Enhanced endothelialization of Dacron grafts by external vein wrapping

Effect of intra-arterial environment on endothelialization and basement membrane organization in polytetrafluoroethylene grafts

PURPOSE

To determine if a complete intra-arterial environment affects endothelialization rate and basement membrane organization in polytetrafluoroethylene (PTFE) grafts.

METHOD

Thirty dogs underwent either infrarenal abdominal aorta PTFE interposition (12) or intraluminal stented (18) grafting. Grafts were explanted at 4 and 8 weeks and rate of endothelial ingrowth calculated. Endothelial cells were identified and basement membrane organization assessed using antibodies against endothelial cell-specific surface antigen CD31, type IV collagen, and laminin.

RESULTS

Endothelialization rates, expressed as percent graft surface area coverage per week, were 3.7% +/- 0.62% (4-week control), 12.9% +/- 0.58% (4-week stented), 4.2% +/- 0.62% (8-week control), and 10.0% +/- 0.54% per week (8-week stented grafts). Endothelial repaving rates were constant for control and increased in all stented grafts (P <0.01). At 4 weeks, laminin was identified in all control (6 of 6) and no stented grafts. Staining was confined to the 20% of the hyperplastic intimal area immediately below the endothelium. At 4 weeks, type IV collagen was present throughout the entire hyperplastic intima in control specimens but was confined to a discrete subendothelial zone in stented grafts. By 8 weeks, type IV collagen became concentrated in the luminal one third of the intima in control grafts.

CONCLUSION

Intra-arterial graft location is associated with early formation of an organized basement membrane and accelerated endothelialization in PTFE grafts.

Accelerated endothelialization model for the study of Dacron graft healing

Accelerated endothelialization was studied by creating a vascular tissue environment around porous Dacron grafts. Three study groups, each containing 10 dogs, were divided equally into 2- and 4-week implant periods, with 8 mm x 6 cm knitted Dacron grafts implanted in the abdominal aorta. Grafts in group 1, the control group, were implanted conventionally. In group 2 each implanted graft was completely wrapped in a resected segment of the autogenous inferior vena cava, with its intima against the wall. In group 3 the adventitial side of the vein was wrapped against the wall. The vein wrap produced accelerated endothelialization as follows: endothelial-like cell coverage scores at 2 and 4 weeks were, respectively, 78% and 98% for group 2 and 80% and 95% for group 3, compared to only 14% and 50% for group 1 (p < 0.05). The neointima, which contained smooth muscle cells, was formed as early as 2 weeks in the vein-wrapped grafts. There were no differences in the speed of healing or in healing patterns according to whether the intimal or the adventitial side of the inferior vena cava was placed against the graft. Histologic findings did not support the hypothesis that accelerated flow surface endothelialization results in direct migration of endothelial cells from the intima of the vein wrap, and there was no clear correlation between the surface endothelial-like cell coverage and microostia. To gain further insight into why accelerated healing occurs in this model, earlier observations accompanied by molecular biology analysis are needed, and vein wrap studies provide a method of comparison for this work.

Use of stents covered with polytetrafluoroethylene in experimental abdominal aortic aneurysm

PURPOSE

To establish the effectiveness of covered stents in the treatment of aortic aneurysms, to investigate the histopathologic healing patterns of the device, and to determine the long-term endurance and integrity of modified polytetrafluoroethylene (PTFE).

MATERIALS AND METHODS

Experimental aneurysms were created in dogs by enlarging the aortic lumen with a patch of abdominal fascia. After 5 months, eight animals underwent an endoluminal bypass. The bypass device consisted of a 6-cm-long stent covered with thin PTFE. After surgery, the animals were killed at 3, 6, and 12 months in groups of three, three, and two, respectively. Specimens were processed for luminal surface studies and cross-sectional histologic study. Explanted PTFE material was analyzed for its physical characteristics and performance and was compared with retained control samples.

RESULTS

Before the animals were killed, aortography showed patent bypass conduits in all animals, although two of eight had leaks into the aneurysmal sac. Endothelialized neointima largely covered the luminal surface of the PTFE stent. The percentage of prosthetic surface covered by tissue did not change from 3 months to 1 year. Physical testing of the explanted PTFE material showed no structural deterioration and no change in the internodal distance. Thickness and axial tensile strength varied 12% and 17% from controls, respectively.

CONCLUSION

Thin-walled PTFE seems to have physicochemical characteristics that make this material adequate for endovascular use. Though limited, this study supports the establishment of preliminary clinical evaluation of metallic stents combined with PTFE for the treatment of abdominal aortic aneurysm.