Histopathological evaluation of woven and knitted Dacron grafts for right ventricular conduits: a comparative experimental study

Composite extracardiac conduits consisting of a low-porosity woven graft and a high-porosity knitted double-velour Dacron graft presealed with fibrin glue were implanted between the right ventricle and the pulmonary artery in 6 dogs under partial heparinization. Two grafts were explanted after 6 weeks, 2 after 12 weeks, and 2 after 6 months. The healing properties of both types of prosthesis were studied macroscopically, under light microscopy, and with scatter electron microscopy. Spontaneous peeling of both the inner and outer capsules of the graft occurred in 3 of 6 woven prostheses during transection. In the remaining 3, peeling could be easily induced by blunt dissection; this was impossible in the knitted grafts. Microscopically, in a comparison of the different weaves after identical time intervals, the inner capsule was noticeably thicker in woven than in knitted grafts. Transtitial ingrowth of fibroblastic tissue could be observed in knitted grafts after 6 weeks; only poor transmural tissue bridging was detectable in woven prostheses after 6 months. Neovascularization of the inner capsule was detectable earlier and was more advanced toward the luminal surface of highly porous grafts. In conclusion, knitted grafts in the position of extracardiac right ventricular conduits showed firmer attachment of both inner and outer capsules to the prosthetic material. Also, the inner capsule remained thinner and revealed a higher degree of neovascularization than in the woven Dacron grafts.     

Biological sealants and knitted Dacron: porosity and histological comparisons of vascular graft materials with and without collagen and fibrin glue pretreatments

Woven Dacron used in extracardiac conduits tends to form a poorly adherent, obstructing layer of pseudointima. Knitted high-porosity conduits allow fibrous and vascular ingrowth and more secure anchoring of the pseudointima. However, at the time of insertion, the porosity rate must be sufficiently low to prevent excessive bleeding in the heparinized patient. Pretreatments with biological sealants are available to temporarily reduce the porosity of knitted Dacron at the time of implantation with subsequent resorption of the sealant. We compared in vitro water porosity rates of knitted Dacron pretreated with conventional techniques, fibrin glue, and collagen impregnation. Only the collagen-impregnated and fibrin glue-treated grafts decreased the porosity of knitted Dacron to an acceptable level. To assess the biocompatibility and resorption of biomaterials used in sealing conduits, pretreated Dacron was implanted subcutaneously in weaning rats. Foreign-body response of untreated Dacron was unchanged by conventional techniques. Fibrin glue also resulted in good capillary ingrowth as well as occasional punctate multifocal deposits of calcium phosphate. The collagen-impregnated grafts differed in the intensity of the inflammatory response and tissue adhesion, possibly related to the degree of collagen cross-linking. This may have important implications regarding structure of the pseudointima in vascular grafts sealed with these materials.     

Unsatisfactory Clinical Experience with a Collagen-Sealed Knitted Dacron Extracardiac Conduit

A clinical trial of a collagen-sealed knitted Dacron conduit (Tascon Medical Technologies) in 86 patients has revealed a high incidence of early reoperation for conduit stenosis. At 3 years, the actuarial incidence of freedom from conduit replacement was 67 +/- 14% for valved conduits, and 66 +/- 20% for nonvalved conduits. Seven of eight conduits that were replaced had a thick, weakly adherent pseudointima. Comparison of the current series with a previous series of patients receiving tightly woven low-porosity Dacron conduits is complicated by the young age, small size, and greater complexity of the current group. Nevertheless, the findings are consistent with the results of two laboratory studies performed at this hospital which suggest that the collagen used in the Tascon conduit undergoes particularly slow resorption, resulting in weak adhesion between the pseudointima and conduit. This allows hemorrhagic dissection to occur deep to the pseudointima. These data suggest that alternative methods of sealing knitted Dacron conduits should be used.     

Heat-denatured albumin-coated Dacron vascular grafts: physical characteristics and in vivo performance

Dacron fabrics with a wide range of porosities were autoclaved for 3 minutes after being soaked in serum, 5% albumin, or 25% albumin. Porosity of compound Dacron grafts made with 25% albumin was less than 1 ml/min/cm2 regardless of the fabric base, whereas porosity of grafts made with serum or 5% albumin was proportional to the porosity of the base fabric. Porosity of the compound grafts remained stable for more than 48 hours and to pressure greater than 450 mm Hg, if the grafts were kept moist. Tubes of Marlex mesh coated with heat-denatured albumin, implanted as infrarenal aortic replacements in dogs, showed complete albumin absorption by 3 weeks. However, perigraft tissue reaction and graft incorporation were minimal and extensive false aneurysm formation resulted. Knitted filamentous Dacron 6 mm tubes coated with heat-denatured albumin were implanted as iliofemoral bypass grafts in 12 dogs, with blood-preclotted knitted filamentous Dacron grafts implanted as contralateral control grafts. Comparison of the albumin-coated grafts with the blood-preclotted control grafts showed no differences in healing or patency at 4 to 6 months. Heat-denatured 25% albumin forms a strong and hemostatic coating regardless of fabric base. Albumin-Dacron compound grafts are easily and rapidly made in the operating room, handle well, and are suitable for large and medium-sized arterial replacements without changes in healing or patency. Because of slow tissue incorporation, however, albumin-coated knitted Dacron grafts should be avoided in patients who require long-term anticoagulation therapy.     

Pseudointima formation in woven and knitted dacron grafts. A comparative ultrastructural analysis

Twelve specially designed crimped aortobifurcated grafts (aorta, woven 200 porosity; right limb, woven 500 porosity; left limb, knitted 1200 porosity) were implanted into mongrel dogs that were sacrificed at two, six, eight and twelve-month intervals. Pseudointima from the mid portion of each type of graft was studied with light (LM) and transmission electron microscopy (TEM). An ocular micrometer was used to determine the thickness of the pseudointima. A trilaminar cytoarchitecture resembling the wall of a medium-sized artery existed in the pseudointima. This was characterized by an inner cellular layer, outer fibrocollagenous layer, and multi-interlamination of cells and glycosaminoglycan in the middle layer. The cellular inner layer was thickest in the knitted and thinnest in the woven 200 porosity. A zone of acellularity separated the luminal cells from the underlying myoblasts and myofibroblasts. Specificity of varying types of cell distribution occurred in the pseudointima. Myoblasts and myofibroblasts were located near the lumen while fibroblasts and mesenchymoid cells were situated near the graft. The luminal cells had the combined features of endothelial cells and myofibroblasts. Collagen and glycosaminoglycans were the dominant extracellular matrix. Despite the difference of fluid permeability in high porosity woven and knitted dacron grafts, the pseudointima was well formed and revealed no striking general cytoarchitectural difference between these two types of grafts.     

Capillary and venule proliferation in the healing process of Dacron venous grafts in rats.

A woven, noncrimped graft (made of Dacron and coated with an elastomer) that was 1.5 mm in diameter and 15 mm long was capable of replacing the vena cava in rats. The elastomer mixture consisting mainly of silicone rubber was necessary to bond the woven Dacron fibrils of the woven graft to prevent fraying at the anastomoses and bleeding through the interstices of the lightly woven, fairly high porosity (500) conduit. Sequential histologic studies showed that patency was associated with the ingrowth of small venules and the spreading of endothelial cells from each anastomosis toward the center. Small venules appeared in loose connective tissue, forming a pseudointima 12 days after grafting. This process occurred in the midportion of the graft before pannus endothelial growth covered the endothelial surface. Various thicknesses of the external polymer coat were studied for their influence on healing. None of the grafts developed a thrombus in these latter studies, and regardless of the thickness of the external elastomer, endothelial resurfacing was complete at 30 days. However, the graft with the thinnest external elastomer coating had the best-formed vasa vasorum, and the intima at both the anastomoses and midportions of the graft was significantly thinner than intima found in grafts of other composition. We conclude that this woven Dacron polygraft provides a surface resistant to early thrombosis; that healing occurs mainly by pannus ingrowth, but external and interstitial factors are also important; and that properties inherent in the polygraft wall determine the size of the residual lumen.     

An experimental study of the strength in arterial prosthetic anastomoses: relationship between anastomotic tensile strength and its organization]

Five types of 6-mm diameter arterial prostheses, i.e.m woven Teflon, woven Dacron, velour woven Dacron, velour knitted Dacron and expanded polytetrafluoroethylene (E-PTFE), were studied in the dog to assess anastomotic tensile strength and its organization. Five types of composite grafts, about 3.0cm in length, were implanted in the abdominal aorta of fifty-two adult mongrel dogs using absorbable polyglycolic acid suture. At 16 weeks after the implantation, if the composite graft was patent, tensile strength of the bilateral anastomoses were measured and its organizations were examined microscopically. The mean forces required to disrupt the anastomoses of velour woven Dacron, velour knitted Dacron and E-PTFE were greater than those of woven Teflon and woven Dacron. At the disrupted sites of woven Teflon and woven Dacron, which were non-velour grafts, fibrous outer sheath were remarkably separated from the grafts. This results indicate that organization and fixation of fibrous outer sheath are the most important factors in the strength of arterial prosthetic anastomoses and velour grafts have superiority over non-velour grafts. The organization of E-PTFE was poor, but its tensile strength was equal to velour grafts. It is considered E-PTFE has an advantage due to non-crimped structure and requires further evaluation over longer period.     

An in vitro study of the properties influencing Staphylococcus epidermidis adhesion to prosthetic vascular graft materials.

This study examines the influence of the properties of various vascular graft materials on the bacterial adherence process of two different strains of Staphylococcus epidermidis (mucous and normucous producing). Dacron grafts (both knitted and woven), Teflon grafts, and Dacron grafts coated with one and two layers of silicone were studied because these materials differ significantly in porosity, hydrophobicity, and surface charge (zeta potential). Graft segments were immersed in 3H-labeled bacteria solution for periods ranging from 5 to 180 minutes and liquid scintillation techniques were used to quantify bacterial adherence. The porous knitted Dacron material had a significantly higher rate of bacterial adherence than either the woven Dacron or Teflon (p less than 0.05). Silicone coating (either one or two layers) reduced adherence by a factor of four for the knitted Dacron (p less than 0.05) and by a factor of two for woven Dacron (p less than 0.05). The mucous producing strain of S. epidermidis displayed significantly better adherence to woven and knitted Dacron than the normucous producing strain, but only when 0.25% dextrose was added to the bacteria solution. These findings indicate that the highly porous knitted Dacron grafts have the highest propensity for bacterial adhesion. Graft materials with the most negative zeta potentials are more resistant to bacterial adherence. Silicone coating of Dacron material significantly changed adherence characteristics, suggesting that this may be a viable strategy for protecting implantable medical devices containing materials to which bacteria readily adhere.     

Indium 111-labeled platelet deposition in woven and knitted Dacron bifurcated aortic grafts with the same patient as a clinical model

A study was designed to compare platelet deposition between knitted and woven Dacron grafts in the same patient. Twenty patients received aortoiliac or aortofemoral bifurcated Dacron grafts, each composed of one woven and one double-velour knitted limb. External nuclear graft imaging was carried out after injection of autologous platelets labeled with indium 111. The patients were studied postoperatively in time periods ranging from 6 days to 42 months. Platelet accumulation was almost identical in knitted and woven limbs in all patients. This study appears to indicate that there is no difference in thrombogenicity between knitted and woven bifurcated Dacron grafts in the aortoiliac or aortofemoral positions measured by platelet accumulation.     

Development of a "leak-proof," knitted Dacron vascular prosthesis

We have developed a technique to render knitted Dacron prostheses totally impervious to blood leak by impregnating the cloth with collagen. These grafts were prepared by the manufacturer, sterilized, and have an indefinite shelf life. The objective of this study was to determine whether collagen impregnation has a deleterious effect on surface thrombogenicity and graft healing. The infrarenal aortas of 30 mongrel dogs were replaced with 6 cm X 6 mm Dacron grafts (15 collagen-impregnated and 15 control). The collagen-impregnated prostheses were visibly indistinguishable from the control. After blood flow was restored, no interstitial bleeding occurred in the collagen grafts, in contrast to the control grafts that initially leaked profusely. When they recovered, the dogs were divided into three groups: group I (five collagen, five control), group II (five collagen, four control), and group III (five collagen, three control). The grafts were harvested at 3, 6, and 9 months, respectively. Retroperitoneal healing response, capsular thickness and adherence, completeness of neointimal healing, surface fibrinolytic activator activity, and sections taken for light and scanning electron microscopy were studied and compared in a blinded fashion. In a separate set of experiments, experimental and control grafts were compared for in vivo surface thrombogenicity. The healing data were identical in the experimental and control prostheses in all parameters. Of particular interest was that initial capsule adherence was better and surface thrombogenicity less in the collagen-impregnated prosthesis. We conclude that collagen impregnation of a knitted Dacron prosthesis has no deleterious effect for the term of observation of this experiment.     

Comparative analysis of pseudointima biogenesis in Gelseal coated Dacron knitted graft versus crimped and noncrimped graft

Twenty-one grafts (knitted crimped, 7; knitted Gelseal, 7; knitted no crimp, 7) of 7 cm long were inserted into 6 month-old mongrel puppies as infrarenal aortic tube grafts and retrieved at two-month intervals. All grafts were perfusion fixed in situ. Ex vivo arteriograms were obtained prior to processing for light (LM) and transmission electron microscopic (TEM) studies of tissue ingrowth into the grafts. All grafts were patent at the time of harvesting. The thickness of pseudointima plateaued at 10 weeks. There was no demonstrable discrepancy in thickness between grafts (cell 72 +/- 13). The luminal cells were modified myofibroblasts that contained short microvilli, gap junctions, myofilament, and incomplete basal laminae. The pseudointima consisted of an interlamination of myofibroblasts alternating with extracellular matrix of collagen and ground substances. It was more cellular near the lumen and more fibrocollagenous near the graft. Myofibroblasts were found near the lumen and fibroblasts near the graft. Macrophages, vasa vasorum, leukocytes and fibroblasts occupied the interstitial space between the graft fibrils. Similar cellular and extracellular composition existed in the three types of grafts. There was no local inflammatory reaction encountered in the Gelseal treated graft. Gelseal, crimped and noncrimped knitted Dacron grafts had pseudointima of comparable architecture, thickness, cellular and noncellular composition. Gelseal did not hinder pseudointima formation or induce local inflammatory reaction.     

A new sealant for knitted Dacron prostheses: minimally cross-linked gelatin

There has been a resurgence of interest in the concept of presealing high-porosity knitted Dacron prostheses with an absorbable biologic material. Such a material should provide reliable porosity control, preferably reducing water porosity from 2000 ml/cm2/min to less than 50 ml/cm2/min. It should not interfere with the fibrous and vascular ingrowth that securely anchors the developing pseudointima. In previous studies, we have examined fibrin glue and two forms of aldehyde cross-linked insoluble collagen used as Dacron sealants. We concluded that delayed resorption of the sealant as seen with glutaraldehyde cross-linked insoluble collagen results in undesirable healing characteristics, particularly lack of adhesion between pseudointima and the luminal surface of a prosthesis. This study examines a new sealant. Soluble collagen (gelatin) is treated to reduce the number of free amino groups available for aldehyde cross-linking. It is then weakly cross-linked with an aldehyde mixture and applied to a knitted Dacron prosthesis. Water porosity studies have confirmed satisfactory porosity control. Both rat subcutaneous and canine circulatory implants for 6 months reveal relatively rapid and complete sealant resorption without undesirable modification of the normal healing process of knitted Dacron.     

Biological sealants and knitted Dacron conduits: comparison of collagen and fibrin glue pretreatments in circulatory models

To assess the healing characteristics of knitted Dacron conduits sealed with fibrin glue, collagen cross-linked with glutaraldehyde, and collagen cross-linked with formaldehyde, composite conduits, each constructed from two of the three test materials, were implanted in the pulmonary and systemic circulations of sheep and dogs for 3 and 6 months. Relative rates and morphologies of sealant resorption and other histological findings were predicted by a previous study involving subcutaneous implantation of the same graft materials in the rat. In particular, both studies revealed that delayed resorption of collagen cross-linked with glutaraldehyde and collagen cross-linked with formaldehyde resulted in lack of adhesion between inner capsule and conduit, thereby causing focal hemorrhagic dissection in the circulatory implants. There were no important qualitative or quantitative differences in healing between sheep and dogs or in pulmonary versus systemic location of the conduit. Rendering an animal cyanotic by placing a pulmonary arterial-left atrial shunt or by increasing or decreasing the intraconduit systolic pressure did not obviously affect healing. The results emphasize that the biological consequences of sealed grafts are strongly dependent on the sealant material. Porosity control of knitted Dacron with biological sealants that undergo minimal or delayed resorption may lead to poor tissue adhesion and resultant complications.     

Biotolerant and haemodynamic effects of copolymerization with acrylic acid on Dacron arterial prostheses.

The biotolerant and haemodynamic effects of three types of woven polyethylene terephthalate (Dacron) arterial prostheses were compared. Two of these were unmodified, one low, one high porosity; and the third was the latter type radiation graft copolymerized with 5% acrylic acid. The prostheses were implanted as aorto-aortic bypass grafts, in dogs, for periods ranging from three to seventeen months. The copolymerized prosthesis exhibited the smallest rise in circumferential elastance from the 'as supplied' state, as well as a significantly slower pulse wave velocity, a significantly thinner internal lining, and a markedly lower longitudinal impedance. Of the unmodified prostheses, the low porosity graft had a significantly greater circumferential elastance, the poorest handling characteristics, and a tendency for pseudo intimal detachment. It was concluded that, although there was little difference between the two unmodified prostheses, the low porosity one had little to recommend it for use in small calibre bypass grafting. Copolymerization with acrylic acid produced better haemotolerant and haemodynamic results as well as surface properties which appeared to discourage fibrin and platelet adherence, and promote a more complete and organised pseudointima.     

Experimental evaluation of ten clinically used arterial prostheses

The arterial grafts currently in use are classified into five basic categories; 10 different commercially available prostheses were chosen to represent these categories. The Vascutek and Bionit are made from knitted Dacron and have medium porosity, requiring preclotting by the surgeon. The DeBakey Soft Woven and Plasma TFE grafts are made from woven Dacron and have low porosity, not requiring preclotting under most circumstances. Also studied were woven and knitted grafts with leakage resistance referred to as impervious Dacron grafts: the Vascutek Gelseal, the Bard Albumin Coated DeBakey Vasculour II, the Microvel with Hemashield, and the albumin saturated, autoclaved DeBakey Soft Woven graft. Gore-Tex and Impra are expanded polytetrafluoroethylene grafts which do not require preclotting. For each type, five grafts 6 cm long and 8 mm in diameter were implanted in the descending thoracic aorta of healthy adult dogs for 16 weeks. The physical characteristics, biocompatibility, and healing patterns varied according to the structure and treatment of the grafts. Pretreatment with biomaterials during manufacture is quite effective in preventing transinterstices blood loss during implantation, but results in altered physical qualities, increased thrombogenicity and delayed healing in comparison to the effects of preclotting with autogenous blood at the time of implantation.     

Infection of vascular prostheses caused by bacterial biofilms

A canine model was developed to study the efficacy of graft replacement as treatment for vascular prosthesis infections from Staphylococcus epidermidis. Infrarenal aortic graft infections were established in 18 dogs by implantation of Dacron prostheses colonized in vitro with a slime-producing strain of S. epidermidis to form an adherent bacteria-laden biofilm (5 X 10(6) colony-forming units/cm2 graft). Study animals developed a graft infection with anatomic and microbiologic characteristics typical of late prosthetic graft infections in humans (sterile perigraft exudate, absent graft incorporation, and normal serum leukocyte count and sedimentation rate). The S. epidermidis study strain was isolated from 14 of 18 explanted grafts (78%) by mechanical disruption of the graft surface biofilm and culture in broth media. Four dogs with sterile graft cultures had histologic evidence of bacterial infection. The established prosthetic surface biofilm infection was treated by graft excision, parenteral cefazolin, and graft replacement with a Dacron or polytetrafluoroethylene (PTFE) vascular prosthesis. One month after graft replacement, no PTFE graft had signs of infection, but perigraft exudate and inflammation involved three of nine Dacron grafts (33%). The study strain was recovered from four of nine PTFE grafts (44%) and two of nine Dacron (22%) replacement grafts (p greater than 0.05). Prosthetic replacement of Dacron prostheses infected by S. epidermidis as a bacteria-laden surface biofilm can result in early graft healing, but persistent colonization of one third of replacement grafts signify that recurrent clinical infection remains a risk.     

Primary anastomotic bonding in polytetrafluoroethylene grafts?

Previous studies have demonstrated that standard knitted and woven fabric grafts are forever dependent on the suture material for anastomotic tensile strength. Clinical experience with polytetrafluoroethylene (PTFE) and double velour knitted grafts have shown that there is extensive fibrous capsular bonding between the graft and the surrounding tissues. This would lead to increased anastomotic tensile strength. To test this theory, 34 mongrel dogs underwent replacement of their infrarenal aortas with grafts made of PTFE (10 dogs), of double velour knitted Dacron (DVD, 11 dogs), of single velour knitted Dacron (SVD, 5 dogs), and of woven Dacron (WD, 8 dogs). One anastomosis was constructed with 5-0 Prolene and the opposite anastomosis was constructed with 5-0 Dexon (average absorption time, 21 days). In five grafts each of PTFE and DVD, as well as in all eight WD grafts, the midgraft was divided and resutured with 5-0 Dexon. All grafts were harvested together with adjacent proximal and distal aorta between 3 and 10 months from the time of implantation. The tensile strength of each anastomosis was measured with a tensiometer. The mean graft-to-artery (absorbable suture) anastomotic tensile strength, in pounds, for PTFE (14.3) and DVD (12.6) was significantly higher than that for SVD (6.9) or WD (7.2) (p less than 0.003). Graft-to-graft anastomotic tensile strength for PTFE (mean 17.3) was significantly better than that for DVD (mean 9.0; p less than 0.03) or WD (mean 7.9; p less than 0.001). Analysis of anastomotic tensile strength as a function of time revealed continued increase in PTFE in contrast to a slow decline with time in DVD.(ABSTRACT TRUNCATED AT 250 WORDS)     

Experimental comparison of albumin-sealed and gelatin-sealed knitted Dacron conduits. Porosity control, handling, sealant resorption, and healing.

Two high-porosity knitted Dacron vascular grafts sealed with aldehyde cross-linked gelatin or albumin were compared with respect to the following characteristics. Porosity control by the absorbable sealant was assessed with a water porosity meter at 120 mm Hg pressure. Ease of suturing was determined by an objective needle penetration test. Sealant resorption was assessed histologically in a subcutaneous immature rat model as well as in circulatory implants. Gross and microscopic healing characteristics were compared in circulatory implants in the thoracic aorta of sheep with use of a composite conduit in every animal, which allowed direct comparison of the two graft materials and minimized differences in healing between individual animals. Both grafts demonstrated excellent porosity control and better handling characteristics than woven Dacron. Sealant resorption was generally rapid, although residual albumin sealant was often seen adjacent to anastomoses. Residual sealant appeared to result in focally poor healing with focal loss of adhesion of surrounding tissue to graft. We conclude that details of sealant preparation and application can importantly influence the performance of presealed knitted Dacron grafts and should be carefully evaluated in the laboratory before clinical implantation is begun.     

External velour dacron grafts in renal artery reconstruction.

Aortorenal interposition grafts were performed in dogs using a new external velour knitted Dacron graft. Encapsulation and endothelialization of these grafts occurs as early as two weeks postoperatively. Complete healing is seen by six weeks, and the cellular lining of these grafts appears to be continuous with, and histologically similar to, the endothelium of contiguous arteries. The healed nonthrombogenic characteristics of this graft material represents a considerable improvement over graft materials of conventional design.     

Patency rate of bifurcated aortic grafts: comparative analysis of woven versus knitted prostheses in the same patient

To investigate the difference in patency rate between woven and knitted aortofemoral or aortoiliac prosthetic grafts, a special vascular prosthesis was manufactured with one limb of the graft knitted and the other, woven. The prosthesis was implanted in 143 consecutive patients with occlusive aortoiliac arteriosclerotic disease or aneurysms. Detailed statistical analysis failed to reveal any difference in the patency rate between the woven and knitted limbs of the grafts during an observation period ranging from one month to two years.