Biomedical Engineering Reference
In-Depth Information
This chapter discusses several important points related to successful implan-
tology and surface interactions with biological environments.
5.3 REQUIREMENTS FOR SUCCESSFUL IMPLANT SYSTEMS
The implantation of devices for the maintenance or restoration of a body func-
tion imposes extraordinary requirements on the materials of construction. Fore-
most among these is an issue of biocompatibility. There are interactions between
the
foreign
material and the surrounding
host
living tissue, fl uid and blood ele-
ments. Some of these are simply adaptive. Others constitute a hazard, both short
and long term, to the survival of the living system [Long et al., 1998; Brunski
et al., 2000]. There are mechanical and physical properties which the material
must provide, and a structural nature which the system should exhibit. Some of
these govern the ability of the device to provide its intended function from a
purely engineering viewpoint.
Others — such as tribology (specifi cally friction and wear), corrosion and
mechanical compliance — signifi cantly relate to biocompatibility concerns. Human
implantation applications impose more stringent requirements on reliability than
does any other engineering task. In most applications, an implanted device is
expected to function for the life of the patient. As the medical profession becomes
more emboldened, the device lifetime must stretch to more than 30 years, if follow-
up maintenance is carefully and thoroughly performed. Yet, there are very few
engineering devices which have been designed to function for more than 30 years.
It is necessary to think in terms of reliability of performance of thousands of
devices for the lifetime of a patient and a tolerable expectation of failure of perhaps
no more than one in one thousand [Bannon et al., 1983; Brunski et al., 2000].
One of many universal requirements of implants, wherever they are used in
the body, is the ability to form a suitably stable mechanical unit with the neigh-
boring hard or soft tissues. A loose (or unstable) implant may function less effi -
ciently or cease functioning completely, or it may induce an excessive tissue
response. In either case, it may cause the patient discomfort and pain. In several
situations, a loose implant was deemed to have failed and needed to be surgically
removed. For a long time it has been recognized that any types of implants (for
both dental implants and orthopedic implants), should possess a biological com-
patibility against an implant receiving surrounding hard/soft tissues.
There are at three least major required compatibilities for placed implants
to exhibit biointegration to receiving hard tissue and biofunctionality thereafter.
They include biological compatibility, mechanical compatibility, and morphologi-
cal compatibility to receiving host tissues [Oshida et al., 1994; Oshida, 2000].
5.3.1 Biological Compatibility
Corrosion is one of the major processes that cause problems when metals are
used as implants in the body. Their proper application to minimize such problems
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