Biomedical Engineering Reference
In-Depth Information
3.2
Types and Solubilities of Calcium Phosphates
Phosphate ions, which comprise a phosphorous ion with a charge of
C
5 and an oxy-
gen ion with a charge of
2, are grouped on the basis of their chemical composition
into orthophosphates, metaphosphates, pyrophosphates, and polyphosphates. The
phosphate ions described in the present chapter belong to the orthophosphate group.
Orthophosphate ions, which have the chemical composition PO
4
3-
, are a component
of all the calcium phosphates that are described.
One of the characteristics of biomineralization is the variety of chemical com-
positions and crystal structures of the substances produced. Calcium phosphate is
considered to be a representative biomineralization compound. Primary substances
that are a calcium phosphate with a known chemical composition and structure
and which can be precipitated as crystals between room temperature (about 25
ı
C)
and human body temperature (about 37
ı
C) are dicalcium phosphate dyhydrite
(DCPD:
CaHPO
4
2H
2
O),
dicalcium
phosphate
anhydrous (DCPA:
CaHPO
4
),
“
-Ca
3
(PO
4
)
2
), octacalcium phosphate (OCP:
Ca
8
(HPO
4
)
2
(PO
4
)
4
5H
2
O), and hydroxyapatite (HAP: Ca
10
(PO
4
)
6
(OH)
2
)[
3
,
4
]. In
addition, while it does not itself form a crystal structure, one of the main precipitates
is ACP. The following two paragraphs will briefly explain HAP and ACP, which
are covered primarily in the present chapter, as well as the solubility of calcium
phosphate salts. A recent detailed review of ACP can be found elsewhere [
5
].
In some instances, the crystal structure of HAP takes on the monoclinic system
(
P
2
1
/
b
)[
6
], while in others it takes on the hexagonal system (
P
6
3
/
m
)[
7
]. The
monoclinic structure is considered to be stable; however, the HAP in organisms
has a hexagonal structure. The unit cell constants of HAP with hexagonal system
are
a
D
b
D
0.94302,
c
D
0.68911 nm, and
-tricalcium
phosphate
(
“
-TCP:
“
”
D
120
ı
, while the monoclinic HAP
has a symmetry in which the length of the
b
-axis is twice that of the
a
-axis, with
a
D
0.98421,
b
D
2
a
,
c
D
0.68814 nm, and
”
D
120
ı
[
8
]. Structural analysis using
synchrotron radiation has shown that the
b
-axis is slightly longer than 2
a
[
9
].
Although the stoichiometric composition of HAP is Ca
10
(PO
4
)
6
(OH)
2
, hexagonal
HAP with this composition is not found in the human body. HAP substitutes
various ions within its core structure, so its chemical composition varies widely
(for example, [
3
]). The CO
3
2-
can substitute for PO
4
3-
and a proportion of the
OH
groups, and F
and Cl
can substitute for a proportion of the OH
groups.
The substitution of CO
3
2-
in HAP, which constitutes bone and teeth, is particularly
common. It is generally thought that HAP in bone and teeth contains 4-5 wt.% and
3-5 wt.% carbon dioxide, respectively [
10
,
11
]; this HAP is sometimes referred to
as carbonate-containing HAP. The columns created by Ca
2C
in a crystal structure
form a channel parallel to the
c
-axis, and the separation of ions from the channel and
substitution of different ionic species readily occur. Instances are known in which
gaps created by the separation of Ca
2C
are replaced by cations such as Na
C
,Sr
2C
,
and Mg
2C
. In these cases, a large stoichiometric discrepancy occurs, resulting in
Ca-deficient HAP. The HAP in bone is a classic Ca-deficient HAP in which the
Ca/P ratio does not attain the stoichiometric 1.67 value. When the calcium content
