New Observations on the Healing Process in Prosthetic Substitution of Large Veins by Microporous Grafts - Animal Experiments

Abstract

Based on the experimental experiences in more than 180 implantations of different materials as venous substitutes segments of the inferior vena cava have been replaced in 34 dogs by Polyurethane (low microporosity) and modified e-PTFE prostheses (increased microporosity of 60 microns and 90 microns fibril length). The 12 months patency rate didn't differ between both tested optimized materials and ranged from 43 to 50%. After a follow-up of 12 months the grafts were taken out and analysed by light, immunofluorescence microscopy, scanning and transmission electron microscopy. In addition a new technique of microcorrosion casts was used for SEM-analyses. As a result a transmural microvessel system in the microporous meshwork of the prostheses with multiple orifices at the inner surface of the grafts could be demonstrated. Complete endothelialization was only observed in e-PTFE prostheses of high microporosity (greater than 60 microns fiber length). There is strong evidence that a full tissue incorporation of microporous artificial grafts mainly depends on a sufficient primary intramural deposit of blood components (fibrin, platelets, leucocytes), which initiates cell invasion from the surrounding tissue, accompanied by a highly developed microvessel network. A multifocal endothelialization takes place from the numerous microvascular orifices on the inner surface of the prostheses. Other sources such as pannus invasion or adhesion of multipotent cells from the blood stream play probably a very limited role.

Prosthetic vascular grafts: Wrong models, wrong questions and no healing

“…The fibrin layer itself is not as mighty as on knitted Dacron, some times giving a 'ghost impression' of the underlying prosthetic fibres. The outside capsule and scar formation is usually thin and dense [147,148,150] with occasional FBGC wrapping around PU fibres [73]. At 2 weeks the FBGC become more prevalent increasing until week 6 [153].…”

“…With very rare exceptions fibrillar PU grafts are largely impenetrable for transmural tissue ingrowth [91,147,148]. Only particularly large inter-fibrillar spaces allow complete fibro-connective tissue penetration throughout the wall thickness [49,73,93].…”

“…Particularly, anastomotic pannus tissue tends to be thicker on PU [149] than on expanded Teflon with capillary orifices opening onto the anastomotic endothelium [148,150]. From day one the midgraft region is continuously covered by a fibrin layer [151] hardly diminishing in thickness during the course of the 1st year [71,72,91,147,148,[150][151][152]. The fibrin layer itself is not as mighty as on knitted Dacron, some times giving a 'ghost impression' of the underlying prosthetic fibres.…”

Cardiovascular biomaterials: when the inflammatory response helps to efficiently restore tissue functionality?

“…After implantation, the mid‐graft region of the fibrillar polyurethane is covered by a fibrin layer that is thinner than on knitted Dacron, and the outer surface is encapsulated by thin and dense scar formation containing FBGC (Zilla et al ., ). Therefore, fibrillar polyurethane grafts are largely impenetrable for transmural tissue ingrowth (Hess et al ., ; Kogel et al ., ). Instead, some ingrowth occurs in microporous foamy polyurethanes whereas it is possible to reduce the inflammatory FBGC reaction by increasing pore size (Zilla et al ., ).…”

Effect of porosity on small-diameter vascular graft healing

“…Despite these encouraging results, transmural growth of perigraft capillaries has not resulted in complete graft healing in humans. [13][14][15] The influence of porosity on graft healing, particularly ePTFE graft healing, remains unclear, even though several investigators [16][17][18] have suggested that the mechanisms of endothelial coverage of prosthetic surfaces can be radically altered simply by changing the porosity of the graft. In the clinical setting, all synthetic grafts derive limited luminal endothelium from the cut edges of the adjacent artery.…”