Cardiovascular stents - Biomaterials and Devices for the Circulatory System - page 174

Stewart et al. , 2009). As with PTCA restenosis, intimal hyperplasia is the
major component of lumen occlusion after stent implantation (Lui et al. ,
1999). Although stent implantation can prevent remodelling and elastic
recoil, the problem of ISR and consecutive need for re-intervention remains
the major limitation of the stenting procedure (Lui et al. , 1999; Lowe et al. ,
2002). Details about the biochemical and cellular mechanisms leading to
ISR can be found in the review paper by Santin et al. (2005).

7.3 Biomaterials for cardiovascular stents

The material and confi guration are important factors to be considered for
optimal stent design. The backbone of the stent material requires suitable
mechanical properties in order to sustain expansion and oscillations and to
remain at the required size on deployment. Clinical studies have clearly
demonstrated that stent construction and design infl uence the risk of stent
thrombosis, the degree of vessel wall injury, and subsequent intimal prolif-
eration (Lau & Sigwart, 1991; Lau et al. , 2004) . In addition, certain metallic
properties have been implicated in thrombosis, infl ammation and adverse
cellular responses with the release of different types of growth factors, all
circumstances thought to trigger the mechanism of fi bromuscular prolifera-
tion and the formation of vessel ISR (Al-Lamee, 2000). A further critical
consideration in terms of any implanted material is the consideration that
its stability will be signifi cantly infl uenced by the surrounding milieu as well
as any additives, impurities, manufacturing process and sterilisation. Any
material intended for prosthesis, including stents, needs to be chemically
and mechanically stable in the biological environment for long periods of
time. Therefore, a dual approach must be taken when evaluating any poten-
tial biomaterial, (i) study of the local and systemic effects of the biomaterial
and its biodegradation in the hostile but sensitive environment of the body
(biocompatibility) and (ii) study of the behaviour of the biomaterial over
time (biofunctionality).
Three main types of corrosion resistant alloys are used to produce the
many different designs of metallic stents. These are iron-chromium nickel
alloys such as stainless steel, cobalt-chromium based alloys and titanium
and its alloys. The most commonly used metallic materials for stents are
stainless steel, nickel-titanium (nitinol) and tantalum. These materials offer
good mechanical properties together with stability in the face of biodegra-
dation (Gotmann, 1997).

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7.3.1 Medical grade 316L stainless steel

The numerous different types of stainless steel available are categorised
into generic groups dependent on the relative amounts of the main phases