Detergent-extracted small-diameter vascular prostheses

The exact mechanism that leads to thrombosis of small-diameter vascular prostheses (<4 mm × >6 cm) is unknown. This report presents preliminary patency and healing data on a sequential detergent-extraction technique for the preparation of autogenous small-diameter microvascular grafts. Fifteen carotid interpositional allografts (3 to 4 mm × 4 to 6 cm) were implanted in 15 mixed species adult greyhounds. Ninety days after implantation grafts were perfusion-fixed in situ, harvested, and evaluated by light microscopy and scanning and transmission election microscopy. Two categories of acellular vascular matrix grafts were evaluated: non—cross-linked and cross-linked (1% carbodiimide). By objective morphologic analysis with blind random view, histologic sections were rated from 0 to 4 in five categories believed to be important for graft healing and patency. Overall graft patency was 8% (12 of 15), and there was no significant difference between cross-linked and non-cross-linked grafts. Non—cross-linked grafts were superior to cross-linked grafts in all areas of histologic evaluation except immunogenicity (p < 0.01). Most important, non—cross-linked grafts demonstrated complete endothelial coverage (p < 0.001). There was no significant difference (that is, normal) between control autografts and non—cross-linked grafts; however, there was a significant difference between control autografts and cross-linked grafts in all parameters except immunogenic reaction (p < 0.01). (J. VASC SURG 1984;1:181-91.)     

Optimal graft diameter: effect of wall shear stress on vascular healing

Arterial walls tend to adapt to maintain a specific wall shear stress. The formation of neointimal hyperplasia and endothelial cell healing of polytetrafluoroethylene grafts may also be governed by wall shear stress, which suggests that an optimal graft diameter may exist. To test this, 40 polytetrafluoroethylene grafts with internal diameters of 3, 6, and 8 mm were inserted end to end in the femoral and carotid arteries of 10 mongrel dogs. Total flow and diameter were measured, and grafts were stained with Evans blue dye, fixed by pressure perfusion, and analyzed by computer for anastomotic neointimal thickening, graft pseudointimal thickening, and degree of endothelial coverage. Mean calculated shear stress was 41 dyne/cm2 for the 3 mm grafts, 7 dyne/cm2 for the 6 mm grafts, and 3 dyne/cm2 for the 8 mm grafts. Fifteen weeks later the patency rate was 0 of 10 for the 3 mm grafts, 16 of 20 for the 6 mm grafts, and 7 of 10 for the 8 mm grafts. The mean graft shear stress was calculated to be 10 dyne/cm2 for the 6 mm grafts and 4 dyne/cm2 for the 8 mm grafts. Pseudointima lining the graft was composed of disorganized protein and cell remnants. The rough surface contained no overlying endothelium. Anastomotic neointima contained a layer of well-organized smooth muscle cells covered by a single layer of polygonal-shaped endothelial cells. A transition zone of thrombus, which is sandwiched by a wedge of smooth muscle cells near the graft surface and covered by endothelial cells, is described. Mean thickness of pseudointima of the patent 8 mm grafts was 150 microns thicker than that of the 6 mm grafts. Anastomotic neointimal thickness was 110 microns thicker in the 8 mm grafts compared with the 6 mm grafts. Among the 6 mm grafts, the carotid grafts had an average initial shear stress of 10 dyne/cm2, whereas the femoral grafts averaged a lower 5 dyne/cm2 and yielded pseudointima and neointima that were 40 microns thicker. The percent graft surface area covered with neointima did not differ among the grafts of differing diameter either proximally or distally. Lower shear stresses produced greater amounts of pseudointimal thickening within polytetrafluoroethylene grafts and neointimal thickening at their anastomoses. Conversely, the high shear stress from small-diameter grafts was associated with poor graft patency. These results suggest that an optimal graft diameter may help to prevent neointimal hyperplasia and graft thrombosis.     

Tissue engineered small-diameter vascular grafts

Arterial occlusive disease remains the leading cause of death in western countries and often requires vascular reconstructive surgery. The limited supply of suitable small-diameter vascular grafts has led to the development of tissue engineered blood vessel substitutes. Many different approaches have been examined, including natural scaffolds containing one or more ECM proteins and degradable polymeric scaffolds. For optimal graft development, many efforts have modified the culture environment to enhance ECM synthesis and organization using bioreactors under physiologic conditions and biochemical supplements. In the past couple of decades, a great deal of progress on TEVGs has been made. Many challenges remain and are being addressed, particularly with regard to the prevention of thrombosis and the improvement of graft mechanical properties. To develop a patent TEVG that grossly resembles native tissue, required culture times in most studies exceed 8 weeks. Even with further advances in the field, TEVGs will likely not be used in emergency situations because of the time necessary to allow for cell expansion, ECM production and organization, and attainment of desired mechanical strength. Furthermore, TEVGs will probably require the use of autologous tissue to prevent an immunogenic response, unless advances in immune acceptance render allogenic and xenogenic tissue use feasible. TEVGs have not yet been subjected to clinical trials, which will determine the efficacy of such grafts in the long term. Finally, off-the-shelf availability and cost will become the biggest hurdles in the development of a feasible TEVG product. Although many obstacles exist in the effort to develop a small-diameter TEVG, the potential benefits of such an achievement are exciting. In the near future, a nonthrombogenic TEVG with sufficient mechanical strength may be developed for clinical trials. Such a graft will have the minimum characteristics of biological tissue necessary to remain patent over a period comparable to current vein graft therapies. As science and technology advance, TEVGs may evolve into complex blood vessel substitutes. TEVGs may become living grafts, capable of growing, remodeling, and responding to mechanical and biochemical stimuli in the surrounding environment. These blood vessel substitutes will closely resemble native vessels in almost every way, including structure, composition, mechanical properties, and function. They will possess vasoactive properties and be able to dilate and constrict in response to stimuli. Close mimicry of native blood vessels may aid in the engineering of other tissues dependent upon vasculature to sustain function. With further understanding of the factors involved in cardiovascular development and function combined with the foundation of knowledge already in place, the development of TEVGs should one day lead to improved quality of life for those with vascular disease and other life-threatening conditions.     

Evaluation of a xenogeneic acellular collagen matrix as a small-diameter vascular graft in dogs--preliminary observations.

Autogenous veins are the materials of choice for arterial reconstruction. In the absence of autogenous material, prosthetic materials are used. However, vascular prostheses of less than 0.4 cm in diameter have low long-term patency. This study was designed to determine if cells would infiltrate an engineered xenogeneic biomaterial used as a small diameter arterial graft in dogs and, if so, to determine the phenotype of the infiltrating cells. Nine acellular xenogeneic grafts (0.4 cm in diameter, 5 cm long), composed of porcine collagen derived from the submucosa of the small intestine and type I bovine collagen, were implanted as end to-end interposition grafts in femoral arteries of five male mongrel dogs (total of nine grafts). All dogs received daily aspirin (325 mg). Patency of implanted grafts was monitored weekly by Duplex ultrasonography. After 9 weeks, or earlier in case of blood flow reduction by at least 75%, grafts were explanted and prepared for light or electron microscopy to evaluate cellularization. Eight of nine grafts remained patent up to 9 weeks. At explant, diameters were 0.31 +/- 0.02 cm at the midgraft, and 0.14 +/- 0.01 and 0.19 +/- 0.01 cm at the proximal and distal anastomoses. At explant, cells of mesenchymal origin (endothelial cells, smooth muscle cells, myofibroblasts) were embedded in the extracellular matrix of the graft scaffold. Minimal evidence of cellular inflammatory reaction and no aneurysmal dilatation or thrombus formation was detected. Variable degrees of hyperplasia were present at proximal and distal anastomoses. This preliminary study demonstrates that a collagen-based xenogeneic biomaterial provides a scaffold for cellularization when used for arterial reconstruction in dogs.     

Effect of compliance mismatch on vascular graft patency

The hypothesis that a mismatch in compliance between a vascular graft and its host artery is detrimental to graft patency was tested by implanting paired arterial autografts, prepared with differential glutaraldehyde fixation of carotid arteries in the femoral arteries of dogs. These grafts differed only in circumferential compliance: they were 100% (compliant) vs. 40% (stiff) as compliant as the host artery. Their flow surfaces were equivalent, as determined by physicochemical measurements and scanning electron microscopy; both lacked viable cells, as determined by in vitro cell culture. In 14 dogs, eight stiff and two compliant grafts became occluded within 3 months, the latter doing so within 24 hours after their contralateral counterparts. Cumulative patencies were 85% and 37% for compliant and stiff grafts, respectively (p less than 0.05) and 100% and 43%, excluding the two dogs with bilateral graft failures (p less than 0.01). We conclude that even with near optimal flow surfaces, compliance mismatch is deleterious to graft patency.     

Effect of nerve graft porosity on the refractory period of regenerating nerve fibers

OBJECT: In the present study the authors consider the influence of the porosity of synthetic nerve grafts on peripheral nerve regeneration. METHODS: Microporous (1-13 microm) and nonporous nerve grafts made of a copolymer of trimethylene carbonate and epsilon-caprolactone were tested in an animal model. Twelve weeks after surgery, nerve and muscle morphological and electrophysiological results of regenerated nerves that had grown through the synthetic nerve grafts were compared with autografted and untreated (control) sciatic nerves. Based on the observed changes in the number and diameter of the nerve fibers, the predicted values of the electrophysiological parameters were calculated. RESULTS: The values of the morphometric parameters of the peroneal nerves and the gastrocnemius and anterior tibial muscles were similar if not equal in the rats receiving synthetic nerve grafts. The refractory periods, however, were shorter in porous compared with nonporous grafted nerves, and thus were closer to control values. CONCLUSIONS: A shorter refractory period enables the axon to follow the firing frequency of the neuron more effectively and allows a more adequate target organ stimulation. Therefore, porous are preferred over nonporous nerve grafts.     

Influence of prosthetic graft fabrication on graft healing,I

The object of this report was to analyze graft healing by comparing the content of collagen and noncollagen protein among seven commonly used prostheses: light-weight knitted dacron concentric crimp (USCI), Sauvage filimentous external velour knitted dacron with concentric crimp, random crimp, or no crimp (USCI), microvel double-velour knitted dacron concentric crimp (Meadox), PTFE Impra, and PTFE Gortex. Grafts (6 mm) were used to replace 4-cm segments of infrarenal abdominal aorta in 85 experimental dogs and five autograft aorta control animals. At 3 and 6 months postimplantation the grafts were aseptically harvested, inspected for completeness of neointimal healing and submitted for biochemical analysis. After 3 months implantation, uncrimped external velour grafts had less hydroxyproline than all dacron grafts except random crimp external velour (P < 0.05) but not less than both PTFE grafts and most dacron and PTFE grafts were not significantly different in noncollagen protein. After 6 months implantation, all dacron grafts had more hydroxyproline than both PTFE grafts (P < 0.03) and most dacron grafts had significantly more noncollagen protein than PTFE Impra (P < 0.05). Of all the grafts analyzed, only uncrimped external velour knitted dacron grafts had both rapid neoendothelial healing and a significant increase in both hydroxyproline and noncollagen protein between 3 and 6 months after graft implantation (P < 0.01). These data indicate that dacron prostheses heal in a manner different from that of PTFE grafts, that there may be differences among the various types of dacron prostheses, and that external velour uncrimped grafts would appear to have the best healing characteristics.     

Effects of antiplatelet agents in combination with endothelial cell seeding on small-diameter Dacron vascular graft performance in the canine carotid artery model

The purpose of this study was to assess the success of endothelial cell-seeded and non-seeded small-diameter vascular grafts in dogs medicated with antiplatelet agents. Eighty dogs underwent bilateral carotid artery replacements with 6 cm lengths of 4 mm I.D. double-velour Dacron grafts. In each dog one graft was seeded with enzymatically derived autologous endothelial cells; the contralateral graft was nonseeded. The following anti-platelet medications were administered beginning 4 days preoperatively: aspirin (5 grains every day); dipyridamole (50 mg twice a day); aspirin plus dipyridamole (5 grains each day plus 50 mg twice a day); aspirin (1.25 grains every other day); ibuprofen (10 mg/kg/day); U-53,059, a cyclooxygenase inhibitor (3 mg/kg/day); and U-63557A, a thromboxane synthase inhibitor (10 mg/kg/day). Grafts were harvested 5 weeks postoperatively. Graft success was evaluated by patency, thrombus-free surface area, area endothelialized, and graft production of prostacyclin. None of the medications prevented neoendothelialization of seeded grafts. Mean patencies of endothelial cell-seeded grafts from medicated dogs were significantly greater than mean patencies of endothelial cell-seeded grafts from nonmedicated dogs. The cyclooxygenase inhibitors best maintained patency in nonseeded grafts. Thrombus-free surface areas of endothelial cell-seeded grafts from medicated dogs were significantly greater than from nonseeded control grafts from the medicated dogs. All medications impaired prostacyclin synthesis. We conclude that the combination of endothelial cell seeding plus antiplatelet medication is most efficacious in small-vessel grafting success and that high levels of prostacyclin production by vascular grafts are not necessary to maintain patency in dogs medicated with antiplatelet agents.     

Microvascular endothelial cell seeding of small-diameter Dacron vascular grafts

One obstacle to the clinical implementation of endothelial cell seeding of vascular prostheses is the difficulty in derivation of large numbers of autologous endothelial cells from blood vessels of patients requiring vascular grafting. Capillary endothelial cells obtained from fat have been suggested as an abundant alternative to large-vessel endothelium for graft seeding. The object of this study was to evaluate the performance of 4-mm internal diameter (ID) Dacron Microvel grafts seeded with omentally derived microvascular endothelial cells. Six-cm lengths of the test grafts were implanted bilaterally into canine carotid arteries. One of each pair of grafts was seeded with endothelial cells (means = 8.4 x 10(6)) derived from collagenase digestion of autologous omental fat samples. The contralateral graft of each pair was nonseeded. At 5 weeks postoperatively, the grafts were harvested and evaluated. The mean patencies of both the seeded and nonseeded grafts were 89 percent. The mean thrombus-free surface area for seeded grafts was 95 +/- 11 percent. This value was significantly different statistically from the mean thrombus-free surface area of nonseeded grafts, which was 43 +/- 19 percent (P less than .05). Histologically, midgraft regions of seeded grafts were cellular, stained positive for collagen, and were characterized by inner capsules ranging in thickness between 35-94 microns. Luminal cells were identified as endothelial by peroxidase antiperoxidase staining techniques. Midgraft regions of nonseeded grafts demonstrated thrombus accumulation, limited cellularity, and inner capsules between 59-194 microns thick. Scanning electron microscopy of seeded grafts revealed smooth luminal surfaces with tight junctions between adjacent cells; surface cells were not present on midgraft regions of nonseeded grafts. In conclusion, endothelial cells derived from omental fat successfully surfaced on Dacron grafts and imparted characteristics to the graft that would predict long-term graft success.     

Use of myxalin for improving vascular graft healing: evaluation of biocompatibility in rats.

Myxalin is a glycopeptide extracted recently from a gram-negative bacterium. It has blood anticoagulant properties and can enhance endothelial cell growth. With the ultimate objective of using this bioactive molecule to promote vascular graft healing, this study assessed its biocompatibility in vivo by comparing the cellular and immunological responses of gelatin-coated knitted polyester grafts with and without myxalin following implantation in the peritoneal cavity of rats for prescheduled periods of 3 days and 1, 2, and 4 weeks. A nongelatin-coated virgin polyester graft was included as the reference material. The biological response to gelatin alone was characterized by a slower rate of cellular infiltration into the implant, reduced collagen synthesis, and higher levels of acid phosphatase and esterase activity in the surrounding tissue. The addition of myxalin to this coating resulted in a significant reduction of hydrolase secretion in the tissue surrounding the implant and an enhancement of cellular ingrowth.     

髓核组织对椎间植骨融合的影响

目的：观察椎间盘髓核组织对椎体间植骨融合的影响。方法：11头体重约35kg的丹麦猪接受L3／4和L5／6两个节段的前路椎体间融合器植骨融合手术。其中一个节段在植骨的同时把本节段的全部髓核混合于植骨材料内（A组），另一节段则去除髓核组织行单纯植骨（B组），手术后6周处死猪，分别进行椎间融合器内骨置DEXA骨密度的检查和骨组织计量学检查。结果：A组和B组骨密度分别为0．109和0．150mg/cm^2，两组之间统计学上无显著性差异（P＞05），A组和B组新生骨和未吸收骨骨量分别为9．2％和9．3％，11．1％和12．3％，无显著性差异（P＞0．05），结论：在6周的观察时间内，猪的髓核组织并不影响椎体间植骨的融合。     

Small intestinal submucosa as a small-diameter arterial graft in the dog

Autogenous saphenous vein, human umbilical vein, modified bovine collagen, Dacron, and PTFE have been used as small-diameter arterial grafts with moderate success. We tested autogenous small intestine submucosa as a small-diameter arterial graft in both a carotid and femoral artery (mean ID 4.3 mm) of 18 dogs (total of 36 grafts). All dogs received aspirin and warfarin sodium for the first 8 weeks after surgery. Graft patency was evaluated by Doppler ultrasound techniques and angiography. Two grafts ruptured and 5 grafts occluded by 21 days after surgery. One graft became occluded at 14 weeks. Fifteen dogs were sacrificed at periodic intervals until 48 weeks after surgery. Patent grafts had no evidence of infection, propagating thrombus, or intimal hyperplasia. Graft aneurysmal dilation occurred in 4 grafts (11%). The grafts were composed of a dense organized collagenous connective tissue with no evidence of endothelial cell growth on the smooth luminal surface. Three dogs are alive at 76 to 82 weeks after surgery. Overall, graft patency was 75%. Graft patency after cessation of anticoagulation therapy was 92.3% (12 of 13 grafts). We conclude that autogenous small intestinal submucosa can be used as a small-diameter arterial graft in the dog and is worthy of further investigation.     

Prosthetic vascular graft

Since the first application of the prosthetic vascular graft for the operation of aneurysm of the abdominal aorta, surgeries with prosthetic grafts were carried out among 573 patients, that is, 445 with aortic diseases, 48 with diseases of the periferal arteries and 75 with congenital heart diseases, respectively. In this paper, our clinical experiences of using the prosthetic grafts during last 25 years were described at first, and then, some problems of the prosthetic graft in the present and future were discussed.     

Effect of vascular endothelial growth factor on laryngeal wound healing in rabbits.

OBJECTIVE: Study the effects of vascular endothelial growth factor (VEGF) on laryngeal wound healing in a rabbit model. STUDY DESIGN: Prospective, randomized, blinded. METHODS: The anterior cricoid cartilage of 10 rabbits was split and a VEGF-soaked collagen sponge was sewn between the cut edges. In 10 control animals, the collagen sponge was soaked with phosphate-buffered saline solution. The larynx was harvested on day 10. The degree of epithelial closure, the degree of soft tissue closure, and the presence of inflammatory cells was graded. RESULTS: There was complete epithelial closure in the control group. There was a slightly higher, but not statistically significant, grade of soft tissue closure in the experimental group. The experimental group had a lower but not statistically significant acute inflammatory response score. CONCLUSIONS: The topical application of VEGF through an implanted collagen sponge to an anterior, subglottic incision in a rabbit has no significant effect on tracheal luminal epithelial closure, acute inflammatory response, or soft tissue repair at postsurgical day 10.     

Composite porosity of expanded polytetrafluoroethylene vascular prosthesis

Polytetrafluoroethylene (PTFE) prostheses were modified to produce two types of composite porosity PTFE grafts: type I — inner 60 μm/outer 20 μm and type II — inner 20 μm/outer 60 μm. These composite porosity PTFE designs were investigated for bleed through, graft healing and patency and compared with high porosity PTFE (60 μm) and standard PTFE (20 μm) grafts. The grafts were implanted into the carotid and femoral arteries of dogs and retrieved after 4, 12 and 18 weeks. Both composite porosity grafts showed significantly less bleed through than standard and high porosity grafts after reperfusion. In composite grafts, the 60-μm layer allowed fibrovascular tissue and histiocyte ingrowth from perigraft tissue, but the 20-μm layer did not. Neointima formation occurred earlier and endothelialization was more extensive in high porosity grafts, but seroma formation occurred in 25% of cases. In type I composite porosity grafts, smooth muscle cells of neointima migrated into the pores of the graft, providing a firmly anchored intima. Type II composite porosity grafts allowed better ingrowth of fibrovascular tissue at the outer layer from the perigraft tissue; however, endothelialization was not completed. Composite porosity grafts should be considered for evaluation in specific clinical situations.     

The value of vein graft surveillance in bypasses performed with small-diameter vein grafts

We assessed the impact of preoperative diameter of the venous conduit on reintervention rate and outcome following infrainguinal vein graft bypass. Consecutive infrainguinal vein bypasses between January 2001 and December 2006 were reviewed. All patients underwent preoperative measurement of vein graft diameter (VGD). Grafts were classified into those with VGD <3.5 mm and those with VGD > or =3.5 mm. All patients were enrolled in a duplex surveillance program. The association between VGD and reintervention rate was assessed. Graft patency and amputation rates were compared. There were 377 bypasses followed up for a median of 23 months (range 8-67). VGD was <3.5 mm in 139 grafts (36.9%) and > or =3.5 mm in 238 grafts (63.1%). A higher proportion of smaller vein grafts (32.3%) required reintervention to maintain graft patency compared with larger conduits (20.2%) (chi(2) = 7.7, p < 0.001). VGD (odds ratio [OR] = 2.87, 95% confidence interval [CI] 1.63-3.81; p < 0.001), smoking (OR = 1.83, 95% CI 1.39-3.20; p = 0.02), and type of bypass (OR = 1.86, 95% CI 1.49-2.47; p = 0.02) were variables associated with higher reintervention rate. There was no difference in graft patency (p = 0.13) or amputation rates (p = 0.35) between the two groups. Use of smaller vein grafts was associated with a higher reintervention rate. Provided that these grafts are surveyed and where necessary repaired, the use of smaller vein grafts is successful and expands the availability of autogenous conduit for infrainguinal arterial reconstruction.     

Sequential studies of healing in endothelial seeded vascular prostheses: Histologic and ultrastructure characteristics of graft incorporation

Chronological events leading to incorporation of endothelial cell seeded prosthetic vascular grafts were documented in this investigation. Forty-one adult dogs underwent thoracoabdominal bypass using double-velour Dacron grafts. Experimental grafts were preclotted with blood containing enzymatically derived endothelium immediately after derivation, or after 14 days of cultivation. Control grafts were preclotted without addition of endothelial cells. Grafts were studied grossly as well as by light, scanning electron, transmission electron, and fluorescence microscopy, 1 to 28 days postimplantation. Control graft healing proceeded from pannus and perigraft ingrowth. Experimental grafts healed from seeded cells as well. Platelets covered all grafts by Days 1 and 2. Thrombus accumulations on control grafts, first evident on Day 4, became maximal by Day 14. Seeded grafts appeared relatively thrombus free with patches of endothelial cells noted by 4 days. These cells were initially separated by gaps, often containing leukocytes. Endothelium became densely packed with cellular migration and proliferation. Subendothelial tissues were composed of fibrin and smooth muscle. Control and experimental grafts were approximately 10 and 80% endothelialized, respectively, by Day 28. Smooth muscle dominated subintimal tissue in experimental grafts. These cells initially appeared fibroblastoid. Endothelial seeding enhances both pseudointimal development and rapid graft incorporation.