It was originally thought that a high porosity (knitted) graft was necessary to allow endothelial
cells and their subendothelial matrix to invade and adhere to the inner graft surface, reducing
thromboembolism and increasing patency. Although prosthetic vascular grafts in canine and
non-human primate models may develop a neoendothelium, re-endothelialization occurs only
sporadically or not at all in man. The patency of woven and knitted grafts was compared in a
study in which bifurcation grafts, one limb of which was knitted and one limb woven, were
implanted in 143 consecutive patients with aortoiliac atherosclerosis or aneurysms. There was
no difference in patency rates between the woven and knitted limbs during observation
periods of 1 month to 2 years.
Knitted grafts are designed for abdominal and peripheral vascular procedures. In general,
they are easily sewn to blood vessels because of their soft, compliant nature. The compliance
of the graft allows a snug fit, even when the native vessel is heavily calcified, and minimizes
bleeding at anastomotic sites. However, removal of the clamps is followed by bleeding through
the graft until clot forms in the interstices. Blood loss can be considerable, and use of these
grafts is contraindicated in patients receiving heparin, due to the likelihood of uncontrollable
haemorrhage. A modification of the standard knitted graft is the double velour graft (Medox)
which incorporates a velour inner and outer pile to enhance the incorporation of clot in the
graft interstices. Dacron grafts of less than 5 mm in diameter should not be used since they are
associated with a high incidence of thrombosis. Such grafts are therefore not suitable for the
treatment of obstructive disease below the knee.
Woven grafts are designed specifically for use in patients who require systemic heparinization
during graft insertion. They are thus ideally suited for replacement of the thoracic aorta. Graft
stiffness is not usually a problem when reconstructing the aorta after resection of fusiform or
sacular aneurysms, as the resection margins tend to be thick and fibrous, and incorporate
sutures and graft well. In patients with ascending thoracic aortic dissection, however, the
aortic tissues are extremely thin, tenuous, and friable. Attempts to sew a stiff, non-compliant
graft into these tissues are often fraught with further tearing, sometimes to the point of
avulsion of a suture line. On the other hand, low porosity is a requirement since patients
undergoing surgery for Type A dissections are on cardiopulmonary bypass and often develop
severe pre- or intraoperative coagulopathies. A satisfactory compromise has been developed
by Medox in the form of the low porosity Veri-Soft woven graft. In this case, the Dacron yarn is
soft enough to allow good flexibility, but the weave is tight enough to prevent haemorrhage
through the interstices. The Veri-Soft graft is effective as a replacement for the ascending
aorta; however, we also toughen the aorta with glutaraldehyde to ensure the stability of the
aortic tissues prior to performing the anastomosis.
All grafts, whether knitted or woven, can be preclotted, thereby eliminating the problem of
haemorrhage through the graft. Although routinely preclotting of Veri-Soft woven grafts is not