Vascular implants for peripheral arterial bypass and aortic aneurysm repair - Biomaterials and Devices for the Circulatory System - page 235

and composition of the oxide layer are known to infl uence the corrosion
behaviour, manufacturers can apply selective surface treatment such as
chemical passivation and electropolishing to improve corrosion resistance.
However, when the stent strut is subjected to pulsatile deformations, the
oxide layer may be disrupted with each deformation and the passivation
and repair process is therefore not fully completed between deformation
cycles. Further optimisation of the surface treatment or the use of protec-
tive coatings may be required in order to improve the biostability of the
stents.

8.8.4 Fenestrated endovascular aneurysm repair

In a patient with a short aneurysm neck, the use of a bare metal stent
extending above the fabric at the top of the stent-graft can recruit aorta at
and above the level of the renal arteries for enhanced fi xation. The bare
metal stent struts may lie across the origins of the renal arteries but since
the wires are thin, they appear not to cause early or late occlusion of these
arteries (Burks et al. , 2002; Morrissey et al. , 2002). With standard endolumi-
nal devices, a minimum length of healthy aorta between the lowest renal
artery and the aneurysm, i.e. a neck, of 10 mm is required (Leurs et al. , 2006)
to provide secure fi xation and good sealing. However, patients with juxtare-
nal aneurysm with either a short or absent neck cannot be treated by EVAR
with conventional devices. In recent years, fenestrated stent-grafts have
been introduced to treat these aneurysms. These devices are individually
customised with fenestrations (circular, elliptical or scallop shape) to pre-
serve fl ow to essential aortic side branches while ensuring an adequate
proximal seal at and above the level of the renal arteries. In order to main-
tain the strength of the fabric and ensure that the fenestrations remain fully
open, nickel titanium wire reinforcements, which are securely fi xed around
the periphery of the openings, are provided. One potential complication
with fenestrated stent-grafts is that of target vessel loss. This may occur
during the procedure as a result of 'shuttering' of the vessel orifi ce due to
malpositioning of the device. Later vessel loss is associated with migration
which leads to compression or even fracturing of the stent within the
fenestration.

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8.9 Conclusions

Although ePTFE and Dacron are widely used and perform well in large
diameter surgical revascularisation procedures, they are much less success-
ful for small-diameter applications. The autologous vein remains the conduit
of choice for vascular grafts of diameter of less than 6 mm. Precuffed and
heparin-bonded ePTFE grafts have been introduced to reduce IH. In the