Cardiovascular stents - Biomaterials and Devices for the Circulatory System - page 195

Spaulding et al. , 2007; Lagerqvist et al. , 2007; Mauri et al. , 2007; Windecker
& Meier, 2007). Indeed, very late stent thrombosis in fi rst generation DESs
is more common than in bare metal stents due to delayed healing and
re-endothelialisation.
Initial concerns relating to the risk of death on intermediate/long-term
follow-up in patients receiving DESs rather than BMSs, was shown by one
study to equate to an adjusted relative risk for death of 1.32 (95% CI,
1.11-1.57) between six months and three years (Lagerqvist et al. , 2007). The
main factor believed to account for such discrepancy was the enhanced
effect of anti-platelet drug cessation and subsequent occurrence of delayed
stent thrombosis in DESs versus BMSs (Pfi sterer et al. , 2006), but stent
diameter and left ventricular ejection fraction have also been identifi ed as
predictors of thrombosis. Therefore, despite improving on BMSs, it is clear
that drug elution fails to provide defi nitive answers to ISR. Understanding
the mechanism underlying neointimal formation thus becomes ever more
important in underpinning future stenting advances.

7.6.2 Second generation drug eluting stents

A second generation of stent coatings to allow drug release has been devel-
oped with the application of a new range of polymeric carriers developed
with the benefi t of lessons learnt from the clinical outcome of their prede-
cessors (O'Brien & Carroll, 2009). Abbott Laboratories have developed
the Xience V ® Everolimus-eluting stent that uses PMBA as its initial layer
on top of the cobalt-chromium stent surface, followed by a co-polymer
of polyvinylidenefl uoro-hexafl uoropropylene (PVDF-HFP) as the drug
holding matrix. Unlike the earlier described DES, this system does not use
a coating layer to control the drug elution rate. This second generation stent
has been shown to be associated with low rates of angiographic restenosis
and a reduction in repeat vascularisation rates compared with Taxus ® pacli-
taxel eluting stents (Serruys et al. , 2006a) and more inclusive randomised
trials including unselected patients are in progress to further evaluate the
effi cacy and safety of these stents.

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7.6.3 Biodegradable coatings for drug eluting stents

Owing to the concerns surrounding durable polymer coated DESs, biode-
gradable polymer-based DESs have been developed to provide improved
clinical effi cacy and safety. Two of the most ubiquitous polymers exploited
in biodegradable biomaterial applications are polylactic acid and polyg-
lycolic acid together with their co-polymer polylactic-co-glycolic acid.
As the polymer matrix degrades, producing carbon dioxide and water, the
entrapped drug is released at a rate dependent on the rate of hydrolysis of