An Experimental Study of Laser in situ Fenestration of Current Aortic Endografts.

OBJECTIVE/BACKGROUND: Laser in situ fenestration (LISF) is emerging as an immediately available alternative in the endovascular treatment of complex aortic aneurysm. However, its biomechanical features remain poorly understood. The aim of this study was to experimentally evaluate textile damage secondary to LISF and to compare LISF with mechanical in situ fenestration (MISF). METHODS: An in vitro study evaluated the damage created by LISF on endograft fabrics versus MISF using a needle. Five different models of commercially available aortic endografts were used (32 samples of polyethylene terephthalate and expanded polytetrafluoroethylene fabrics). Tensile strength tests were performed on the fabrics before and after in situ fenestration, to determine the loss of mechanical strength. Integral water permeability tests at the stent-fenestration interface evaluated the watertightness of junctions. Stability of the connection was assessed with a fatigue bench test flexing the branch on the fenestration. In a second step, an in vivo study evaluating LISF in sheep was conducted. RESULTS: Resulting holes had circular and cauterised edges following LISF, whereas fabric filaments were pushed aside after MISF. Tensile tests demonstrated a 34% and a 27% mechanical resistance loss after LISF (p = .004) and MISF (p = .001) compared with non-fenestrated samples. A non-significant global decrease of 7% in mechanical resistance was found following LISF compared with MISF (p = .520). Water permeability tests highlighted that leak rates were higher following LISF than with MISF with regard to multifilament specimens (p < .05). Fatigue tests induced modification of the morphology of fenestrations. The surface area of the fenestration was increased for all samples after 170,000 cycles. Regarding the in vivo study, 14 LISF were performed in 12 sheep with a technical success rate of 88%. CONCLUSION: This study demonstrates that both LISF and MISF create substantial damage to all available endograft fabrics. Until comparisons with reinforced fenestrations are performed, LISF and MISF should not be used outside investigational studies.

Poor long-term stability of the Corvita abdominal stentgraft.

INTRODUCTION: The endovascular treatment of abdominal aortic aneurysms was introduced in the early 1990s, with different generations of devices using various options for either the stent skeleton or the membrane. REPORT: Corvita generated one of these devices using braided stainless steel and a porous spun polycarbonate urethane membrane. DISCUSSION: In this report, we describe a case involving Corvita stentgraft explantation for complete aneurysm reperfusion after 13 years, demonstrating major degradation of the polyurethane membrane.

Expanded polytetrafluoroethylene arterial prostheses in humans: histopathological study of 298 surgically excised grafts.

The expanded polytetrafluoroethylene vascular prosthesis is considered to be the best synthetic alternative for peripheral arterial reconstruction. Most studies on the healing characteristics of expanded polytetrafluoroethylene prostheses have been carried out on animals, and very few data are available on prosthesis implanted in humans long term. We implanted 298 expanded polytetrafluoroethylene grafts as arterial substitutes in humans. The mean duration of implantation was 523 d and the grafts were implanted mainly for infrainguinal or axillofemoral bypass. The cellular and collagen infiltration of the microporous expanded polytetrafluoroethylene structure was generally poor. Infiltration occurred mainly in the external region of the prosthetic wall and increased with the duration of implantation. The external reinforcement was not a major factor in limiting tissue infiltration. The luminal surfaces were covered with a thin, irregular layer of organized fibrin, interspersed with exposed expanded polytetrafluoroethylene areas. Mineral deposits were observed in five cases. Despite poor healing, the clinical performance of expanded polytetrafluoroethylene vascular prostheses is relatively good. Since the chief advantage of this material is good mechanical stability in vivo, any modifications of the graft to improve healing characteristics or thrombogenic properties should not be made at the expense of stability in vivo.

Expanded polytetrafluoroethylene arterial prostheses in humans: chemical analysis of 79 explanted specimens.

The expanded polytetrafluoroethylene (ePTFE) vascular prostheses are widely used as small and medium diameter blood conduits when an autologous venous material is not available or is not suitable. The long-term performance of a prosthesis is dependent on several factors, including its healing characteristics and its stability in vivo. This study was undertaken to assess whether chemical degradation of ePTFE occurs when such arterial substitutes are implanted in humans. Seventy-nine ePTFE grafts excised for complications were analysed using the following techniques: measurement of the contact angle (theta), electron spectroscopy for chemical analysis (ESCA or XPS), Fourier transform infra-red spectroscopy (FTIR) and differential scanning calorimetry (DSC). The results were compared with those obtained from virgin ePTFE and virgin ePTFE washed prostheses. The measurement of the contact angle (theta) permits the comparison of the level of hydrophobicity of material after in vivo residency. The contact angles of explanted ePTFE grafts are greater than those of virgin ones but remain close to those of washed virgin prostheses. The ESCA method allowed investigation of the chemical changes which occur on the surface of ePTFE prostheses after implantation because of the low penetration of the X-ray (about 50 A). This study did not reveal any chemical degradation of the ePTFE with time of implantation for periods up to 6.5 yr. Changes in the surface composition were probably related to lipid and/or protein uptake. The FTIR spectroscopy provides information about the chemical composition of material. Compared with the virgin ePTFE prostheses, the FTIR spectra of explanted prostheses showed specific bands which are characteristic of lipid and/or protein absorptions. The bulk properties of ePTFE studied by DSC did not show any significant changes with time of implantation. It is concluded that ePTFE grafts remain stable in vivo for periods up to 6.5 yr.

Carbodiimide cross-linked gelatin: a new coating for porous polyester arterial prostheses.

The performance of a polyester arterial prosthesis impregnated with gelatin and cross-linked with carbodiimide (Uni-graft) was compared with its porous parent graft (Protegraft) using a canine thoraco-abdominal bypass model. The grafts were investigated in terms of their handling characteristics, imperviousness at implantation, surface thrombogenicity and healing behaviour. Prostheses 30 cm in length were implanted for the following periods: 4, 24 and 48 h, 1, 2 and 4 weeks, 2, 3, 4, 5 and 6 months. Both types of graft had good handling characteristics. The ready-to-use impregnated graft provided satisfactory haemostasis at implantation with no blood permeating through the wall after flow was restored. Both grafts exhibited low surface thrombogenicity, as determined by the uptake of labelled fibrin and platelets, and the healing sequence of the impregnated graft after resorption of the gelatin was equivalent to that of the preclotted control. Biodegradation of the gelatin was complete within 1 month of implantation with the subsequent development of a collagenous internal capsule at both anastomoses. Endothelial cells were observed between 4 and 6 months, but were confined to small islets distributed along the luminal surface. The prostacyclin/thromboxane A2 (PGI2/TXA2) ratio, which gives an indication of the level of endothelial cell activity, was greater than 1.0 after 1 week of implantation for the control graft. For the impregnated graft it reached 1.0 only after 3 months of implantation, but remained above 1.0 for periods of up to 6 months.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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.

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.

[Vascular graft prosthesis]. / Substituts vasculaires.

Performed since the 1950s, vascular grafting has opened modern era of vascular surgery. Autologous venous grafts are of first choice for revascularisation of small arteries. Synthetic grafts are mainly modelled using microporous polytetrafluoroethylene or terephtalate polyethylene. These prosthesis are mainly used for revascularization of medium and large size arteries.

Properties and challenges in materials used as vascular and endovascular devices.

In the last decades, main evolutions in the field of vascular surgery have been correlated to the development of devices allowing more reliable and safe sustainable treatment. First devices that have been proposed were vascular prostheses made of polymeric materials. The second generation of devices was stents made of metals and alloys. The third generation, endografts, associated these both materials. Materials used as vascular and endovascular devices must meet a number of requirements based on dimensional, physical and mechanical criteria. Ideally, they should demonstrate a behavior as close as possible as that of human arteries in terms of mechanical properties such as compliance, long-term durability, and in terms of biological properties such as biocompatibility, luminal surface healing and thrombogenicity. We propose in the present manuscript a review of properties of materials currently used for the construction of vascular and endovascular devices, future challenges in the fields of new materials and scientific approaches and tests to understand and predict the behavior of the next generations of devices.

1H-NMR spectroscopic study of the effect of aging vascular prostheses made of poly(ethylene terephthalate) on the macromolecular weight.

Trichloroacetyl isocyanate reacts rapidly and quantitatively with both acid and hydroxyl chain ends to form derivatives that can be readily determined by (1)H-NMR spectroscopy. This method provides a convenient mean for characterization of polyethylene terephthalate (PET) end-groups. The (1)H-NMR spectroscopy has been applied to describe the chemical aging of the PET vascular prostheses by determination of the hydroxyl and carboxyl end-group concentrations and therefore the macromolecular weight. To validate (1)H-NMR results, we used chemical titration of the end-groups and classical viscosimetric method as complementary techniques. The analyses made on the explants of different lifetime demonstrated a significant deterioration compared with the virgin prostheses. A high degradation of macromolecular weight is observed. This phenomenon is explained by a random scission of the ester linkages.

What can we learn from explanted endovascular devices?

OBJECTIVE: To examine the durability of the Stentor and Vanguard endovascular devices in human implants. METHODS: The textile covering, the polypropylene ligatures and the stent metal of 34 devices (25 Stentor, 9 Vanguard) with a mean duration of implantation of 28.8 +/- 16 months was examined by means of stereomicroscopy and scanning electron microscopy. RESULTS: The polyester textile covering showed gaps along the sutured seam and isolated holes in the fabric. All of the examined polypropylene ligatures were worn, some ruptured. Four different types of stent corrosion were classified--pits (100%), bizarre craters (68%), large deficiencies (14%) and fractures (32%). CONCLUSION: Holes in the polyester fabric and frame dislocations are specific for the design of Stentor and Vanguard grafts. The early corrosion of the stent metal Nitinol in these devices is surprising. Until more experience is gained with other devices, we have to be reminded, that the "gold standard" for the long-term durability of artificial vascular grafts is still "today's" conventional graft.

Fat- and bone marrow-impregnated small diameter PTFE grafts.

OBJECTIVES: to evaluate an alternative and simple technique which consists in impregnation of a synthetic prosthesis with either autogenic omental fat or bone marrow. These tissues have been selected based on previous works and because they contain multiple cellular and extracellular compounds with biological healing properties (i.e. angiogenesis, endothelialisation, etc.). DESIGN: PTFE grafts of Group 1 were impregnated with fatty tissue, those of Group 2 with bone marrow and those of Group 3 served as controls. MATERIALS: nine mongrel dogs divided among these three groups. PTFE grafts are 3 mm in diameter. METHODS: in each animal, both iliac arteries were submitted to an end-to-side ilio-iliac bypass. At 3 months, pathology assessment was performed. RESULTS: group 1: all grafts were thrombosed and intimal hyperplasia was found occluding the anastomotic sites. Group 2: 4/6 grafts were patent and their mid-portion presented a thin neointima which did not totally cover the anastomotic sites. Group 3: 2/5 grafts were patent and their mid-portion as well as the anastomotic sites were covered with neointima which was hyperplastic in some areas. CONCLUSIONS: addition of bone marrow cells may contribute to improve the quality of the healing process.

Corroded nitinol wires in explanted aortic endografts: an important mechanism of failure?

PURPOSE: To analyze surface alterations and fractures observed in the nitinol stent wires of explanted endovascular grafts used for treatment of abdominal aortic aneurysm. METHODS: Twenty-one explanted Stentor devices and 1 Cragg stent were received from investigators in Germany and France. After macroscopy and photography, the explants were cleaned and the polyester coating removed. The frame was examined completely by stereomicroscopy, and irregularities were assayed by scanning electron microscopy and energy dispersive x-ray analysis (EDAX). The observed alterations were classified according to stereomicroscopic and electron microscopic morphology. RESULTS: The mean implantation interval for the endografts was 29.1 +/- 13.2 months (range 5-46). All examined explants, even those retrieved after only a few months in situ, showed pitlike surface damage 10-25 microm in diameter. Larger, irregularly shaped surface alterations were observed in approximately 70% of the explants. Older explants (age >32 months) presented vast regions of decay, with bending of the wire and stress cracks in some areas. EDAX examination revealed decreased nickel concentration in the corroded regions. CONCLUSIONS: Corrosion of the nitinol wire in endovascular grafts is confirmed. Presumably, the observed pitting and irregularly shaped corrosion defects are the precursors of material failure. They weaken the thin wire, which leads to stress cracks and eventually fracture of the stent wire under circulatory pulsation. Cell-induced electrochemical corrosion and active cellular destruction of surfaces are well-known mechanisms that must be investigated for their possible roles in the corrosion of stent metals.

[Late complications after alloplastic vascular replacement]. / Spätkomplikationen nach alloplastischem Gefässersatz.

The follow-up of 273 AAA patients operated 1981-1985 showed a mean dilation of polyester grafts of 17.4% in 12 days and 34.8% in 3 years. This early dilation is due to mesh expansion of the warp knitted grafts. It has no clinical relevance. In contrary the late dilation after approximately 10 years is due to degradation. The evaluation of 436 explanted grafts attained from 75 hospitals showed graft rupture to be the cause of the explanation for 42/255 (16%) polyester grafts and 2/42 (5%) PTFE grafts. The most ruptures were observed in the surrounding of the inguinal ligament after 10 to 20 years of duration. In comparison to modern endovascular grafts the conventional polyester and PTFE grafts still are the gold standard of durability.

[Injury of polyester grafts by vascular clamps]. / Schädigung von Polyestergefässprothesen durch Gefässklemmen.

UNLABELLED: Protected vascular clamps are not new. Clamp associated damage of human arteries has already been published over 20 years ago. The necessity of protective clamps seems to have been forgotten. In our explant archive (230 explants) we have observed an accumulation of graft ruptures in the groin (13 of 25 ruptures). We presume a multifactorial process. Clamp damage could be part of it. The aim of this study is to prove the clamp induced damage of polyester vascular grafts and to examine whether protected clamps can reduce this. METHOD: Five unprotected (Aesculap(R) FB512R, FB502, FB517, Ulrich CC1235, CV3535) and 5 protected vascular clamp types (Aesculap(R) FB667, FB668, Edwards(R) - formally Baxter(R) - Fogarty(R) CV5050, CV5201, Edwards(R) Cosgrove(R) CV1033) were tested. A longitudinal burst test was performed after maximal clamp closure on 6 different, multifilament polyester yarns of 2 different vascular grafts manufacturers (B. Braun(R), Edwards(R)). RESULTS: The yarn tests with protected clamps showed no difference to those of the unclamped yarns. After clamping with unprotected vascular clamps the stress-strain-diagrams differed significantly. The mean, maximum burst strength was up to 75 % lower. Video documentation revealed filament ruptures. Damage of the yarn surface was seen on a simple woven graft in scanning electron microscopy (SEM). DISCUSSION: The application of unprotected vascular clamps on polyester vascular grafts is common in Germany (56 %). The observed damage of multifilament polyester yarns makes it necessary to re-consider the use of unprotected vascular clamps. The benefit for biological vessels has already been shown.

Albumin-impregnated polyester vascular prosthesis for abdominal aortic surgery: an improvement?

AIM: To compare the peroperative blood loss and the postoperative systemic inflammatory reaction in patients receiving either a Vasculour II Albumin pre-impregnated prosthesis (VA group, n = 32) or a preclotted Vasculour II prosthesis (V group, n = 33) for elective surgery of the abdominal aorta. SETTING: University Hospital. DESIGN: Prospective, randomised study. METHODS: Peroperative blood loss was measured over two different periods: Phase I from the beginning of the operation to the completion of the proximal anastomosis, when blood loss cannot be related to the model of prosthesis implanted and phase II after the completion of the proximal anastomosis to the end of the operation. Postoperative blood loss was evaluated by the determination of the retroperitoneal drainage volume over a period of 2 days immediately following the operation. The presence of periprosthetic fluid was measured with echography at days 4, 9, 30 and 60. The postoperative systemic inflammatory reaction was evaluated by measuring the sedimentation rate and the C reactive protein levels daily from day 1 to day 9, and at days 14, 21, 28, 45, and 60, and by measuring the body temperature daily from day 1 to day 9. RESULTS: No significant differences of peroperative blood loss were observed. The same proportion of patients (35%) in both groups received homologous transfusion. The mean number of units of homologous blood transfused per patient was respectively 0.77 and 0.91 for the VA and the V group. The retroperitoneal drainage volume and the percentage of patients with periprosthetic fluid did not differ significantly. No significant differences in systemic postoperative inflammatory reaction were observed. CONCLUSION: There were no benefits in using albumin-impregnated prosthesis as opposed to preclotted prosthesis in terms of peroperative and postoperative blood loss, or by looking at the incidence of homologous blood transfusion. However, the glutaraldehyde cross-linked albumin did not induce any systemic inflammatory reaction.

Sealing of polyester prostheses with autologous fibrin glue and bone marrow.

The purpose of this study was to develop a sealing technique for polyester prosthetic grafts able to promote healing and reduce intimal hyperplasia. The porcine experimental model was aortoiliac bypass with a 6-mm diameter knitted polyester prosthetic graft implanted for 14 and 90 days. Animals were divided into three groups according to sealing technique as follows: pre-clotting with blood (group I, n = 12), sealing with autologous fibrin glue (group II, n = 14), and sealing with autologous fibrin glue and bone marrow cells (group III, n = 16). Feasibility and quality of sealing were evaluated by scanning electron microscopy prior to implantation and by assessment of blood loss. After removal, prostheses were cut into three segments comprising the proximal anastomosis, midsection, and distal anastomosis. Pieces were fixed, embedded in paraffin, and serially sectioned for histologic study. Histological study focused on the degree of stenosis and hyperplasia of the neointima of each prosthesis. The results of this short-term study indicate that sealing of polyester vascular prosthetic grafts with autologous fibrin glue and bone marrow cells is effective in reducing intimal hyperplasia. However further study will be needed to assess long-term healing.