Peripheral nerve revascularization: histomorphometric study of small- and large-caliber grafts

The revascularization of nerve grafts was investigated using histologic and morphometric techniques. Small-diameter nerve grafts (sciatic in the rat and sural in adult ewes) were studied, as was a large-diameter peroneal nerve graft in the ewe. Ninety-six hours after sciatic nerve engraftment, rats were injected with an intravascular fluorescent tracer. Evans blue albumin (EBA). Specimens were observed for the number of vessels perfused. Analysis showed no difference in vascular pattern between the grafted nerves and their control nerves, suggesting that spontaneous revascularization had occurred to establish a vascular tree essentially identical to the native nerve. Sural and peroneal nerve grafts were evaluated in adult ewes at 7 or 40 days post-nerve grafting. Similar to the rat sciatic nerve, the small-diameter sural nerve grafts were completely revascularized, with an equal number of perfused vessels at both time periods, with respect to control specimens. In contrast, the larger-caliber peroneal nerve grafts were not perfused at 7 days, and very poorly perfused at 40 days. This correlated with scant neural regeneration at 40 days. The finding suggests that small-diameter nerve grafts spontaneously revascularize, and revascularization using microvascular techniques is not necessary. In contrast, the larger-diameter nerve graft did not revascularize well. Such a large-diameter nerve graft would provide a suitable model to investigate the potential merits of a vascularized nerve graft.     

The effect of wall mechanical properties on patency of arterial grafts

Normal arteries have properties which match the low output impedance of the heart to the high peripheral impedance. These properties can be assessed in terms of compliance (% diameter change per unit pressure change) as well as by other haemodynamic parameters. Experiments were designed using vein, Dacron and expanded polytetrafluoroethylene (PTFE) in a low flow canine femoral artery bypass model. No graft group achieved perfect patency. At twelve weeks 80% of vein grafts, 30% of Dacron grafts, and 15% of PTFE grafts remained patent. The compliance of vein grafts was maintained despite marked thickening of the wall. Patency was correlated at a highly significant level with compliance. The studies demonstrate that the matching of the mechanical properties of grafts to host arteries is important in the design of successful synthetic arterial grafts.     

Dacron arterial grafts: the influence of porosity, velour, and maturity on thrombogenicity

It has been claimed that the neointimal healing of Dacron arterial prostheses can be enhanced by increasing porosity and including both an internal and an external velour layer. To test this, 24 patients received at random either woven (USCI, DeBakey, C. R. Bard, Inc.) or more porous, double-velour, knitted (Microvel, Meadox Medicals, Inc.) Dacron aortobifemoral prostheses. Graft thrombogenicity was measured using autogenous 111In-labeled platelets shortly following surgery and 6 to 9 months later. The thrombogenicity index was defined as the mean daily rise in the ratio of emissions over the graft to emissions over a reference area (aortic arch) and is a measure of platelet deposition. At early study the mean (+/- SE) thrombogenicity index was similar in woven and knitted graft patients at 0.19 +/- 0.4 and 0.14 +/- 0.2, respectively. In both groups it was lower (P less than 0.05) 6 to 9 months later at 0.06 +/- 0.2 (woven( and 0.08 +/- 0.1 (knitted), with again no difference between materials. Although platelet survival was restored to near normal values in both groups by 6 to 9 months, only one woven graft failed to demonstrate continued platelet accumulation by gamma-imaging. Thrombogenicity in Dacron grafts diminishes in the early months of maturation but is not affected by porosity and velour. Moreover, this thrombogenicity persists beyond the period of altered platelet survival.     

The relative influence of arterial pressure versus intraoperative distention on lipid accumulation in primate vein bypass grafts

Atherosclerotic degeneration has been well documented to be the limiting factor for long-term function of aortacoronary vein bypass grafts. Injury, including that induced by pressure distention in preparation for grafting, is thought to play a role in this degeneration. Injury can be minimized by limiting the distending pressure, but vein grafts are chronically subjected to arterial pressures that far exceed native venous pressure. We evaluated the relative influence of arterial pressure and of higher pressure of distention on cholesterol and apolipoprotein-B accumulation by grafts in our established animal model of graft atherogenesis. Grafts were interposed in the femoral arteries of eight normolipemic stump-tailed macaque monkeys. Before insertion, each vein was distended at 125 mm Hg (arterial pressure) for 1 minute with autologous blood, followed by distention of one half of the vein at 350 mm Hg for 1 additional minute. Grafts and ungrafted control vein were removed 3 months later. Cholesterol concentration in grafts distended at 125 mm Hg was 213% (p less than 0.01) and apolipoprotein-B concentration was 430% (p less than 0.001) of that in ungrafted control veins, whereas in grafts distended at 350 mm Hg cholesterol was 250% (p less than 0.01) and apolipoprotein-B was 925% (p less than 0.001) of the control concentrations. Although morphologic differences between the two groups of grafts were less profound than biochemical differences, foam cells were observed more frequently in grafts distended at 350 mm Hg than in those distended at 125 mm Hg. These data demonstrate that chronic exposure to arterial pressure has a significant effect on graft cholesterol that is proportionally greater than that caused by intraoperative distention at moderate pressure. Nevertheless, the detrimental effects of excessive distending pressures should not be ignored.     

The effects of laser energy on the arterial wall

Laser energy has been proposed as a method of resecting atherosclerotic plaque since the mid 1960s. However, only over the past several years have we come to understand some of the unique interactions of the laser with cardiovascular tissue. In laser angioplasty a major challenge has been choosing the optimal laser and duration of laser exposure to achieve adequate resection of plaque, while minimizing such complications as thrombosis, perforation, embolization, aneurysm formation, and accelerated atherosclerosis. Ultimately we must develop a more selective laser that resects plaque while leaving adjacent arterial wall uninjured. This review describes the physics of laser energy, the different lasers available for use in the cardiovascular system, laser-arterial wall interactions, and some of the limitations of laser angioplasty.Section Editor - Bruce L. Gewertz, MD, (Chicago, Illinois)     

The influence of denier and porosity on performance of a warp-knit Dacron arterial prosthesis.

The study objective was to determine the optimal water porosity for a warp-knit, externally veloured Dacron arterial prosthesis. Preclotting efficiency, surgical handling characteristics, thrombogenicity, and 56-day healing in the descending thoracic aorta of mongrel dogs were measured for seven different arterial prostheses of the same basic design constructed to test the influence of decreased porosity produced by increased yarn denier at a constant needle density. The six prototypes tested were 160, 180, 200, 220, 240, and 280 denier. The 140 denier prosthesis (Bionit, C. R. Bard, Inc., Billerica, MA) served as the control. The water porosity of these prostheses varied from 2036 ml/cm2/min at 120 mmHg for the control to 286 for the 280 prototype. The preclotting efficiency increased with decreased water porosity and with increased outer-wall filamentousness. Desirability of surgical handling characteristics decreased with lowered water porosity, while the degree of outer-wall filamentousness had little influence. All the prostheses showed excellent through-wall healing at 56 days, including endothelialization. The 160 prototype represented an optimal blend of high preclotting efficiency, desirable surgical handling characteristics, and excellent 56-day healing. These studies are pertinent to the future development of porous, warp-knit Dacron arterial prostheses.     

The effect of antiaggregating drugs on the patency of grafts in the arterial system

In experiments in dogs we studied the effect of anti-aggregating agents on the patency of prosthetic vascular grafts (4 mm in diameter) and vein grafts, both placed in areas with a rate of flow ranging between 50–60 ml/min. All 6 prosthetic grafts became occluded in the nonmedicated controls, 1 of 5 prosthetic grafts remained patent for a prolonged period of time in dogs receiving 250 mg acetylsalicylic acid 3 times a day, while 5 of 6 prosthetic grafts remained patent in animals receiving an antiaggregating mixture. Follow-up studies showed that antiaggregating medication had no effect on the patency of vein grafts, but it prevented the formation of mural thrombi and subendothelial proliferation.

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Résumé Nous avons étudié chez le chien l'effet d'agents anti-aggrégants sur la perméabilité de greffes vasculaires prothétiques (4 mm de diamètre) et veineuses ayant des débits de 50–60 ml/min. Les greffes prothétiques se sont toutes obstruées chez les 6 témoins non traités; chez 5 animaux recevant 250 m g d'acide acétylsalicylique 3 fois par jour, 1 greffe est restée perméable à long terme; sur les 6 animaux recevant l'agent anti-aggrégant, 5 greffes sont restées perméables. Pour les greffes veineuses, la médication anti-aggrégante n'a aucun effet sur la perméabilité; mais elle prévient la formation de thrombi muraux et la prolifération sous-endothéliale.

     

Microporous, complaint, biodegradable vascular grafts for the regeneration of the arterial wall in rat abdominal aorta

Microporous, complaint, biodegradable vascular grafts prepared from mixtures of polyurethane (PU) and poly-L-lactic acid (PLLA) can function as temporary scaffolds for the regeneration of the arterial wall in small-caliber arteries. This study was undertaken to determine the most suitable composition for PU/PLLA vascular grafts to ensure an optimal regeneration. Four types of PU/PLLA vascular grafts differing in percent weight of the PU/PLLA mixture, molecular weight of PLLA, and pore size were implanted into the abdominal aorta of rats (n = 32). Six weeks after implantation two implants of each graft type were evaluated by means of scanning electron microscopy and six implants were evaluated by means of light microscopy. In two types of the PU/PLLA vascular grafts, both of which were prepared from a 95%/5% weight PU/PLLA mixture with PLLA of molecular weight 500,000 but which had a different pore size, there was (I) absence of aneurysm formation and maintenance of arterial implant pulsations, (II) regeneration of a complete antithrombogenic neointima, (III) regeneration of a neomedia of comparable thickness to the media of normal rat abdominal aorta with the regeneration of elastic laminae almost throughout its thickness, and (IV) regeneration of a sufficiently supporting neoadventitia. These results demonstrate that a 95%/5% weight PU/PLLA mixture with PLLA of molecular weight 500,000 is the most suitable composition for PU/PLLA vascular grafts to ensure an optimal regeneration of a neoarterial wall that is of sufficient strength, compliance, and thromboresistance to function as a small-caliber arterial substitute. Pore size of these PU/PLLA grafts does not affect regeneration.     

The influence of thoracotomy and laparotomy on the chest wall movement

The influence of thoracotomy and laparotomy on the chest wall movement was examined using newly produced 6-channeled linear magnetometer. Two groups (one is the group of the patients who had undergone thoracotomy and laparotomy for esophageal cancer and the other is the group of the patients who had undergone laparotomy) were examined 2 to 8 weeks after operation. The chest wall movement was restricted analward at the operative wound of the chest wall. The other points had no remarkable restriction. From the view point of the chest wall movement this study shows thoracotomy and laparotomy at the same time and only laparotomy had almost the same influence.     

The influence of avascularity on the mechanical properties of human bone-patellar-tendon-bone grafts

Our aim was to analyse the effect of avascularity on the morphology and mechanical properties (tensile strength, viscoelasticity) of human bone-patellar-tendon-bone (BPTB) grafts in vitro. These were harvested at postmortem and stored submerged in denaturated human plasma at a constant pH, pO2, pCO2, temperature and humidity under sterile conditions. Mechanical testing was performed two and four weeks after removal of the graft. The mean ultimate strength was 1085.7 +/- 255.8 N (control), 1009.0 +/- 314.9 N (two weeks cultured) and 1076.8 +/- 414.8 N (four weeks cultured). There was no significant difference in linear stiffness or deformation to failure between the groups. There was a difference in viscoelasticity between the control group and the avascular grafts and the latter had significant lower peak load-to-load ratios after 15 minutes compared with the control group. After two and four weeks the graft contained viable fibroblasts. There was regular cellularity in the superficial layers and decreased cellularity in the midportion. The structure of the collagen including the crimp pattern appeared to be normal in polarised light. We conclude that avascularity does not significantly affect ultimate failure loads or stiffness of BPTB grafts. Slight changes in viscoelasticity were induced, but the significance of the increased stress relaxation is not fully understood.     

The influence of quality on the long-term patency of arterial grafts: the surgeon's key role in improving results in medium and small caliber vessels

The long-term patency of small and medium caliber grafts is directly affected by the surgeon's informed selection. In addition, when knitted synthetics are the appropriate choice, the methods used to preclot them for implantation will have a direct effect on their function.