Calcium Orthophosphates as Bioceramics - Calcium Orthophosphates: Applications in Nature, Biology and Medicine - page 372

those of calcium orthophosphates, which in the course of time may
be resorbed completely [589].

4.6.1 Interaction with Surrounding Tissues and the Host

Responses

An interaction between an implant and surrounding tissues is a
dynamic process. Water, dissolved ions, biomolecules and cells
surround the implant surface during initial few seconds after the
implantation. It has been accepted that no foreign material placed
within a living body is completely compatible. The only substances
that conform completely are those manufactured by the body itself
(autogenous), while any other substance, which is recognized as
foreign, initiates some types of reactions (a host-tissue response).
The reactions occurring at the biomaterial/tissue interfaces lead to
time-dependent changes in the surface characteristics of both the
implanted biomaterials and the surrounding tissues [58, 590].

In order to develop new biomaterials, it is necessary to understand

the host responses. Like any other species, biomaterials and

bioceramics react chemically with their environment and, ideally, they
should neither induce any changes nor provoke undesired reactions
in the neighboring or distant tissues. In general, living organisms
can treat artificial implants as biotoxic (or bioincompatible [54]),
bioinert (or biostable [43]), biotolerant (or biocompatible [54]),
bioactive and bioresorbable materials [4-7, 39, 40, 45, 49-54, 77, 531,
589-591]. Biotoxic (e.g

in vivo

, alloys containing cadmium, vanadium, lead
and other toxic elements) materials release to the body substances
in toxic concentrations and/or trigger the formation of antigens
that may cause immune reactions ranging from simple allergies to
inflammation to septic rejection with the associated severe health
consequences. They cause atrophy, pathological change or rejection
of living tissue near the material as a result of chemical, galvanic or
other processes. Bioinert [592] (e.g

.

. , zirconia, alumina, carbon and

titanium) and biotolerant (e.g , polymethylmethacrylate, titanium

and Co-Cr alloy) materials do not release any toxic constituents but
also do not show positive interaction with living tissue. They evoke a
physiological response to form a fibrous capsule, thus, isolating the
material from the body. In such cases, thickness of the layer of fibrous
tissue separating the material from other tissues of an organism
can serve as a measure of bioinertness. Generally, both bioactivity

.