoracic or abdominal aorta. Anaconda prostheses were implanted transfemorally for prescheduled periods (1 or 3 months). Aneurysm exclusion and stent-graft patency were monitored angiographically. Healing was assessed with histological analysis and scanning electron microscopy (SEM). Textile analysis determined the physical and chemical stability of the woven polyester material, while the biostability of the nitinol wires was evaluated with SEM and spectroscopy. Results: All prostheses were intact at explantation. After 1 month, endothelial-like cells were migrating in a discontinuous manner both proximally and distally over the internal collagenous pannus at the device-host boundary. After 3 months, endothelialization had reached the midsections of the devices, with a thicker collagenous internal capsule. Patches of endothelial-like cells were sharing the luminal surface with thrombotic deposits. However, the wall of the device at the level of the aneurysm was generally poorly healed, with multiple thrombi scattered irregularly over the luminal surface. The polyester fabric was intact except for some filaments that were ruptured adjacent to the sutures and some abrasion caused by the nitinol wires. No evidence of corrosion was found on the nitinol stents. Conclusions: This Anaconda stent-graft has demonstrated its ability to exclude arterial aneurysms. The device used in this study was an experimental prototype, and the manufacturer has incorporated new immobilization features into the model for clinical use. The constituent materials appear to be suitable in terms of biocompatibility, biofunctionality, and short-term durability.

Transrenal Deployment of a Modular Stent Graft to Repair AAAs with Short Necks: Experiments in Dogs

Severely angulated (> 60°) or short (< 15mm) proximal necks remain significant anatomical limitations for endovascular stent-graft repairs for abdominal aortic aneurysms. Ensuring proper proximal fixation of the stent-graft to the host artery without the short-or long-term risks of endoleak or migration represents a particular technical challenge for these anatomical circumstances. An innovative balloon expandable stent combined with a weft-knitted prosthesis was specifically designed for these situations by modelling the stent to the neck anatomy without overdistension or potential barotrauma allowing better incorporation of the device. The Latecba stent-graft consists of a 2 parts modular design. The first one, Module A, is deployed at the transrenal level and consists of a Palmaz type stent whose first half is bare and second half is sutured to a crimped weft-knitted polyester graft whose distal end holds a constriction. The second Module B is a non-crimped weft-knitted graft attached to 2 stainless steel stents. The first stent is entirely contained in the proximal textile tube, allowing fixation to module A. The second stent, which is left uncovered over the distal third, ensures proper fixation of the stent-graft distally. Following the creation of a prosthetic aneurysm in the infrarenal aorta in 32 dogs, 29 received the Latecba stent-graft for scheduled durations of 10 days, 1 month, 3 months and 6 months. Proper deployment of the stent-grafts was achieved without difficulty. All 29 animals survived and the devices were all patent at sacrifice. No device defects or migrations were observed and the stent-grafts proved to be efficient in this setting to exclude the aneurysm. Analyses of the explanted devices (gross observations, RX, CT scan, IVUS, angioscopy) confirmed the stability of this modular stent-graft. Further on-going clinical investigations are warranted to validate this concept before this stent-graft becomes commercially available without any restriction.

Analysis of Explanted Latecba Modular Stent-grafts Deployed Transrenally to Repair AAAs with Short Necks in 29 Dogs

Twenty-nine modular stent-grafts deployed transrenally to repair AAAs with short necks in dogs were harvested at autopsy of the animals after scheduled durations of implantations of 10 days, one month, three months, and six months. Analyses of the explanted devices included non-destructive techniques such as gross observations, X-rays CT scan, IVUS and angioscopy. Further to appropriate dissection, histological investigations were carried out by means of scanning electron microscopy (SEM) and light microscopy. All the 29 specimens were extensively encapsulated with fibrous tissues but the fibrous capsule was thin in six of them; four capsules were ulcerated. The X-rays confirmed the stability of the devices that were still straight (12), slightly bent (12) or bent (4). The modules were misaligned in only one case. IVUS and angioscopy confirmed the patency of all the stent-grafts with thin internal capsules both proximally and distally with variable capsulation in the mid-section of the grafts. The left renal artery orifices were found to be patent at dissection with no obstruction to flow. The luminal flow surface of the stent-grafts was smooth and glistening proximally and distally containing endothelial like cells and vasa-vasorum. Poor healing was noted in the aneurysm area. Transrenal deployment of this modular stent-graft is feasible and gave excellent results with regard to biofunctionality and biocompatibility. The device proved to be safe and efficient.

Polymethylmethacrylate (PMMA) as an embedding medium preserving tissues and foreign materials encroaching in endovascular devices.

Problems of displacement, poor healing, degradation of the polymers and corrosion of the metallic frame in endovascular devices still require in–depth investigations. As the tissues and the foreign materials are in close contact, it is of paramount importance to efficiently investigate the interfaces between them. Inclusion in polymethymethacrylate (PMMA) permits us to obtain thin slides and preserve the capacity to perform the appropriate stainings. An AneuRx prosthesis was harvested in bloc with the surrounding tissues at the autopsy of a patient 25 months post deployment in a 5.7 cm diameter AAA and sectioned in the direction of the blood flow in two halves. A cross-section of the encapsulated distal segment together with the surrounding aneuryshmal sac was embedded in polymethylmethacrylate (PMMA). Further to complete polymerization, slices of the specimen were cut on a precision banding saw under coolant. They were affixed onto methacrylate slides with a UV cured adhesive. Binding and polishing were done on a numeric grinder and slices 25 to 30 µm in thickness were stained with toluidine blue prior to observation in light microscopy. Additional slices were prepared for scanning electron microscopy and X-ray energy dispersive spectrometry for determination of the elemental composition of the Nitinol stent. The aortic wall did not demonstrate complete integrity along with its circumference. Some areas of rupture were noted. The content of the sac was heavily shrunk and was mostly acellular. The walls of the device were very well encapsulated. The PMMA embedding permitted the polyester wall, the Nitinol wire and the collagen to keep in close contact. Scanning electron microscopy involved backscattered electrons and confirmed the corrosion the Nitinol wire at the boundary with living tissues. Based upon the results obtained, we believe that PMMA embedding is the most appropriate method to process endovascular devices for histological and material investigation. Needless to say, that paraffin embedding would have not been feasible for such a big size specimen involving different materials.

Vascular prostheses for open surgery

Abstract: The chapter discusses the advantages and limitations of arterial prostheses (or vascular grafts) in terms of their biocompatibility, biofunctionality and biostability. Criteria for biomaterials selection and prosthesis design that have enabled patients to recover more rapidly without any device-related complications are reviewed, and developments are considered that may lead to future improvements in healing and clinical outcomes for the next generation of vascular prostheses.

Ossicular Implants (Prosthesis) – from Infancy to Maturity: Presenting Author: Mario Milkov

Learning Objectives: The idea of ossicular chain reconstruction emerges and develops as a result from the creation of Wulstein and Zollner’s concept of tympanoplasty. Initially, doing the ossicular reconstruction a transposition of incus mainly is performed. Bone or cartilage material has been used as a material. In 1956 Wulstein reported the use of a vinyl-acrylic device as acoustic transmitter between the mobile footplate and the tympanic membrane graft. However, the results were poor. Shea moved from the concept of a graft to that of a bioprosthesis. Shea first began a successful series of ossicular reconstructions. Shea first implanted Teflon prosthesis on a 48-year-old female patient who had widespread otosclerosis, throughout the oval window rim and footplate. As a result, the patient’s hearing improved. Today the implantation of ossicular prostheses to replace non-functional and pathologically altered ossicular chain is a well-accepted surgical technique. The limited graft applications in the reconstructive auditory-chain surgery stimulated the search for new prosthetic medical device. Several questions are of paramount interest for solving this problem: 1. Biomaterial selection for the ossicular implants (prosthesis) construction; 2. Design of the ossicular implants (prosthesis); 3. Validation of the ossicular implants (prosthesis); 4. Monitoring of the patients with implanted ossicular prostheses. In modern otosurgery, a large variety of biomaterials were made use. None of them is, however, useful for any applications. In general, the biomaterials used for ossicular chain reconstruction should possess a good biocompatibility and biostability. They must be well osteointegrated, with minimal risk of ankylosis. Surface properties, particularly structural characteristics, critically influence the quality of the implant-biological interface. The biomaterials need to be easily processed and retain their shape already acquired. A proper sound transmission requires biomaterials of low mass and high hardness. None of them is, however, useful for any applications. In the literature available, usage of different animals for biomaterial validations has been reported. In experiments on the guinea pigs bulla mastoidea model, introduced in the experimental medical practice from Assoc. Prof. Mario Milkov, MD, Ph.D., gold, Teflon, hydroxyapatite, and ceromer were used convincing us in the good qualities of the guinea pig to serve as a model for testing the ossicular prostheses. The idea of ossicular chain reconstruction emerges and develops as a result from the creation of Wulstein and Zollner’s concept of tympanoplasty. Initially, doing the ossicular reconstruction a transposition of incus mainly is performed. Bone or cartilage material has been used as a material. In 1956 Wulstein reported the use of a vinylacrylic device as acoustic transmitter between the mobile footplate and the tympanic membrane graft.Shea first began a successful series of ossicular reconstructions. Today the implantation of ossicular prostheses to replace non-functional and pathologically altered ossicular chain is a well-accepted surgical technique. The limited graft applications in the reconstructive auditory-chain surgery stimulated the search for new prosthetic medical device. Several questions are of paramount interest for solving this problem: