Biomedical Engineering Reference
In-Depth Information
osteoblasts [627]. Finally but yet importantly, both nano-structured
rough surfaces and a surface charge on implants were found to cause
an asymmetrical division of the stem cells into osteoblasts, which is
important for osteoinduction [622].
4.6.3
Biodegradation
Shortly after implantation, a healing process is initiated by
compositional changes of the surrounding bio-fluids and adsorption
of biomolecules. Following this, various types of cells reach the
bioceramic surface and the adsorbed layer dictates the ways the cells
respond. Further, a biodegradation of the implanted bioceramics
begins. This process can occur by either physicochemical dissolution
with a possibility of phase transformations or cellular activity (so-
called bioresorption), as well as by a combination of both processes.
Dissolution is a physical chemistry process, which is controlled by
some factors, such as solubility of the implant matrix (Table 1.1),
surface area to volume ratio, local acidity, fluid convection and
temperature. For HA, the dissolution process in acids has been
described by a sequence of four successive Eqs. (7.1-7.4) [628, 629].
See section 7.2.5 Chemical model for details.
With few exceptions, dissolution rates of calcium orthophosphates
are inversely proportional to the Ca/P ratio, phase purity and
crystalline size, as well as it is directly related to the porosity and
surface area. Phase transformations might occur with OCP, DCPA,
DCPD, α-TCP, β-TCP and ACP because they are unstable in aqueous
environment under the physiological conditions. Bioresorption is
a biological process mediated by cells (mainly, osteoclasts and, in a
lesser extent, macrophages) [630-632]. It depends on the response of
cells to their environment. Osteoclasts attach firmly to the implant and
dissolve calcium orthophosphates by secreting an enzyme carbonic
anhydrase or any other acid, leading to a local pH drop to ~4-5
[633]. Furthermore, small particles of calcium orthophosphate can
also be phagocyotosed by osteoclasts, i.e. they are incorporated into
cytoplasm and thereafter dissolved by acid attack and/or enzymatic
processes. In any case, biodegradation of calcium orthophosphates
is a combination of various non-equilibrium processes, occurring
simultaneously and/or in competition with each other.
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