Vascular implants for peripheral arterial bypass and aortic aneurysm repair - Biomaterials and Devices for the Circulatory System - page 213

respiratory, renal and cardiac complications precluding the elderly and
patients with signifi cant co-morbidities. Endovascular aneurysm repair is a
minimally invasive procedure introduced in the 1990s to exclude the aneu-
rysm from the circulation and prevent rupture while the aneurysm sac and
its contents remained in situ . This is achieved by means of a device consist-
ing of a fabric graft and a metal stent structure. The device, called a 'stent-
graft', is introduced through one of the femoral arteries and guided to the
aneurysm where it is anchored in place. The procedure is performed under
X-ray fl uoroscopy.
The aim of this chapter is to review the vascular implants currently
used in open vascular surgical reconstructions and endovascular proce-
dures for treating infrainguinal arterial disease and aortic aneurysm
repair.

8.2 Vascular stents

Stents are classifi ed as either balloon-expandable or self-expanding accord-
ing to their method of deployment. The choice of materials for both types
of stents is dependent on several factors, e.g. mechanical and physical prop-
erties, availability of fabrication techniques, good resistance to corrosion
and fatigue, biocompatibility and non-thrombogenicity.
Balloon-expandable stents require materials that can be deformed plasti-
cally through the infl ation of a balloon. The ideal material should therefore
have a low yield stress so that expansion of the stent can be achieved at
acceptable balloon pressure. On the other hand it should have high elastic
modulus so as to achieve high radial strength with low profi le stent struts
and minimal recoil after the stent is fully deployed. Stainless steel 316 L is
the standard material for balloon-expandable stents since it fulfi ls these
requirements well. Cobalt-chromium-based alloys which have higher elastic
modulus, tensile strength but similar yield stress to 316 L are also commonly
used.
Self-expanding stents are fabricated in their expanded size then com-
pressed into a delivery system. As they are released from the delivery
system, they self-expand to the preset size, relying on the elastic properties
of the material. These stents undergo large strains which should be fully
recoverable and the material that is most suited for this purpose is super-
elastic nickel-titanium alloy, nitinol.
The required properties such as high radial strength, low recoil, good
fl exibility and trackability are derived not only from the material used but
also the dimensions of the stent struts, the geometry (number of cells or
repeating units in the circumferential and longitudinal directions) and the
manufacturing process used to produce the stents.

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