Healing mechanisms of high-porosity PTFE grafts: significance of transmural structure.

A high-porosity structure facilitates endothelialization of polytetrafluoroethylene (PTFE) grafts. The mechanism for endothelial coverage, however, has been controversial. This study was designed to clarify the healing mechanisms of high-porosity PTFE grafts. Four types of PTFE grafts (n = 48) were studied after implantation in both carotid and femoral arteries of dogs. The grafts were standard (ST) PTFE [mean internodal distance (MID): 30 microm]; high-porosity (HP) PTFE (MID 90 microm), composite porosity (CP) PTFE (MID: inner layer, 90 microm; outer layer, 30 microm); and polyurethane-coated high-porosity (PCHP) grafts which had the outer surface covered with nonporous polyurethane. Patency rates at 18 weeks were ST 6/12, HP 8/12, CP 6/12, and PCHP 3/12 (P = 0.290). The rates of endothelialization (%, mean +/- SD) in the patent grafts were ST 25 +/- 7, HP 75 +/- 20,* CP 57 +/- 15,* and PCHP 17 +/- 5 (*P < 0.05 vs ST or PCHP). Rich transmural tissue incorporation was observed in the HP grafts. In contrast to the HP grafts, PCHP grafts had no outer tissue ingrowth, inner tissue incorporation was scant, and anastomotic intimal hyperplasia was pronounced. The edges of pannus ingrowth in the PCHP grafts were irregular, such that the anchoring of pannus was weak and easily detachable. Well-developed endothelial cells were observed in the HP and CP grafts, but nonendothelialized areas always occurred in the center of the grafts. Capillary openings were noted in the HP grafts; however, their number was small (0-8/graft) on electron microscopy and did not account for the degree of endothelialization observed. We conclude that the principal mechanism for endothelialization of PTFE grafts is ingrowth from the anastomoses rather than transmural endothelialization, even in high-porosity grafts. Transmural fibrocapillary incorporation of PTFE vascular grafts provides a key supporting structure essential to the progression and attachment of endothelial ingrowth from the anastomoses.