Biomedical Engineering Reference
In-Depth Information
difficult. Hence, it is worth stressing that the ease of ionic substitutions in
the Ha structure makes bone an important system for keeping toxic heavy
metals out of physiological fluids.
the ability to host ions in this structure allow synthetic calcium phosphates
to be designed with improved properties for specific biomedical applications.
the ionic substitutions can modify the surface structure and electric charge
of HA, with a potential influence on the material in biological environments.
in addition, the presence of certain chemical elements that are released
during the ceramic resorption facilitates the bone regeneration carried out by
osteoblasts. in this sense, amounts of strontium, zinc or silicate stimulate new
bone formation. Carbonate and strontium ions facilitate apatite dissolution
and, consequently, the resorption of the implant. Silicates in the network
increase the mechanical strength of apatite, a very important factor in porous
ceramics, and accelerate its bioactive response.
54-56
thus, the current trend
is to obtain synthetic calcium phosphates partially substituted for use in
implants.
Biphasic mixtures of calcium phosphates
Most popular materials of this type are based on Ha and b-tricalcium
phosphate, b-Ca
3
(PO
4
)
2
, (b-TCP) mixtures.
57-62
Such mixtures evolve to
HCa under physiological conditions. the chemical reactions take place
under equilibrium conditions between the more stable Ha and the more
resorbable b-TCP. The mixture is gradually dissolved, acting as a stem
for newly formed bone by the releasing of Ca
2+
and PO
4
3-
to the local
environment. this material can be injected, used as coating or as bulk in
bone replacement: forming of bulk pieces, filling of bone defects, and so
on.
63
Moreover, many other biphasic mixtures were investigated including
calcium phosphates and other second generation ceramics such as bioactive
glasses, calcium sulphates, and so on.
64-66
Bone cements based on calcium salts
Cements based on calcium phosphates, calcium carbonates or calcium
sulphates, have attracted much attention as biomaterials owing to their
excellent biocompatibility and bone-repair properties.
67-70
these cements must
not be delivered in a prefabricated form and this is a remarkable advantage
of the cements over the conventional bioceramics. Most of the injectable
calcium phosphates used evolve to an apatitic calcium phosphate during the
setting reaction. the physical-chemical properties of these materials, such
as the setting time, porosity and mechanical behaviour, depend on cement
formulation and the presence of additives.
71-74
These cements cure in field, are
biocompatible and can be resorbed slowly. During this gradual process, the
