Human Umbilical Vein and Other Biografts for Atherosclerotic Lower Extremity Occlusive Disease

Linda M. Harris and Maciej Dryjski

The choice of intervention and conduit for lower extremity arterial reconstructions when the ipsilateral greater saphenous vein (GSV) is not present remains controversial. Many reconsider less optimal endovascular interventions, and others opt for use of the contralateral GSV, followed by other autogenous veins, sometimes used as composite grafts. When no autogenous conduit is available, options for revascularization include prosthetic grafts, modified biologic grafts, endovascular recanalization, or limb amputation.

This has led to exploration of other biologic tissues, such as human umbilical vein (HUV) grafts, fresh and cryopreserved venous and arterial homografts, and bovine heterografts. The potential benefit of biografts is the availability of an off-the-shelf graft that handles similarly to autogenous tissue and that has presumed improved patency and decreased risk of infection.

There has been variable level of enthusiasm regarding modified biologic grafts over the past several decades. The development of new endovascular techniques and the inherent problems with biologic grafts have diminished their role in treatment of vascular diseases. The choice to use a biologic graft currently is usually influenced by tissue loss, active infection, and small vessel caliber. Unfortunately, few studies directly compare the efficacy of modified biografts against prosthetic grafts.

Human Umbilical Vein Grafts

The use of glutaraldehyde-tanned umbilical grafts was first reported by Dardik in 1976. The original umbilical grafts were prepared by manually stripping the human umbilical cord tissue and stabilizing the vessel wall with glutaraldehyde. More recently, grafts were prepared with computer-controlled mechanical lathes, giving a uniform wall thickness, and a Dacron mesh was added on the outer surface to decrease risk of aneurysmal degeneration. The process of glutaraldehyde tanning resulted in a conduit devoid of endothelium. The graft was not at increased risk for thrombosis caused by rejection, but it lacked the antithrombotic benefits of an endothelial lining. Previously, the HUV graft was available from Synovis Life Technologies (St. Paul, MN), but it is currently off the market and there are no imminent plans to return it, mainly because of U.S. Food and Drug Administration (FDA) requests for tracing the umbilical cord to the infant to decrease risk of transmittable diseases.

Technical Features

HUV grafts do not tolerate traction in the tunneling process and require passage through a tube during their placement to prevent damage to the graft. Similarly, standard vascular clamps were not applied to these grafts; tourniquet control was used to avoid damage to the conduit. Thrombosed grafts were difficult to declot with balloon catheters. To decrease the risk of anastomotic aneurysm formation, the manufacturer suggested that the entire wall, including supporting Dacron mesh, be incorporated in the suture line. Finally, postoperative anticoagulation was also strongly recommended.

Patency Data

Dardik, in his initial experience with 907 bypasses performed for critical limb ischemia in 95% of patients, found a 5-year assisted primary patency rate of 57% for femoropopliteal and 33% for femorocrural bypasses. After the graft was modified, the same group reported on 71 additional infrageniculate bypasses, with a 58% primary patency for below-knee popliteal reconstructions and 36% for crural reconstructions. Secondary patency was 64% and 41%, respectively. Results of patency and limb salvage from other studies of HUV were similar for bypasses above the knee but were variable for more distal bypasses (Table 1).

TABLE 1

Patency for Human Umbilical Vein Grafts

Author	Year	No. of Grafts Implanted	Patency of Popliteal Bypass (%)	Patency of Crural Bypass at 5 Years (%)
Dardik	1988	907	57	33
Batt	1990	105	—	29 (3 years)
Sommeling	1990	227	55% below knee, 57% above knee	19
Dardik	1995	71	58	36
Sato	1995	111	—	60.9
Johnson (VA)	2000	261	49	—
Dardik	2002	283	56	43

Comparative Studies

In 1982, Cranley and colleagues reported a 3-year patency of 59% for saphenous vein, 35% for polytetrafluoroethylene (PTFE), and 31% for HUV for tibial reconstructions for critical limb ischemia. The New England Society of Vascular Surgery Registry in 1991 compared HUV and extended PTFE (ePTFE) and found improved 5-year patency with HUV over ePTFE for femoropopliteal above-knee bypass (69% vs. 45%) and for below-knee bypass (45% vs. 22%). A Veterans’ Affairs (VA) randomized prospective study of HUV found a 5-year assisted primary patency of 53% for above-knee bypasses with HUV versus 73% for GSV and 39% for ePTFE. However, patients with HUV had the highest early thrombosis and amputation rate. Other randomized studies comparing HUV to ePTFE have found higher patency rates for HUV at 3 to 6 years, ranging from 42% to 75% versus 22% to 34% for PTFE.

Complications

Complications associated with umbilical vein grafts included thrombosis, infection, and aneurysm formation. The incidence of infection for umbilical vein grafts was found to be similar to that for saphenous vein grafts, at 3% to 4.3%.

The primary concern with HUV has been aneurysmal degeneration, which initially developed after 5 years. Dardik, in his initial study, found that after 5 years, up to 36% of patent grafts became aneurysmal. However, the second generation of HUV, modified in 1990, appeared to have a reduced risk of biodegradation. Dardik followed patients over 10 years since the modification of the graft and reported on 283 lower extremity bypasses, with no aneurysmal degeneration.

Cryopreserved Vascular Conduits

The initial idea to use venous allografts as a vascular conduit was explored in 1912 by Carrel in canine experiments. Linton in 1955 and Dye in 1956 reported the first clinical experiences with these allografts. The cryopreservation process was initially thought to maintain viability of the endothelial cells while decreasing antigenicity. However, it is now well known that cryopreservation does not decrease the immune reaction. Most studies have documented at least partially intact endothelium after cryopreservation. Endothelial cells are key in maintaining antithrombotic properties, fibrinolytic activity, and prostacyclin production, which have been documented in cryopreserved veins. However, cryopreserved veins do appear to accumulate low-density lipoprotein (LDL) cholesterol at a higher rate than nonpreserved veins in vitro, suggesting some loss of endothelial integrity. The smooth muscle cell layer in implanted grafts is eventually replaced by fibrotic layer a few months after implantation. This is most likely a result of an immune reaction resulting in cellular destruction and fibrosis.

Preparation of the Allograft

All allografts are tested for communicable fungal, bacterial, and viral diseases (e.g., hepatitis, HIV). Consequently, there has been no known case of viral or bacterial transmission of infection with cryopreserved vascular implants.

Cryolife (Kennesaw, GA) provides commercially available allografts that are shipped frozen in a solution of dimethyl sulfoxide (DMSO)/chondroitin sulfate until ready for use. Over the years, several grafts have been available, including greater saphenous vein, femoral vein, femoral artery, and aorto-bi-iliac arteries. Different lengths of greater saphenous vein are available, but the vein itself is harvested from the level of the femoral vein to the ankle. Cryopreserved femoral veins are shorter, but the minimum diameter is 5 mm and the minimum length is 5 cm. Femoral arterial cryografts are also available and are harvested from the iliac artery to the popliteal fossa. The aorto-bi-iliac grafts include the renal arteries and iliac branches. All grafts are harvested from organ donors within 24 hours of cessation of circulation. Subsequently, this tissue is placed in cryoprotectants and stored in a vapor phase of liquid nitrogen in temperatures between −110°F and −196°F.

The grafts should be seromatched for ABO and Rh blood type to decrease risk of rejection. After proximal and distal target vessels have been identified, the graft is thawed by submersion in a warm water bath (37°C to 42°C), which allows rapid thawing without cellular damage. The graft is then rinsed in a series of three provided solutions before implantation. The Cryovein handles similarly to autogenous vascular tissue. The graft is placed through a tunnel in similar fashion to the normal GSV. A subcutaneous tunnel is preferred to allow easier access for late revisions. Patients are heparinized and the vein is typically placed in a reversed fashion.

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