Page 323 - Biodegradable Polyesters

12.2 Stenting Development 301

a clot, or thrombus, to form at the stented site – a phenomenon called throm-
bosis. This is extremely dangerous as a thrombus may break away from the stent
and cause a heart attack [8, 9]. The occurrence of stent thrombosis soon (within
30 days) after implantation is common in the case of BMSs, although the intro-
duction of antiplatelet therapy for patients who received BMS implantations has
greatly reduced the occurrence rates [9].

12.2.2
Coated Metal Stents

Owing to the common occurrence of restenosis and thrombosis after BMS
implantation [10], a need was seen to alter the surface characteristics of BMSs
without changing their mechanical properties as it seemed that restenosis
resulted mainly from the body’s chemical interaction with BMSs. Experimenta-
tion with coating BMSs began and a new category of stents was formed called
coated metal stents (CMSs). Several different coatings have been applied to
stents, including platinum, carbon, gold, silicon carbide, phosphorylcholine,
polymers, and titanium-nitride-oxide [8, 11] in an attempt to lower restenosis
and thrombosis rates. The results of coating stents were mixed and in some cases,
such as in the case of gold-coated stents, restenosis rates were higher than those
of uncoated stents [12].

12.2.3
Drug-Eluting Stents

The first stent coatings were passive and the train of thought had been to make
stents as benign and stable as possible to prevent undesirable responses within
patients’ vessels. Coated stents eventually evolved by way of a new train of
thought: what if the coating could play an active role in preventing the body’s
responses responsible for complications after stent implantation? Thus began the
era of drug-eluting stents (DESs), which are metal stents loaded with drugs which
are gradually released to provide local treatment to prevent restenosis. DESs were
a significant improvement over BMSs and CMSs but a problem which persisted
was the presence of a permanent, foreign object in the body. Furthermore, studies
on the outcomes of treatment with DESs revealed that late stent thrombosis rates
were sometimes higher than those of BMSs [13]. The cause of this may be that
endothelialization of the stent struts is delayed by drug elution [14].

12.2.4
Recap and the Next Phase of Stent Evolution: Biodegradable Stents

Taking a step back and considering the challenges faced by stent technology
throughout its evolution from BMSs to DESs, we notice that the initial complica-
tions faced after BMS implantation were acute, occurring soon after implantation,