Tissue responses to implanted materials - Biomaterials and Devices for the Circulatory System - page 10

systemic infl ammatory response to these devices involves activation of the
endothelium and the coagulation system. Implantation of VAD results in
increased expression of adhesion molecules on endothelial cells. Markers
of thrombin generation and fi brinolysis are also elevated for at least 3
months (John et al. , 2009).

1.4 Hypersensitivity response

The majority of data reporting hypersensitivity reactions to implanted
devices is based on small studies. Implantation of cardiovascular devices
introduces a variety of potentially toxic materials to the body. This includes
Dacron polyester and polytetrafl uoroethylene in vascular grafts; titanium
alloys in circulatory assist devices, heart valves, conductive leads and pace-
maker cases; cobalt-chromium alloys for heart valves and circulatory assist
devices; and stainless steel for circulatory assist devices, guide wires and
vascular stents (Rosenberg, 2000; Ravi and Aliyar, 2005). Although there is
no evidence to suggest the presence of systemic toxicity from the implanta-
tion of these devices, implant materials have the potential to cause hyper-
sensitivity reactions, particularly in individuals who have had previous
exposure to the same material. Immunological effects may also be produced
by low molecular weight chemical stabilisers that are contained in trace
amounts within implanted devices (FDA, 1999).
There are four types of reaction, types I to IV, of which types II and III
have been considered to be less relevant to bioprosthetic implants by the
US Food and Drug Administration (FDA, 1999). Type I reactions are short
onset (within minutes) and mediated by IgE with clinical manifestations
including anaphylactic responses of vasodilation, oedema, smooth muscle
contraction and infl ammation. Type IV or delayed-type hypersensitivity
reactions are the commonest reactions associated with implanted materials.
They are mediated by T cells, occur at least 12 hours after exposure to an
allergen and can cause cellular and tissue injury. They classically result in
skin swelling, induration or eczema although also occur in deep tissues, as
in the case of cardiovascular implants. Adverse consequences of hypersen-
sitivity reactions can be classifi ed into two different entities: (1) symptoms
due to localised or systemic hypersensitivity response and (2) device mal-
function or failure (Honari et al. , 2008).
An example of the role of hypersensitivity in cardiovascular implants is
the potential association between hypersensitivity and complications of
coronary stents. Allergic reactions to bare metal coronary stents are most
likely caused by metallic ions. In-stent restenosis is probably caused partly
by contact sensitivity to these metallic components (Honari et al. , 2008).
Metals are also potential allergens in drug-eluting stents (DES). Polymers
used in the composition of DES can also induce infl ammatory reaction.

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