Biomedical Engineering Reference
In-Depth Information
phase disappeared giving rise to the final CDHA phase [25]. Thus,
all existing formulations of calcium orthophosphate cements can be
divided into two major groups: apatite cements and brushite cements
[133]. The final hardened product of the cements is of the paramount
importance because it determines the solubility and, therefore,
in
vivo
bioresorbability. Since the chemical composition of mammalian
bones is similar to an ion-substituted CDHA, apatite-forming cement
formulations have been more extensively investigated. Nevertheless,
many research papers on brushite cements have been published as
well.
All self-setting calcium orthophosphate formulations are
made of an aqueous solution and fine powders of one or several
calcium orthophosphate(s). Here, dissolution of the initial calcium
orthophosphate(s) (quickly or slowly depending on the chemical
composition and solution pH) and mass transport appear to be
the primary functions of an aqueous environment, in which the
dissolved reactants form a supersaturated (very far away from the
equilibrium) microenvironment with regard to precipitation of the
final product(s) [135, 136]. The relative stability and solubility of
various calcium orthophosphates (see Table 1.1) is the major driving
force for setting reactions that occur in these cements. Therefore,
mixing of a dry powder with an aqueous solution induces various
chemical transformations, where crystals of the initial calcium
orthophosphate(s) rapidly dissolve(s) and precipitate(s) into
crystals of CDHA (precipitated HA) or DCPD with possible formation
of intermediate precursor phases (e.g.,
ACP [113] and OCP [25, 129-
132]). During precipitation, the newly formed crystals grow and
form a web of intermingling microneedles or microplatelets of the
final products, thus provide a mechanical rigidity to the hardened
cements. In other words, entanglement of the newly formed crystals
is the major reason of setting (Fig. 4.9). For the majority of apatite
cements, water is not a reactant in the setting reaction. Therefore,
the quantity of water, actually needed for setting of apatite cements,
is very small [22, 135, 137]. However, for brushite cements, water
always participates in the chemical transformations because it is
necessary for DCPD formation. Due to this reason, brushite cements
are always hydraulic, while usually this term is not associated with
apatite cements.
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