Cardiovascular stents - Biomaterials and Devices for the Circulatory System - page 179

Gold coatings

The coating of stainless steel stents with gold has been reported to reduce
thrombogenicity, thrombus mass and decrease neointimal formation and
infl ammation, possibly due to the smooth coated surface and lack of impuri-
ties (Tanigawa et al. , 1995; Alt & Schömig, 1998; Edelman et al. , 2001), and
impart improved visibility under fl uoroscopy (Herman et al. , 1998; Edelman
et al. , 2001). However, clinical experience of gold coated stents has been
less than impressive. Reports from clinical trials associated the use of gold
coated stents with increased neointimal proliferation and higher rates of
angiographic restenosis than uncoated stainless steel stents with increased
need of revacularisation procedures (Kastrati et al. , 2000; Park et al. , 2002;
Dahl et al. , 2002; Danzi et al. , 2002; Reifart et al. , 2004). More recently, a 5
year follow-up study confi rmed these earlier fi ndings, reporting that com-
pared with uncoated steel stents, gold coated stents were associated with a
sustained increased overall risk for major cardiac events with enhanced
neointimal hyperplasia over a long period of time and that the sustained
risk of restenosis may increase the long-term mortality risk (Pache et al. ,
2006).

Effect of biological components on metal ion release

Metallic materials themselves do not show toxicity, but dissolved metal ions
and corrosion products may show toxicity when bound with biomolecules
(Hanawa, 2002). Indeed, the ability of the essential trace elements, which
constitute stainless steel, to promote not only different disease processes
(Fraga & Oteiza, 2002; Rocher et al. , 2004) but also cellular activation has
been reported (Niki et al. , 2003; Voggenreiter et al. , 2003). However, it is as
yet unknown whether metal ions released from stent devices cause long-
term complications in the progression of ISR. Studies have shown metal
ions to (i) promote fi broproliferative responses around implants (Heintz,
2001), (ii) stimulate the production of pro-infl ammatory cytokines (Lui
et al. , 1999), (iii) be toxic to the cells (Cragg et al. , 1993) and (iv) cause
overall immunogenic reactions (Bertrand et al. , 1998; Koster et al. , 2000).
Body fl uids contain ions H + , Cl − , Na + , PO 4 3− , HPO 4 2− , H 2 PO 4 − , all of which
are known to infl uence metallic corrosion. In addition, the concentration of
dissolved oxygen in venous blood is a quarter that of air, and a reduction
in oxygen is known to accelerate corrosion of some metals as oxygen
replenishment maintains the protective oxide fi lms (Trethewey & Cham-
berlain, 1998). Nickel, chromium and molybdenum have all been found to
elute from stainless steel stents (Gutensohn et al. , 2000).
It is known that coronary artery stenting can induce the host response
(Flesser & Leclerc, 1997). The release of metal ions by biomaterials is

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