Biomedical Engineering Reference
In-Depth Information
The degree of porosity of an ePTFE graft is controlled by the distance
between the nodes. In clinically used grafts this is of the order of 30
m
although experimental studies have shown that grafts with larger internodal
distance are associated with higher degree of endothelialisation (Hirabayashi
et al. , 1992). Expanded PTFE grafts are usually provided with an external
support coil to improve their resistance to kinking. They may in addition
have a thin reinforcing layer of low porosity ePTFE on the external surface.
Carbon and heparin coated ePTFE grafts have been introduced in
order to reduce the thrombogenicity of the base biomaterial and thereby
improve the patency of infrainguinal bypass. Prospective randomised
multicentre trials have shown that carbon coated grafts produced a small
benefi t but the results did not show statistical signifi cance (Barcourt,
1997; Kapfer et al. , 2006). Covalent bonding of heparin to the luminal
surface of ePTFE grafts has been shown to provide patency rates com-
parable to the autologous saphenous vein at one and two years for
above and below-knee femoropopliteal and femorocrural bypass (Bosiers
et al. , 2006; Dorrucci et al. , 2008; Daenens et al. , 2009). These studies
were non-randomised. However, a prospective randomised multicentre
trial was reported in the Gore Propaten ® Instructions for Use document
(W. L. Gore & Associates, 2006), involving 200 patients requiring above-
knee femoropopliteal bypass. The study compared the heparin bonded
Gore Propaten ® graft against the standard Gore-Tex Stretch graft (W.
L. Gore & Associates, Inc., Flagstaff, Arizona, USA). The primary and
secondary patency rates and limb salvage were similar and the heparin
did not appear to confer a signifi cant advantage over the standard ePTFE
graft.
μ
8.3.3 Polyethylene terepthalate (Dacron) grafts
Polyethylene terepthalate (PET) fi bres are used to manufacture fabric
grafts in two basic constructions: woven or knitted. The woven graft consists
of yarns of PET fi bres running alternately over and under each other, result-
ing in a tight and strong structure with relatively low porosity. Knitted grafts
are produced by looping yarns around needles. The number and size of the
needles and the diameter of the yarns determine the spacing between the
yarns and consequently the porosity of the fabric. Since the yarns of knitted
grafts are less tightly packed, knitted grafts are more conformable and are
easier to handle than woven grafts. Knitted grafts are much more porous
and require preclotting with the patient's blood prior to implantation.
However, the need for preclotting is obviated by the use of grafts pre-
impregnated with gelatine, albumin and collagen. Both woven and knitted
grafts are provided with circumferential crimps which are usually circular
but sometimes helical in order to improve their surgical handling and
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