Thrombin-free fibrin coating on small caliber vascular prostheses has high antithrombogenicity in rabbit model

Fibrin coatings on prosthetic vascular graft, which are conventionally produced by fibrinogen and thrombin, are expected to improve antithrombogenicity and healing characteristics. Thrombin is one of the factors of blood coagulation cascade; however, it has a possibility to play a negative role in the graft antithrombogenicity. The purpose of this study was to evaluate the performance of our new grafts, thrombin-free fibrin-coated small caliber vascular prostheses. Knitted polyester fabric vascular prostheses 2 mm in internal diameter were coated with fibrin coating with thrombin (Graft I) or without thrombin (Graft II). Both grafts were implanted in bilateral common carotid arteries of 35 Japanese white rabbits, with Graft I in one side and Graft II in the contralateral side. Graft patency, histology, thrombin activity, and platelet deposition were compared between both grafts on postoperative days (PODs) 1, 3, 7, 10, 14, 30, and 60. Both grafts were patent without thrombus or stenosis at each end point (maximal period, POD 60). Macro- and microscopic findings revealed that no obvious difference was observed between both grafts. Before graft implantation, thrombin activities in Grafts I and II were 0.711 +/- 0.086 and 0.009 +/- 0.007 optical density at 405 nm, respectively. Thrombin activity of Graft II was significantly less than that of Graft I in every period after graft implantation, and platelet deposition of Graft II was significantly less than that of Graft I until POD 30. Thrombin-free fibrin-coated vascular prostheses have superior performance of antithrombogenicity to conventional fibrin-coated vascular prostheses with thrombin.     

Homologous and heterologous connective tissue tube for vascular substitute

A newly-developed vascular graft having many advantages over the fabric vascular prosthesis was evaluated as aortic and vena caval substitutes. Alcohol-preserved connective tissue tubes, obtained from dogs and calves, supported by polyester mesh were implanted into the thoracic aortae of 61 dogs and the superior vena cava of 24 dogs. In the case of the aortic experimant, all the tested animals were healthy and each graft was patent at the time of removal. In the case of the vena caval experiment, all the tested animals were healthy, but 10 of 24 grafts were occluded. Aneurysmal dilatation, immune response and regressive changes of newly formed neointima were not observed in any case. Moreover, no blood leakage through the graft wall was observed. Leakage at the line of anastomosis was very little. The great advantage of the graft was good endothelization of the neointima. Between the grafts obtained from dogs and calves, there was no difference in the process of their healing.     

Homologous and heterologous connective tissue tube for vascular substitute.

A newly-developed vascular graft having many advantages over the fabric vascular prosthesis was evaluated as aortic and vena caval substitutes. Alcohol-preserved connective tissue tubes, obtained from dogs and calves, supported by polyester mesh were implanted into the thoracic aortae of 61 dogs and the superior vena cava of 24 dogs. In the case of the aortic experiment, all the tested animals were healthy and each graft was patent at the time of removal. In the case of the vena caval experiment, all the tested animals were healthy, but 10 of 24 grafts were occluded. Aneurysmal dilatation, immune response and regressive changes of newly formed neointima were not observed in any case. Moreover, no blood leakage through the graft wall was observed. Leakage at the line of anastomosis was very little. The great advantage of the graft was good endothelization of the neointima. Between the grafts obtained from dogs and calves, there was no difference in the process of their healing.     

Development and evaluation of a new polymeric material for small caliber vascular prostheses

Polyethylene oxide (PEO), because of its low levels of protein and cellular adsorption, may provide a suitable coating for synthetic small caliber vascular prostheses. PEO/polysiloxane networks were synthesized via an acid-catalyzed epoxy/hydroxyl crosslinking reaction and used to produce conduits with a 4-mm internal diameter. Three networks with nominal PEO molecular weights of 2000, 8000, and 20,000 and all 65% PEO by weight were studied. Blood compatibility was assessed by measuring 111In-platelet and 125I-fibrinogen deposition in a baboon ex vivo shunt, over a 1-hr time period and at a flow rate of 50 ml/min. Differences in material performance were noted particularly after the initial 30-min blood contact period. Materials in the mid and high PEO molecular weight range (8000 and 20,000) had significantly lower levels of platelet adsorption than networks of low PEO molecular weight (2000) at 30 min (P less than 0.005) and 60 min (P less than 0.05). The lowest level of platelet deposition was noted on networks of high PEO molecular weight (20,000). During the observation period, platelet accumulation on this surface was less than one platelet per 1000 microns. Platelet deposition on Gore-Tex was two and three orders of magnitude greater than that on the high molecular weight PEO material at 30 and 60 min, respectively (P less than 0.001). Fibrinogen adsorption was also lower on materials of mid and high PEO molecular weights, when compared with low molecular weight networks (P less than 0.05) and Gore-Tex (P less than 0.05). Scanning electron micrographs confirmed these observations. Overall, platelet and fibrinogen depositions are low for PEO networks, particularly for materials of high PEO molecular weight. This latter observation may be related to increased surface molecular mobility and a relative enhancement of PEO content at the blood-material interface.     

Tissue engineering of small caliber vascular grafts.

OBJECTIVE: Previous tissue engineering approaches to create small caliber vascular grafts have been limited by the structural and mechanical immaturity of the constructs. This study uses a novel in vitro pulse duplicator system providing a 'biomimetic' environment during tissue formation to yield more mature, implantable vascular grafts. METHODS: Vascular grafts (I.D. 0.5 cm) were fabricated from novel bioabsorbable polymers (polyglycolic-acid/poly-4-hydroxybutyrate) and sequentially seeded with ovine vascular myofibroblasts and endothelial cells. After 4 days static culture, the grafts (n=24) were grown in vitro in a pulse duplicator system (bioreactor) for 4, 7, 14, 21, and 28 days. Controls (n=24) were grown in static culture conditions. Analysis of the neo-tissue included histology, scanning electron microscopy (SEM), and biochemical assays (DNA for cell content, 5-hydroxyproline for collagen). Mechanical testing was performed measuring the burst pressure and the suture retention strength. RESULTS: Histology showed viable, dense tissue in all samples. SEM demonstrated confluent smooth inner surfaces of the grafts exposed to pulsatile flow after 14 days. Biochemical analysis revealed a continuous increase of cell mass and collagen to 21 days compared to significantly lower values in the static controls. The mechanical properties of the pulsed vascular grafts comprised supra-physiological burst strength and suture retention strength appropriate for surgical implantation. CONCLUSIONS: This study demonstrates the feasibility of tissue engineering of viable, surgically implantable small caliber vascular grafts and the important effect of a 'biomimetic' in vitro environment on tissue maturation and extracellular matrix formation.     

Experimental vascular graft using small-caliber fascia-wrapped fibrocollagenous tube: short-term evaluation

This study was undertaken to evaluate the application of autogenous fascia as a framework for a fibrocollagen tube for small-caliber vascular prostheses in Japanese white rabbits (n = 15). The fascia, measuring 10 x 40 mm, was harvested from the dorsal fascia and was carefully wrapped around a silicone rod of 1.5 mm in diameter. Then, the fascia-wrapped silicone rod was implanted into a subcutaneous pocket on the medial thigh. Four weeks later, the fascia-wrapped silicone rod was removed from the subcutaneous pocket. The rod was removed from the material, and the fascia-wrapped fibrocollagen tube was treated to make it antithrombogenic. Subsequently, the 1.5 x 10-mm fascia-wrapped tube was prepared as an arterial conduit. Using microvascular techniques, the tube was interposed into the divided femoral artery. Eleven of 15 grafts maintained patency over the follow-up period and no aneurysmal formation was found at any graft site. Microscopically, there was an ingrowth of endothelium with fibroblast proliferation from each end of the recipient vessel at 2 weeks after interposition; however, no neointima was found to line the center of the conduit. At 5 weeks after interposition, the neointimal growth rate was 70 percent. There was an ingrowth of endothelium with fibroblast proliferation from each end onto the entire internal surface of the conduit at 8 weeks after interposition. Further study is required before any long-term conclusions can be drawn.     

Promoting nerve regeneration through long gaps using a small nerve tissue graft

BACKGROUND: If nerve tissue is capable of inducing regeneration, as suggested by the neurotropism theory, then even small pieces of nerve tissue should have the potential to induce nerve regeneration. Therefore, long gaps might presumably be bridged via the neurotrophic potential of small pieces of nerve tissue grafted into the middle of the nerve gap. It is necessary to confirm the validity of the neurotropism theory and to also explore the potential usefulness of small nerve grafting through long gaps. METHODS: A small piece of nerve tissue was grafted into a silicone tube bridging a relatively long nerve gap in an attempt to promote nerve regeneration. A 15-mm gap was created in the left sciatic nerve of 31 Wistar rats (8 weeks of age). The experimental groups included one with nonvascularized nerve tissue grafted into a silicone tube with no distal nerve suturing (NV-A), another with vascularized nerve tissue grafted into a silicone tube with no distal nerve suturing (V-A), a third group with nonvascularized nerve tissue grafted into a silicone tube with distal nerve suturing (NV-P), a fourth group with vascularized nerve tissue grafted into a silicone tube with distal nerve suturing (V-P), and a group with no nerve segment grafted into the silicone tube (control). Electrophysiologic and histologic examinations were performed 10 weeks after the operation. RESULTS: No regeneration was obtained in the control group. Nerve regeneration was evident at the proximal end of the tube in the NV-A, V-A, NV-P, and V-P groups, and at the distal end in the NV-P and V-P groups. The degree of distal regeneration was extremely slight in the NV-A and V-A groups. An electrophysiologic examination performed in the NV-P and V-P groups revealed better results in the latter group. CONCLUSION: Small nerve grafts are capable of inducing nerve regeneration even over a long nerve gap, by grafting nerve tissue into the middle of the lesion using a silicone tube.     

Apicoaortic bypass using a vascular graft with a bioprosthetic valve

We report a case of 75-year-old man who underwent an apicoaortic bypass for severe aortic stenosis. The patient had a porcelain aorta accompanied by a severely calcified aortic annulus. We used a woven polyester vascular graft instead of a rigid apical connector because the latter material cannot be obtained in Japan. Postoperative examination showed no compression or stenosis in the apical outflow. A woven polyester vascular graft is therefore considered suitable for an apicoaortic bypass.     

Endoscopic harvesting of saphenous vein with a small caliber

Endoscopic saphenous vein harvesting (ESVH) requires a high degree of technical expertise. However, few studies have investigated the influence of SV caliber on the technical difficulty and outcome of ESVH. We analyzed 86 consecutive patients who underwent ESVH using a VirtuoSaph(trade mark) system. SV caliber was measured in the above-knee portion by ultrasound. Patients were then divided into two groups: group A (SV caliber <3 mm, n=16), and group B (the remaining patients, n=70). ESVH procedure time and SV characteristics were compared between the groups. In group A, the SV had a larger number of side branches (11.7+/-1.2 in A, 9.8+/-0.4 in B, P=0.043) and required a longer operation time (A, 57.5+/-3.7 min; B, 43.9+/-1.9 min, P=0.0024), whereas the time required for endoscopy did not differ. Graft length (A, 27.7+/-5.0 cm; B, 25.7+/-3.7 cm) and ratio of repaired side branches (A, 26.6+/-20.5%; B, 25.7+/-23.9%) showed no significant differences. ESVH using a VirtuoSaph(trade mark) system is feasible, regardless of SV caliber. SV with a caliber of <3 mm has a larger number of side branches, thus prolonging the procedure time.     

Fibrocollagenous tissue tube as a small arterial substitute

A new arterial graft, a fibrocollagenous tissue tube supported by polyester mesh, was evaluated as a substitute for a small caliber artery. The graft of 3 mm in internal diameter and 3.5 to 5.7 cm in length were implanted into the external iliac arteries of 24 dogs. The patency rate of the graft was 73% from 0 to 7 months following the operation. The newly formed vascular wall was soft and stable. the graft showed good endothelization of the neointima. Supported in part by the Scientific Research Encouragement Grant No. 387108 from the Ministry of Education of Japan.     

A baboon flow-regulated shunt for the study of small caliber vascular grafts

Synthetic vascular grafts often occlude when used in low-flow high-resistance reconstructions. In order to study the pathophysiology of graft failure a flow-regulated ex vivo shunt was designed for insertion into the baboon femoral artery and vein. Synthetic graft materials (4 mm i.d.) can be placed into the shunt circuit and studied at known rates of flow for uptake of ¹¹¹In-labeled platelets. Segments of the grafts can be removed from the shunt circuit at specified time intervals for morphologic study with scanning electron microscopy (SEM). In this paper the shunt model is described in detail and early experiments with it are reported. Labeled platelet uptake and SEM studies suggest that flow rate and graft composition influence the deposition of platelets and other blood components on graft surfaces. At high-flow knitted Dacron attracts large numbers of platelets, and becomes covered with a nearly confluent platelet-protein carpet within 1 hr. At low flow platelet uptake and development of the platelet-protein carpet is slower. Polytetrafluoroethylene (PTFE) attracts few platelets at high-flow rates, but at lower-flow rates begins to develop a platelet-protein carpet similar to that seen on knitted Dacron. Flow-related factors influencing platelet deposition are discussed and further experiments to be carried out with this model are described.     

Late occlusion at the anastomosis site of an extended polytetrafluoroethylene (E-PTFE) graft with small caliber

Recently expanded polytetrafluoroethylene (E-PTFE) graft is often used in arterial bypass for atherosclerosis obliterans. In our series of 38 patients (48 legs) undergoing femoropopliteal bypass with E-PTFE graft of 6mm-caliber, a cumulative patency rate was 40.8% at 36 months after operation and a mean duration of patency was 26.3 months. To clarify the cause of occlusion, the diseased grafts were removed and histopathologically examined. Macroscopically, the E-PTFE graft was occluded by the intimal hyperplasia, so called pannus ingrowth from the recipient artery into the graft. The pannus formation of the graft was more remarkable in the distal than in the proximal site of anastomosis, suggesting the influence of blood flow on its formation. Microscopically, no endothelial cells were observed inside the E-PTFE graft except for the surface of pannus. Thus, less extension of endothelialization was thought to be a cause of late occlusion in E-PTFE grafts. Most desirable is the autologous saphenous vein for femoropopliteal bypass. However, when it is not available, E-PTFE graft with a larger caliber must be used in combination with postoperative anticoagulant therapy.     

Endoscopic management of impacted ureteral stones using a small caliber ureteroscope and a laser lithotriptor

PURPOSE: We reviewed our experience with retrograde endoscopy using a small caliber ureteroscope and a laser lithotriptor in 104 consecutive patients with impacted ureteral stones and also analyzed the associated endoscopic findings. MATERIALS AND METHODS: From July 1993 to October 1999 we performed retrograde endoscopic treatment in 104 patients with impacted ureteral stones. Average maximum stone diameter was 15.2 mm. (range 2 to 110), and 9 patients had stones larger than 3 cm. Mean duration of impaction was more than 14 months and the longest was 10 years. We used 6.9Fr rigid or 6.9 to 7.5Fr flexible ureteroscopes and a pulsed dye laser or holmium:YAG laser lithotriptor. RESULTS: Of the 104 ureteral stones 100 (96.2%) were completely fragmented by a single endoscopic procedure. Although 1 uric acid stone could not be disintegrated by either the pulsed dye laser or electrohydraulic lithotripsy early in this series, it was discharged spontaneously 2 weeks after the procedure. Additional extracorporeal shock wave lithotripsy (ESWL*) was required in 3 patients who initially had stones larger than 3 cm. A month after treatment no patient had evidence of residual stones. Using adjuvant ESWL a 100% success rate was attained with minimal morbidity. Endoscopic observation revealed inflammatory polyps of the ureter in 22 patients (21.2%), and stricture adjacent to the stone in 16 (15. 4%). There were no significant complications. CONCLUSIONS: Using a small caliber ureteroscope and a laser lithotriptor we could treat the target stone in all 104 patients with adjuvant ESWL in 3 cases. Endoscopic lithotripsy seems to be effective first line therapy for chronically impacted stones which are frequently associated with chronic inflammation, polyps and strictures, and avoids the futile repetition of ESWL and problems related to the prolonged passage of stone fragments.