Can Magnetic Resonance Imaging Be the Key Technique to Visualize and Investigate Endovascular Biomaterials?

OBJECTIVE

Magnetic resonance imaging (MRI) is an established modality in clinical use but may be potentially underutilized to visualize and investigate biomaterials. As its use is totally contraindicated only for ferromagnetic devices, it was employed to visualize deployment, biofonctionality, healing, and biodurability of a commercially available endovascular device, namely the Medtronic-AVE AneuRx. The quality of the observations coupled with the absence of ionizing radiations are likely to make this technique an attractive imaging modality in the future.

METHOD

The potential benefits of the MRI technique were investigated in a GE Vectra-MR 0.5T MRI for the Medtronic-AVE AneuRx endovascular prosthesis, under different conditions: undeployed i.e., inserted in the delivery cartridge as received from the manufacturer (step 1), deployed in a mock glass-aneurysm tube (step 2), and as a pathological explant harvested at the autopsy of a patient (step 3). The device was submitted to X-rays for examination in addition to MRI. At step 3, the device was further investigated with light microscopy and scanning electron microscopy (SEM) together with X-ray diffraction.

RESULTS

The device which was inserted and pleated in the delivery cartridge did not demonstrate any significant observation either in MRI or in X-rays. When it was deployed in the mock aneurysmal glass tube, light artefacts were associated with the T2 weighed FSE images around the Nitinol whereas X-rays gave images of indisputable interest. Similar results were noted using the explanted device. Very high contrasts were obtained with T1 whereas T2 images were almost defect free. The X-rays allowed to accurate imaging of the Nitinol skeleton but were poor to discriminate between the different tissues. Pathology observations using light microscopy were not really challenged, as the magnetic resonance imaging was performed using a 0.5T machine.

DISCUSSION

The benefits of magnetic resonance imaging as a quality control technique to examine an endovascular device within its cartridge remains ill defined. Similarly, the role of conventional X-rays is unknown. The observation of devices fully deployed in a mock aneurysmal glass-tube under MRI are potentially useful but X-rays images allowed better definition. The MRI examination of the explanted device does permit observations related to the healing of the device that might be obtained in vivo and, thus offers new avenues for the follow-up of implanted devices. The pathological investigations brought additional informations about the tissues and the corrosion of the Nitinol. However, it is unlikely that MRI will permit detailed analysis of the biomaterials and in particular the corrosion process of the stents.

CONCLUSION

These early observations of the follow-up of devices using MRI warrant further investigation. The absence of ionizing radiation with MRI makes this technique particularly attractive. As there is no emission of ionizing radiation associated with magnetic resonance, it is recommended that further investigation using this environment friendly technique for the follow-up of devices made of biomaterials that are MRI compatible.

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 microm 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.

MRI virtual biopsies: analysis of an explanted endovascular device and perspectives for the future

Information that can be obtained by magnetic resonance imaging (MRI) of explanted endovascular devices must be validated as this method is non-destructive. Histology of such a device together with its encroached tissues can be elegantly performed after polymethymethacrylate (PMMA) embedding, but this approach requires destruction of the specimen. The issue is therefore to determine if the MRI is sufficient to fully validate an explanted device based upon the characterization of an explanted specimen. An AneuRx device deployed percutaneously 25 months earlier in a 75-year-old patient was removed en bloc at autopsy together with the surrounding aneurysmal sac and segments of the upstream and downstream arteries. Macroscopic pictures were taken and a slice of the cross-section was processed for histology after polymethylmethacrylate (PMMA) embedding. For the magnetic resonance imaging investigation, the device was inserted in a Biospec 4.7 T MRI system with a 20 mm diameter birdcage resonator used for both emission and reception. A Spin-Echo (SE) was used to acquire both T1 proton density (PD) and T2 weighted images. A gradient-echo (GE) sampling of a free induction decay (GESFID) was used to generate multiple GE images using a single excitation pulse so that four images at different TE were obtained in the same acquisition. The selected explanted device was outstandingly well-healed compared to most devices harvested from humans. No inflammatory process was observed in contact or at distance of the materials. In MRI T1 images display no specific contrast and were homogeneous in the different tissues. The contrast was improved on proton density weighed images. On the T2 weighed images, the different areas were well identified. The diffusion images displayed in the surrounding B region had the greatest diffusion coefficient and the greatest anisotropy. The MRI analysis of the explanted AneuRx device illustrates the possibilities of this technique to characterize the interaction of the endovascular graft with the surrounding tissues. MRI is a breakthrough to investigate explanted medical devices but it also can be advantageously used in vivo to obtain virtual biopsies, because real biopsies to determine the 3 Bs (biocompatibility, biofunctionality and bioresilience) cannot be carried out as they could obviously initiate infection and degradation of the foreign materials.

First-generation aortic endografts: analysis of explanted Stentor devices from the EUROSTAR Registry

PURPOSE

To examine the structure and healing characteristics of chronically implanted Stentor endografts that were explanted due to migration, endoleak, thrombosis, or aneurysm expansion.

METHODS

The devices were harvested following reoperation (n = 5) or autopsy (n = 1) with implantation times ranging from 13 to 53 months. Structural modifications to the metal components were examined using radiography, endoscopy, and magnetic resonance imaging (MRI). Specimens taken from components of the modular stent-grafts were examined histologically and with scanning electron microscopy (SEM) to assess healing behavior. Physical and chemical stability of the nitinol wires and woven polyester graft material was evaluated using SEM and electron spectroscopy for chemical analysis.

RESULTS

Although the endografts were retrieved for a variety of reasons, they exhibited similar healing and structural modifications. The woven polyester sleeve showed evidence of yarn shifting and distortion, yarn damage, and filament breakage leading to the formation of openings in the fabric. The luminal surface endografts showed incomplete healing characterized by a poorly organized, nonadherent thrombotic matrix of variable thickness. Radiographic and endoscopic observations indicated that structural failure of the grafts, particularly in the main aortic component, was related to severe compaction and dislocation of the metallic frame due to suture breaks. Corrosion marks were observed on some nitinol wires in all devices. Chemical analysis and ion bombardment of the nitinol wires revealed that the surface concentrations of titanium and nickel were not homogenous. The first layer was composed of carbon or organic elements, followed by a stratum of highly oxidized titanium with a low nickel concentration; the titanium-nickel alloy lay beneath these layers.

CONCLUSIONS

Although the materials selected for construction of endovascular grafts appears judicious, the assembly of these biomaterials into various interrelated structures within the device requires further improvement.

First-generation aortic endografts: analysis of explanted Stentor devices from the EUROSTAR Registry

PURPOSE

To examine the structure and healing characteristics of chronically implanted Stentor endografts that were explanted due to migration, endoleak, thrombosis, or aneurysm expansion.

METHODS

The devices were harvested following reoperation (n = 5) or autopsy (n = 1) with implantation times ranging from 13 to 53 months. Structural modifications to the metal components were examined using radiography, endoscopy, and magnetic resonance imaging (MRI). Specimens taken from components of the modular stent-grafts were examined histologically and with scanning electron microscopy (SEM) to assess healing behavior. Physical and chemical stability of the nitinol wires and woven polyester graft material was evaluated using SEM and electron spectroscopy for chemical analysis.

RESULTS

Although the endografts were retrieved for a variety of reasons, they exhibited similar healing and structural modifications. The woven polyester sleeve showed evidence of yarn shifting and distortion, yarn damage, and filament breakage leading to the formation of openings in the fabric. The luminal surface endografts showed incomplete healing characterized by a poorly organized, nonadherent thrombotic matrix of variable thickness. Radiographic and endoscopic observations indicated that structural failure of the grafts, particularly in the main aortic component, was related to severe compaction and dislocation of the metallic frame due to suture breaks. Corrosion marks were observed on some nitinol wires in all devices. Chemical analysis and ion bombardment of the nitinol wires revealed that the surface concentrations of titanium and nickel were not homogenous. The first layer was composed of carbon or organic elements, followed by a stratum of highly oxidized titanium with a low nickel concentration; the titanium-nickel alloy lay beneath these layers.

CONCLUSIONS

Although the materials selected for construction of endovascular grafts appears judicious, the assembly of these biomaterials into various interrelated structures within the device requires further improvement.

Endovascular repair of thoracic aortic aneurysm in dogs: evaluation of a nitinol-polyester self-expanding stent-graft

PURPOSE

To validate the ease of deployment and in vivo healing performance of a nitinol-polyester self-expanding stent-graft using a canine thoracic aortic aneurysm model.

METHODS

Arterial aneurysms were surgically created in 8 dogs by sewing a polyester patch onto the anterior side of the thoracic aorta. The nitinol-polyester self-expandable stent-grafts (Cragg EndoPro System 1) were implanted transluminally via the femoral route and deployed at the site of the thoracic aneurysm. Aneurysm exclusion and endograft patency were assessed by angiography after implantation and before animal sacrifice at scheduled periods ranging from 1 week to 3 months. The explanted specimens were examined with magnetic resonance imaging (MRI) to study the position of the stent-graft with respect to the aneurysmal sac. Histological analysis using light microscopy and scanning electron microscopy was performed to examine the inflammatory response and healing characteristics of the device.

RESULTS

Seven of 8 stent-grafts were implanted successfully; a bend occurred within the aneurysmal sac in 1 dog, which led to continued perfusion of side branches. This endoleak sealed spontaneously within 1 week, and complete exclusion of the aneurysms in all 8 animals continued throughout implantation. At the time of explantation, all devices were structurally intact and well positioned in the aneurysmal sac. At 1 week, the luminal surface displayed a thin layer of thrombotic matrix, which was gradually replaced by a collagenous internal capsule with endothelial-like cell coverage along both ends of the stent-grafts at 2 and 3 months. No exacerbated inflammatory reaction due to either the nitinol wires or the polyester sleeve was observed after 3 months of implantation.

CONCLUSIONS

This short-term in vivo study of a nitinol-polyester self-expanding endograft demonstrated the effective exclusion of thoracic aneurysms with a satisfactory healing response and no excessive tissue or inflammatory reactions.

Comparison of healing in fresh and preserved arterial allografts in the dog

The use of aortic allografts for the management of vascular prosthetic infections has recently been reintroduced. Impressive results have been obtained; however, the possibility of late degeneration remains a major concern. The healing behavior of aortic allografts, either fresh or preserved, in antibiotic-supplemented nutrient medium at 4 degrees C for 1 week and used as thoracic aorta substitutes in dogs was investigated after 6 months of implantation. Four dogs received a fresh aortic allograft from four different donors, and four dogs received a preserved allograft from two different donors. Autografts in two dogs were performed as controls. The in vivo investigation was conducted to describe (1) the histological characteristics of the arterial wall, (2) the macroscopic and thrombogenic aspect of the luminal surface, (3) the integrity of the endothelial lining by scanning electron microscopy, and (4) its biochemical function by prostacyclin (PGI2) and thromboxane A2 (TXA2) secretion. Immune-mediated reactions directed toward the grafts were measured by sequential screening of donor-specific serum antibody development. All donor-recipient pairs of dogs were major histocompatibility complex (MHC)-incompatible according to a mixed lymphocyte reaction (MLR) assay. From the results of this study we concluded that although preserved arterial allografts exhibited similar surface characteristics as those of fresh allografts in terms of re-endothelialization and long-term graft function, an elicited immune response, a degenerative process in the media, and a hyperplasic reaction in the intima could not be prevented using this method of preservation.

Low Reynolds number turbulence modeling of blood flow in arterial stenoses

Moderate and severe arterial stenoses can produce highly disturbed flow regions with transitional and or turbulent flow characteristics. Neither laminar flow modeling nor standard two-equation models such as the kappa-epsilon turbulence ones are suitable for this kind of blood flow. In order to analyze the transitional or turbulent flow distal to an arterial stenosis, authors of this study have used the Wilcox low-Re turbulence model. Flow simulations were carried out on stenoses with 50, 75 and 86% reductions in cross-sectional area over a range of physiologically relevant Reynolds numbers. The results obtained with this low-Re turbulence model were compared with experimental measurements and with the results obtained by the standard kappa-epsilon model in terms of velocity profile, vortex length, wall shear stress, wall static pressure, and turbulence intensity. The comparisons show that results predicted by the low-Re model are in good agreement with the experimental measurements. This model accurately predicts the critical Reynolds number at which blood flow becomes transitional or turbulent distal an arterial stenosis. Most interestingly, over the Re range of laminar flow, the vortex length calculated with the low-Re model also closely matches the vortex length predicted by laminar flow modeling. In conclusion, the study strongly suggests that the proposed model is suitable for blood flow studies in certain areas of the arterial tree where both laminar and transitional/turbulent flows coexist.

Totally laparoscopic aortobifemoral bypass: a review of 10 patients

The main purpose of this study is to evaluate the feasibility of totally laparoscopic aortobifemoral bypass for occlusive aortoiliac disease. Ten patients who had incapacitating claudication have been included to date in this investigation. We have designed a transabdominal retroperitoneal technique that allows performance of the procedure without the problems associated with retraction of intraperitoneal organs. During the study, surgery time decreased from 510 to 245 min. Mean total aortic clamping time was 121 min, and the mean time required to perform the aortic anastomosis was 66 min. Mean blood loss was 820 ml. Three patients needed conversion. Postoperative complications developed in three patients. One had an aortoureteral fistula, which needed reoperation; one experienced complications related to a retroaortic left renal vein; and the third had a mild compartment syndrome of the right leg. Totally laparoscopic aortobifemoral bypass is feasible. Laparoscopic aortobifemoral bypass appears to ease the patient's postoperative course and could become in the not so distant future part of the repertoire of the surgeon performing vascular surgery.

Comparison of the in vivo behavior of polyvinylidene fluoride and polypropylene sutures used in vascular surgery

To find a nonabsorbable suture material that is equivalent to polypropylene in ease of handling and tensile properties, and that has low thrombogenicity and tissue reactivity but improved biostability, some researchers and clinicians see merit in considering the suitability of monofilaments made from polyvinylidene fluoride. The current animal study investigated the relative biocompatibility and biostability of these two suture materials by using them to anastomose a polyester arterial prosthesis in a canine thoracoabdominal bypass model for 10 periods of implantation ranging from 4 hr to 2 years. Biocompatibility was assessed with light and scanning electron microscope examinations of the explanted sutures, and biostability of the cleaned sutures was determined by Fourier transform infrared spectroscopy and scanning electron microscope analysis. The polyvinylidene fluoride and polypropylene sutures were found to have similar handling and healing characteristics. During the first months in vivo, both types of suture experienced a temporary increase in carbonyl group absorption that coincided with the duration of the inflammatory response. After 1 and 2 years in vivo, the explanted polypropylene sutures showed visual evidence of surface stress cracking. This was not found with the explanted polyvinylidene fluoride sutures. These results suggest that polyvinylidene fluoride may be more biostable than polypropylene in the long term.

In vitro and in vivo studies of a polyester arterial prosthesis with a warp-knitted sharkskin structure

The present study was undertaken to assess the performance of a new knitted and gelatin-sealed polyester vascular graft that is believed to have greater dimensional stability than current commercial devices. Samples of the uncrimped, crimped, and sealed prosthesis were submitted to a series of in vitro and in vivo trials. Four commercial polyester knitted devices were included as controls for the in vitro tests, which included measurements of the textile and yarn structure and physical, chemical, and thermal properties of the graft, such as water permeability, dilatation, suture retention strength, melting point, and crystallinity index. The in vivo evaluation involved implanting the prototype device as a canine thoraco-abdominal bypass for periods ranging from 4 h to 1 year and assessing the biocompatibility, biofunctionality, and biostability of the explanted specimens. The warp-knitted structure of the prototype device has a unique sharkskin stitch that confers a superior dilatation resistance and suture retention strength to the prosthesis. The animal trial demonstrated that the gelatin ensures initial hemostasis without preclotting. The gelatin is bioresorbed during the first 2 weeks of implantation, which generates a temporary, moderate, acute inflammatory response. An external capsule of granulomatous tissue and an internal collagen capsule are formed between the first and third month. Analysis of the textile and physical properties of the explanted prostheses confirmed there was neither dilatation nor significant changes in structure or mechanical performance during implantation, thus confirming the biostability of this new prototype device and opening the way for clinical trials.

Can the infusion of elastase in the abdominal aorta of the Yucatán miniature swine consistently produce experimental aneurysms?

The intraluminal elastase perfusion model has been proved to be potentially effective in producing abdominal aortic aneurysm in rodents, but it produced unpredictable results in larger animals. The purpose of this study was to explore the potential ability of such a model to produce experimental aneurysm consistently in the Yucatán miniature swine. Six Yucatán miniature swine received infusion with porcine elastase into an isolated segment of the infrarenal aorta. The excised arterial segments were examined macroscopically to assess the luminal surface characteristics and histologically to describe the different pathologic injuries induced by the elastase treatment on the intima, media, and adventitia of the arterial wall. Histologic examination revealed that the elastic network of the media was destroyed. In the first week after perfusion, altered smooth muscle cells were located in the intima and innermost layer of the media in juxtaposition with the occlusive thrombus. Infiltration of inflammatory cells was observed in these regions of elastic network and smooth muscle cell alterations. In the arterial segments of swine sustained for 3 weeks, a reduction of smooth muscle cells was noted in some areas. An important number of necrotic lesions was observed, and they were associated with the development of calcium deposits. Significant intimal hyperplasic reaction was identified at day 19 and again at day 21. However, no aneurysmal development was observed. This study constituted the first experiment with infusion of porcine elastase in the Yucatán miniature swine infrarenal aorta. The present experimental protocol induced important elastic network and smooth muscle cell alterations leading to severe necrotic lesions associated with calcium deposition, but it produced no aneurysmal dilatation. This model requires further testing to obtain a more complete degradation of the elastic network in both the media and adventitia and more significant collagenolysis without early thrombotic events.

Efficiency of an external support to reduce lipid infiltration into venous grafts: in vitro evaluation

Excessive distension of venous grafts due to arterial pressure enhances the convective water transport (filtration flow) through the vessel wall, and thus might affect the infiltration of macromolecules such as lipoproteins. In this paired experimental study, filtration velocities were measured at 100 mm Hg for canine jugular veins with or without external supports of expanded polytetrafluoroethylene (ePTFE) arterial prostheses. In addition, to assess the effect of filtration velocity on lipid infiltration or uptake, canine jugular veins were wrapped over half of their lengths with ePTFE arterial prostheses and perfused with dog serum containing 3H-cholesterol at a pressure of 100 mm Hg. At 100 mm Hg, the average filtration velocity of the wrapped jugular veins was 7.9 +/- 1.3 x 10(-6) cm/s whereas the average filtration velocity of the unwrapped veins was 27.3 +/- 2.7 x 10(-6) cm/s (p < 0.005). Moreover, the unwrapped veins had a significantly higher uptake rate of labeled cholesterol than the wrapped veins (10.9 +/- 7.3 x 10(-4) cm/h and 5.0 +/- 1.6 x 10(-4) cm/h, respectively, p < 0.005). In conclusion, under arterial pressure, veins experience excessive distention, which leads to significant increases in both filtration flow and cholesterol uptake. An external wrap or support of ePTFE material protects veins from excessive distension and thus may prevent atherosclerosis in venous grafts by reducing cholesterol uptake.

Can collagen impregnated polyester arterial prostheses be recommended as small diameter blood conduits?

A collagen impregnated graft and its parent preclotted prosthesis were implanted as thoraco-abdominal bypasses in dogs for periods ranging from 4 hr to 6 months and evaluated for their ease of handling, imperviousness, and healing behavior in terms of luminal surface thrombogenicity using labeled platelets and fibrinogen, prostacyclin (PGI2) secretion, histomorphometric determination of internal capsule thickness, and histopathologic and scanning electron microscopic studies. The collagen impregnated graft was impervious to blood and both grafts showed excellent handling characteristics. Fibrin uptake was negligible on both grafts; however, platelet uptake was higher on the collagen impregnated graft than on the control graft at 4 and 24 hr. The healing behavior of the collagen impregnated graft was also found to be different than that of the control graft between 1 and 6 months post implantation. The development of a host collagenous internal capsule at the anastomoses, and a confluent endothelial lining, was observed in both grafts at 1 month; in later implantation periods, the healing of the medial region was found to be more irregular in the collagen impregnated grafts, showing a lower mean PGI2 secretion than the preclotted control grafts. Histomorphometric analysis showed the internal capsule on the collagen impregnated grafts to be thicker than on the control grafts for most periods of implantation. The current study illustrates that the healing process of collagen impregnated grafts is delayed and that bovine collagen has a stimulating effect on tissue encapsulation. Current impregnated polyester arterial prostheses therefore cannot be recommended as small diameter blood conduits.

An albumin-coated polyester arterial graft: in vivo assessment of biocompatibility and healing characteristics

The albumin-coated vascular graft (ACG) and its uncoated polyester substrate, the Vascular II (V-II), were evaluated in terms of biocompatibility and biofunctionality using two in vivo animal studies. Biocompatibility and immunoreactivity were assessed by implanting intraperitoneally in the rat small segments of the ACG and the V-II graft and harvesting them with their surrounding tissue 3d, 1, 2 and 4 weeks later. Cytofluorometric determination of total T cells (CD3), the ratio of CD4/CD8 subsets and the percentage of IL-2 receptor-positive T cells in the peripheral blood has revealed that no significant difference in any of the T cell populations was found between the ACG and the V-II graft. The cellular reactivity of the ACG in terms of acid phosphatase activity at the implant side was significantly greater at 3 d but not at longer periods. Biofunctionality was evaluated by implanting both grafts as a thoracoabdominal vascular bypass in dogs for 11 different periods ranging from 4 h to 6 months. The rate of albumin resorption was such that traces were still present at 1 month, but no longer observable at 2 months. Tissue incorporation into the graft wall was earlier for the V-II (2 weeks) than for the ACG (4 weeks), which showed complete encapsulation, tissue incorporation and endothelialization after 2 months in vivo. Only small differences were observed between both grafts in terms of platelet and fibrin uptake on the luminal surface. The prostacyclin/thromboxane A2 ratio increased to a level higher that 1.0 aorta within 1 month for the V-II and 4 months for the ACG. In conclusion, the Bard ACG has demonstrated excellent biocompatibility in terms of blood T cell behaviour and acid phosphatase activity at the implant site. Finally, its healing response is equivalent to that of the uncoated Dacron prosthesis once the albumin coating has been resorbed.

Fresh venous allografts in peripheral arterial reconstruction in dogs. Effects of histocompatibility and of short-term immunosuppression with cyclosporine A and mycophenolate mofetil

To date, no arterial substitute has been shown to be as effective as the autologous saphenous vein in peripheral revascularization procedures. In the present study, the venous allograft was evaluated as a vascular substitute in terms of patency and induction of host immune reactivity, whether used in major histocompatibility complex-incompatible, major histocompatibility complex-incompatible dogs. The immunosuppressive drug therapies were given for a period of 31 days, beginning 1 day before transplantation, and consisted of the use of cyclosporine A, mycophenolate mofetil, or a combination of both. All histoincompatible allografts were thrombosed at 4 or 8 weeks after transplantation with antibody development and cell-mediated cytotoxicity in the graft, whereas histocompatible allografts showed late stenosis without immunologic reactions directed toward donor cells. Given alone, neither cyclosporine A nor mycophenolate mofetil improved the overall patency of venous allografts; thrombosis occurred shortly after cessation of immunosuppression. Still, the cyclosporine A-mycophenolate mofetil combination therapy led to a 100% patency rate at 20 weeks after implantation and immune reactions were markedly reduced. This study shows that the fresh vein allograft is still an attractive and functional alternative to the autologous saphenous vein if the host immunologic reactions are controlled by cyclosporine A-mycophenolate mofetil immunosuppression.

Polyvinylidene fluoride (PVDF) as a biomaterial: from polymeric raw material to monofilament vascular suture

This study identified the effects of various manufacturing processes on the crystalline microstructure, mechanical properties, and biocompatibility of a polyvinylidene fluoride (PVDF) suture. To achieve this, changes in the crystalline microstructure and the tensile behavior of PVDF monofilaments were monitored in vitro after different thermal processing, coloration, and sterilization treatments. In addition, the in vivo biocompatibility of the manufactured and sterilized PVDF suture was assessed by using it to anastomose a preclotted polyester vascular prosthesis as a thoracoabdominal bypass in a series of dogs. The tissue response was followed by histologic and scanning electron microscopy over implantation periods ranging from 4 h to 6 months. Differential scanning calorimetry and infrared spectroscopy (FTIR-ATR) showed that thermal processing and the addition of a coloring agent had a direct effect on modifying the crystalline microstructure and hence changing the mechanical properties. For example, thermal processing converted some of the alpha phase into the beta and gamma polymorphs, whereas coloration led only to a major increase in the beta-to-alpha ratio. The tensile properties were found to be optimized when the relative proportion of the beta and gamma phases combined compared to the alpha form gave rise to an FTIR A509/A532 absorption ratio between 4.0 and 4.5. Sterilization was found to cause some modifications to the crystalline microstructure near the surface of the monofilaments, but it did not change their mechanical properties. Pathologic examination of the anastomotic regions after different periods of implantation revealed a minimal cellular response, with no mineralization, intimal hyperplasia, or excessive fibrous tissue reaction. This good biocompatibility, together with other desirable characteristics such as ease of manipulation and satisfactory mechanical strength, makes PVDF an attractive alternative monofilament suture material for cardiovascular surgery.

Polyvinylidene fluoride monofilament sutures: can they be used safely for long-term anastomoses in the thoracic aorta?

Polyvinylidene fluoride (PVDF) represents an attractive alternative to polypropylene as a monofilament vascular suture because of its satisfactory physicochemical properties, it ease of handling, and its good biocompatibility. However, the polymer's ability to remain mechanically and chemically stable when exposed to a mild hydrolytic environment over the long term has yet to be demonstrated. One in vitro study involved the comparison of the long-term relative resistance of PVDF and polypropylene sutures to hydrolysis for a period of 9 years. The PVDF suture showed major molecular rearrangements from the original ratio of three crystalline structures to the single beta crystalline phase. The observation of some surface oxidation and water inhibition did not significantly modify the tensile strength of the PVDF suture, which retained 92.5% of its original value. In contrast, the polypropylene sample did not undergo any recrystallization but was associated with more oxidation byproducts and more water molecules near the surface, which contributed to a 46.6% loss in initial tensile strength. An in vivo study confirmed that PVDF sutures are biocompatible and are able to maintain satisfactory biostability when used to anastomose thoracic aortic allografts for a period of 6 months in the dog. The cellular reaction of fresh allografts as well as the control autografts to PVDF sutures was minimal. In other allografts that had been preserved in a supplemented medium for 1 week prior to implantation, the PVDF sutures healed satisfactorily with the formation of neocollagen and few macrophages surrounding the monofilament. No evidence of instability at the allograft-host artery junction was observed, confirming that the PVDF sutures were able to ensure a secure anastomosis in the thoracic aorta. PVDF sutures have demonstrated superior long-term biostability in vitro and minimal tissue response in vivo. These are two essential requirements when evaluating the use of a suture for vascular surgery in general and thoracic aortic surgery in particular.

The gelweave polyester arterial prosthesis

OBJECTIVE

To determine the effect of the gelatin coating on the efficacy of Gelweave, a new gelatin-sealed woven polyester graft material, as an arterial prosthesis.

DESIGN

In-vitro and in-vivo studies of the prosthesis.

SETTING

A laboratory of experimental surgery in a university teaching institution.

SUBJECTS

After in-vitro testing of the material, eight dogs were subjected to a series of in-vivo tests to evaluate the properties of Gelweave in comparison with its unsealed precursor and a commerically available collagen-coated woven polyester prosthesis.

INTERVENTION

Implantation of the prosthesis as a thoracoabdominal bypass for prescheduled periods ranging from 4 hours to 6 months.

MAIN OUTCOME MEASURES

Physical and chemical properties of the virgin prosthesis compared with the other two prostheses, effects of the gelatin-sealed prosthesis on healing, the hematologic characteristics of the dogs before operation and at sacrifice, microscopic studies, fibrin and platelet uptakes, prostaglandin secretion, and properties of the Gelweave grafts removed at varying periods after implantation.

RESULTS

The gelatin sealant in the Gelweave prosthesis effectively reduced the water permeability of the new prototype to zero. Neither blood loss at implantation nor infection during the postimplantation period was observed. The gelatin impregnation did not cause any adverse response in the dogs and was completely lysed within 2 weeks, thus allowing encapsulation and graft healing to progress satisfactorily. After 2 weeks, the prostacyclin:thromboxane ratio was greater than 1.0, whereas the fibrin and platelet uptakes on the luminal surface of the Gelweave grafts remained low, regardless of the period of implantation. Analysis of the explanted grafts confirmed that this gelatin-sealed prototype prosthesis healed satisfactorily and no adverse biologic response occurred as a result of the gelatin coating. It maintained its biostability during 6 months in situ.

CONCLUSION

The new Gelweave arterial prosthesis is ready for clinical use as a thoracic and abdominal vascular substitute.

Evaluating the Dialine vascular prosthesis knitted from an alternative source of polyester yarns

The sudden and unilateral decision by E. I. DuPont de Nemours & Co., Inc., to withdraw its polymers for use in implantable devices has presented the medical device industry with an immediate and serious challenge to find alternative sources of biomaterials. In France, the company Cardial S.A. has already taken steps to find an alternative polyester yarn to replace Dacron by developing a new arterial prosthesis knitted from polyester yarns supplied by Rhône-Poulenc Fibres. This article describes an in vitro and in vivo study of this French device, called the Dialine prosthesis, with a view to determining its relative performance compared to current American and British prostheses, which rely on DuPont's Dacron yarn. In addition to analyses of the morphology and textile structure, and measurements of its physical and chemical properties, the Dialine graft was implanted as a thoracoabdominal bypass in dogs for periods ranging from 4 h to 6 months. In addition to our pathologic and histologic observations, we cleaned and evaluated the explanted prostheses for in vivo changes in dimensions, strength, and crystalline microstructure. The Dialine graft was found to differ structurally from other polyester prostheses because it is warp-knitted from a mixture of flat and texturized yarns with finer filaments. Its denser structure has a lower water permeability, greater flexibility and ease of handling, satisfactory strength, and dimensional stability, and it presents different textures on its luminal and external surfaces. The in vivo trial demonstrated that it has excellent biocompatibility and biostability over 6 months. With no thrombi observed on the luminal surface after 3 months, it has a faster rate of healing, generates compact external and internal capsules with a thinner neointima, and has an overall milder inflammatory response than is normally observed with Dacron-based prostheses.

Posttransplant antibodies and fresh venous allograft failure in dogs

Small diameter arterial reconstruction is usually achieved by use of the autologous long saphenous vein. As an alternative to this blood conduit, the venous allograft has been used with some success in the past, but is likely to be the target of an immune rejection reaction from the host. This study was designed to characterize humoral immune reactions possibly involved in the outcome of venous allografts. Ten mongrel dogs received a histoincompatible femoral vein allograft and an autograft as interposition grafts to both femoral arteries. They were investigated for donor-specific antibody development using donor splenocytes and cultured vascular endothelial cells (EC). Serum samples were collected at surgery, at 2 weeks, and every month until graft occlusion occurred. All autografts were patent at retrieval except one, and all allografts underwent thrombosis. In all dogs, donor-specific IgG development was observed that appeared specifically at 4 weeks and lasted until graft occlusion was detected. All reactive sera were cytotoxic to donor EC except one, and this reactivity was completely lost after serum absorption on donor splenocytes. This latter absorption resulted in the total loss of flow cytometric reactivity against donor EC in 3 dogs, whereas a low reactivity was still present in 4 dogs. Immunoblotting analysis showed a posttransplant reactivity against various protein bands on donor EC. Absorption of the reactive serum on donor splenocytes resulted in the loss of reactivity to proteins of approximately 40, 30, and 22 kDa in most experiments. Moreover, as demonstrated by immunofluorescence on cryostat sections of explanted grafts, IgG deposition was seen mainly in the media and the adventitia of the allografts but not in autografts. These results suggest that a donor-specific antibody response directed mainly against MHC antigens might play a role in the thrombosis of histoincompatible venous allografts, thus decreasing the patency rate.

Fresh venous allografts as arterial substitutes in dogs: the importance of donor-recipient tissue compatibility

Fresh venous allografts were investigated in dogs matched according to donor-recipient tissue compatibility, either originating from the same litter or chosen at random (pound dogs). Five centimeter long segments of femoral vein were interposed as carotid substitutes in an autografting and allografting manner between paired dogs. During a 5-month implantation period, donor-specific antibody development was measured in the recipient serum by a flow-cytometric assay using cultured donor vascular endothelial cells. Autografts and allografts were investigated in terms of patency, histopathology, and endothelial cell function. Fifteen of 16 autografts remained patent. Allografts between littermate dogs, whether compatible or incompatible, showed no donor-specific antibody development and were all patent at retrieval. Compatible and incompatible allografts in littermates did not show any difference in prostacyclin (PGI2)/thromboxane A2 (TXA2) ratios. In pound dogs, both compatible allografts were patent and one dog developed donor-specific antivascular endothelial cell antibodies. Among incompatible dogs, antibody formation was detected at 1 month in five of six recipients and graft patency was as follows: two partial thromboses, two stenoses, and two patent grafts. The PGI2/TXA2 ratio was significantly lower in incompatible allografts than in compatible ones (p = .028). These results show the importance of donor-recipient histocompatibility matching in improving the outcome of vein allografts.

Why make monofilament sutures out of polyvinylidene fluoride?

In recent years some clinical reports have associated suture failures with polypropylene monofilaments. Therefore there is interest in developing an alternative suture material that is less thrombogenic than polyester and similar in handling characteristics but less prone to mechanical failure than polypropylene. To this end, Peters Laboratoire Pharmaceutique has developed a new monofilament suture material from polyvinylidene fluoride (PVDF), which has been subjected to a special treatment to modify its crystalline form and level of crystallinity. The purpose of this study was to evaluate its mechanical, chemical, and biologic properties and to compare its performance, in a peripheral vascular application, to that of a polypropylene control. A series of in vitro tests were performed to study the morphology, tensile properties, creep, surface chemistry, thermal characteristics, and resistance to iatrogenic trauma. In addition, an in vivo trial was undertaken in which vascular prostheses anastomosed with either PVDF or polypropylene sutures were implanted as a thoracoabdominal bypass for 6 months in the dog. Histologic and degradation analyses were performed on the explants. The results from the mechanical tests on 4-0, 5-0, and 6-0 PVDF and polypropylene sutures demonstrated that although both materials have similar breaking strengths, the PVDF has a higher extension at break, has less delayed extension when under tensile creep testing, and suffers less trauma than the polypropylene when compressed by a standard needle holder. While chemical analyses found evidence of surface oxidation on both types of sutures, thermal analysis confirmed that the level of crystallinity of the PVDF polymer is higher than that of the polypropylene control. During the pilot study in animals, PVDF sutures were found to have good handling and frictional characteristics that facilitated the tying of knots. Histologic analysis of the explants found no inflammatory cells in the tissue surrounding either the PVDF or polypropylene sutures, and scanning electron microscopic examination of the cleaned suture surfaces found no evidence of degradation during 6 months in vivo. Though preliminary in nature, these findings indicate that monofilament sutures made from PVDF provide an attractive alternative to those made from polypropylene for use in cardiovascular surgery. In addition to providing acceptable in vivo behavior and being easy to manipulate and more resistant to iatrogenic injury, PVDF materials can be sterilized by beta or gamma radiation and so can reduce dependence upon ethylene oxide and chlorofluorohydrocarbons.

Erythromycin-associated ergotamine intoxication: arteriographic and electrophysiologic analysis of a rare cause of severe ischemia of the lower extremities and associated ischemic neuropathy

Vasospasm associated with ergotamine is a well-known phenomenon. In this case report we present a rare drug interaction between erythromycin and ergotamine at normal doses causing lower extremity ischemia in a 36-year-old woman. Nitroprusside proved to be the treatment of choice. The response was dramatic and took place in a matter of hours in this patient. Ischemic monomelic neuropathy is a recently described entity in which axonal necrosis is caused by a loss of distal extremity blood flow. The association between erythromycin and ergotamine may be a dangerous pharmacologic combination; drugs that have a hepatic cycle with ergotamine derivatives must be used with caution.

A pathological study of arterial prostheses surgically excised after overt clinical infection

Infection caused regarding vascular grafts in vascular surgery still remains a major problem. To reduce this problem and the complications which follow, the surgeon must be able to apply the best surgical management and also be confident with the vascular substitute used. There are two important factors to be considered: the biomaterial must have low propensity to infection and good stability if and when infected. In an attempt to verify this problem, 93 vascular grafts surgically excised for overt infection were examined. Techniques used for examinations were gross morphology, histopathology and scanning electron microscopy (SEM) evaluation. There were 23 human umbilical vein (HUV) grafts, 51 Dacron grafts and 19 expanded polytetrafluoroethylene (ePTFE) grafts. Histopathological signs of infection were absent in 57% of the ePTFE and Dacron grafts and in 17.4% of the HUV grafts. The latter were more heavily histologically infected and in some cases the walls were destructed. Histopathological signs of infection were seen on all the prosthetic walls in 36% of all the specimens and were mainly on the external portion of the grafts for the remaining prostheses. Bacteria were seen in respectively 21.7, 15.7 and 20% of the HUV, Dacron and ePTFE grafts with the Gram stain and in 86.9, 84.3 and 94.7% with SEM. The implantation period was shorter for the bioprostheses compared to that of the synthetic grafts because of the site and the indication of implantation. The stability of the bioprostheses was lower compared to that of the synthetic grafts when infected, leading to a breakdown of the wall along the length of the graft. The infection was found on the external capsule of the grafts rather than on the luminal surface.

Fresh venous femoral autografts and allografts as carotid substitutes in matched dogs from the same litter

Pathological and biochemical investigations were performed on femoral veins used as fresh autografts and allograft substitutes in the carotid arteries of dogs from the same litter and separated in two groups described as compatible and incompatible after a mixed leukocyte culture (MLC). After 5 months of implantation, all the arterial substitutes were patent. The analyses gave the following results: 1. The haematologic parameters were not affected by the grafts. 2. Light microscopy (LM) and scanning electron microscopy (SEM) indicate a thrombotic accumulation and the presence of inflammatory cells in the compatible and the incompatible allografts. The occurrence of pathological changes was more pronounced in the incompatible group. 3. Immunostaining of anti-factor VIII-related antigen reveals a better preservation of the endothelial cell wall on the autograft. 4. The 6-keto-PGF1 alpha/TXB2 ratios of the autografts and the compatible allografts were similar.

Arterial reconstruction to the foot vessels: is it worth the trouble?

Between 1974 and 1988, 32 patients with severe rest pain and ischemic ulcerations of lower extremities underwent 34 arterial reconstructions to the foot vessels, using, in the majority of cases, the in-situ vein technique. There were 10 early graft failures, resulting in the loss of five extremities. The indication for surgery, presence of diabetes and type of bypass were not predictive of early thrombosis. Five-year primary and secondary patency rates were 52.8% and 62.6%, with a foot salvage rate of 74%. Although a number of these long bypass grafts fail early because of the severity of tibial disease, this procedure appears to be durable in a selected group of patients.

In vivo evaluation of four chemically processed biological grafts implanted as infrarenal arterial substitutes in dogs

Four chemically processed grafts implanted as arterial substitutes in dogs were evaluated as blood conduits in terms of patency rates, healing characteristics and biostability. Omniflow, Biopolymeric, Dardik-Biograft of the second generation and BIMA grafts were implanted as infrarenal aortic substitutes for 4, 24 and 48 h (short-term), 1, 2 and 4 wk (medium-term), 3 and 6 month (long-term), each type of graft being implanted for a complete series. The explanted grafts were evaluated macroscopically and processed for light and scanning electron microscopy. One Omniflow graft was occluded at 6 months and two Dardik-Biografts were thrombosed, one at 24 h and one at 1 month. All explanted grafts in the Biopolymeric series and in the BIMA were patent at the animals' death. Histological studies revealed frequent subintimal fibrosis in the Biopolymeric and Omniflow grafts. A peripheral inflammatory reaction was present in most grafts explanted. Scanning electron microscopy showed an aggravation of flow surface irregularities, after implantation of Omniflow, Biopolymeric and Dardik-Biograft, compared to virgin prostheses. These grafts presented an internal capsulae on the graft flow surface along the anastomoses. The capsulae slightly extended towards the centre of the graft after long-term implantation, (6 month) and corresponded to the pannus. Endothelial-like cells covered this pannus. The BIMA graft performed the most satisfactorily and retained its blood compatibility best, that is, the luminal surface was smooth with only minor thrombotic deposits and a thin pannus along the anastomotic lines.

Accuracy of carotid Doppler peak frequency analysis: results determined by receiver operating characteristic curves and likelihood ratios

To determine the accuracy of peak Doppler frequency measurements in the diagnosis of extracranial carotid arterial disease and to apply the results to the assessment of the individual patient, 397 studies of continuous-wave Doppler frequency analysis were compared with the results of arteriography. The diagnostic accuracy of the measurement of peak frequency was determined in two ways. First, receiver operator characteristic curves were calculated for different threshold levels of peak frequency. The results showed that the measurement of the peak frequency can be related in only an approximate way to the severity of the arteriographic stenosis because of the uncertainty of the probe-to-vessel angle. Nevertheless, the overall maximum diagnostic accuracy was approximately 91% (Kappa = 0.82 +/- 0.05). Second, likelihood ratios were calculated to take into account the degree of abnormality of the peak frequency measurements. The probability of carotid disease increased as the peak frequency increased, giving likelihood ratio ranges from 0.03 to 25.71. To be able to apply the peak frequency measurements to the assessment of the individual patient in the vascular laboratory, our results were used to construct tables that show the posttest probability of carotid disease based on the clinically estimated pretest probability of disease and the measured peak frequency. It is concluded that the measurement of peak Doppler frequency is an adjunct in the diagnosis of carotid artery disease.

An in vitro model and its application for the study of carotid Doppler spectral broadening

A new in vitro model has been developed for studying the changes in the ultrasound Doppler spectrum that occur in the region of a stenosis. Pulsatile flow in rigid acrylic tubes was produced by means of a modified hemodialysis pump. The Doppler spectral waveforms were measured using a continuous wave Doppler system, a probe of a known field pattern, a real-time high resolution frequency analyzer, and a video display and recording system. The flow velocity waveforms were found to be nearly identical to those seen in the human carotid. Measurements were made to determine the critical stenosis and the results are similar to those reported from in vivo studies. In a preliminary study, the extent of spectral broadening was found to be dependent on the recording site in relation to the stenosis, the severity of the stenosis, and the flow rate. Using qualitative methods it was not possible to determine either the influence of the shape of the stenosis or the phase of the cardiac cycle on spectral broadening.

Detection of occlusive disease of the carotid artery with continuous wave Doppler spectral analysis

The accuracy of Doppler spectral analysis for diagnosing stenoses of the carotid artery was evaluated in this study. Two hundred and five patients were examined, using a 4 megahertz continuous wave Doppler velocity meter and a real-time frequency analyzer. By a comparison of the results of arteriography, peak Doppler frequencies of greater than 3.8 kilohertz were diagnostic of internal carotid artery stenoses having residual lumen diameters of 3.2 millimeters, or less, with a sensitivity rate of, 91 per cent and a specificity rate of 92 per cent. Spectral broadening, evaluated by subjective grading, yielded similar results.

Survival in colorectal cancer

The authors studied the hospital records of 258 patients with colorectal cancer diagnosed between 1965 and 1974 at l'Hôpital du Saint-Sacrement, Quebec and computed expected and relative survival rates to adjust for deaths from other causes. Life-table methods based on full intervals only were used to analyse survival. The patients (138 men and 120 women) ranged in age from 24 to 96 years (mean, 64.9 +/- 12.7 years). The 5-year relative survival rate expressed as a percentage was 48.6 +/- 4.7 for the total group and 64.1 +/- 5.6 for patients in stages A, B and C of Dukes' classification. The overall operative mortality rate of 2.7% compared favourably with that reported elsewhere. Among patients with cancer of the sigmoid colon or rectum (the most readily detectable), the diagnosis was made at stage A in one out of nine and at stage D in approximately one out of four. Despite the introduction of universal medical insurance in Quebec in 1970, there has been no significant trend towards earlier diagnosis.

Recurrent bladder papillomata treated with bacillus Calmette-Guérin: a preliminary report (phase I trial)

Six patients with recurrent bladder papillomata were treated both by abdominal scarification with bacillus Calmette-Guérin (BCG) and by intravesical BCG, without resection. Four patients responded favorably (two showing major systemic complications), and the other two, who failed to respond, underwent further surgery.