Biomedical Engineering Reference
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boundary of the subset (Fig. 1.7). This appears to be a neutral ion
cluster of ~ 9.5 Å in diameter, whose composition is expressed by
the formula Ca
, i.e., 3 times of TCP. Since then, in literature
it is defined as a Posner's cluster. It was later suggested that these
clusters in fact possessed a S
(PO
)
9
4
6
symmetry [229, 230]. This subset is
linearly expanded by ~ 3% and its radial distribution function is
similar to that calculated from the diffuse X-ray diffraction profile
of ACP. The authors postulated that at the far-MRO level these
slightly enlarged spherical subsets are randomly clustered in ACP
particles (Fig. 1.7) with water filling the intervening spaces [104,
224, 225]. No data have been found in the references that plasma-
spayed ACPs cannot contain Posner's clusters as the structural units;
however, undoubtedly, they cannot contain water molecules in the
intervening spaces. Interestingly, but the atomic arrangement of
Posner's clusters appears to be analogous to that existing in several
types of crystalline calcium orthophosphates, such as HA, OCP, and
β-TCP. For example, the relationship between the original Posner's
cluster and the atomic structure of HA is represented in Fig. 2.11.
However, one must stress, that this is a model only because the real
arrangements of the clusters into larger structures are not known
yet.
6
Figure 2.11
A model of Posner's cluster (in a circle) showing its relationship
with the HA crystal structure. Black lines forming a rhomb
delimit the borders of one unit-cell of HA. Reprinted from Ref.
[31] with permission. A similar relationship is also available
in Ref. [254]. This correlation between Posner's clusters and
HA structure was first published in Ref. [104]. One should
note, that in Ref. [230] another set of atoms of the HA crystal
structure has been chosen to represent a Posner's cluster.
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