Biomedical Engineering Reference
In-Depth Information
Ca
F
X
O
P
X
X
X
X
Figure 1.9
Three-dimensional structure of
fluorapatite. View down the
c
-axis show-
ing PO
4
tetrahedral ionic groups, Ca-ions,
and channel ions. The parallelogram indi-
cates the outline of the unit cell. The unit
cell consists of two triangular prismatic sub-
cells forming a rhombic prism. Six of the
Ca
2
+
atoms form a sixfold site (indicated
by dashed lines) in which the channel ions
reside (F
−
inthecaseoffluorapatite).These
channels are oriented perpendicular to the
page. Every crystallographic site (including
the channel site) has a certain size, and thus
not every atom or ionic group will fit into
each site (the sizes of atoms are not drawn
to scale). After [82].
preserved in hydroxylapatite. Thus, it has a lower symmetry than fluorapatite. Such
differences in symmetry impact the growth morphology of the crystals, important
to the mechanical properties of a composite material like bone.
Wopenka and Pasteris commented in 2005 that in the contemporary biomedical,
orthopedic, and biomaterials literature, the mineral component of bone is still usu-
ally referred to as
hydroxy(l)apatite
or
carbonated hydroxy(l)apatite
,asifbiological
apatite were a well defined and well understood material. Whereas the Raman
spectra of apatite in enamel, just like those of both geologic OHAp and synthetic
OHAp, show the O-H modes for hydroxyl within the apatite structure, the spectra
for apatite in bone do not. This is the property of all cortical bones of different
mammals that were analyzed in [82].
The crystallographic structure of bone apatite is similar to that of OHAp, but
there are important differences. The Raman spectra of synthetic OHAp, geologic
OHAp, human enamel apatite, and cortical mouse bone apatite provide several
differences between OHAp and biological apatites.
The bone apatite does not have a high concentration of OH groups, which is
the feature of the mineral hydroxylapatite. Some bone apatites may not contain
any OH groups at all. There is growing evidence for the lack of OH in bone
apatite based not only on the results obtained
via
Raman spectroscopy but also on
