Biomedical Engineering Reference
In-Depth Information
early maturation stage, and 1.45
10
6
in erupted enamel [
77
]. Thus, the solubility
of the enamel crystals decreases as maturation proceeds; in other words, the stability
of enamel crystals increases with maturation.
The solubility of F-HAp with different F
10
63
)
when almost half the OH
has been replaced with F
(Ca
5
(PO
4
)
3
(OH)
0.43
F
0.57
)
[
78
]. The relationship between the F
content in enamel and HAP stability
gives important information about the formation and prevention of caries and the
remineralization of tooth enamel. Much information about the composition of
apatites and their properties is readily available (for example, [
79
-
81
]).
contents is minimum (6.55
CO
3
-HAp and Properties of CO
3
2
in CO
3
-HAp
5.3.2
The mineral component of mammalian tooth enamel is generally described as
CO
3
-HAp. The CO
3
2
is an important lattice component because it regulates
the solubility, morphology, and size of biological HAP [
82
,
83
]. Whether CO
3
2
can replace OH
in the HAP lattice was a controversial issue because of the
geometric restrictions caused by the form and size of PO
4
3
(regular tetrahedron),
CO
3
2
(equilateral triangles), and OH
(linear). Strict analysis of CO
3
-HAp
(Ca
10
(PO
4
)
5.99
(CO
3
)
0.86
(PO
4
)
2.384
(OH)
0.26
O
0.02
) showed that one CO
3
2
replaces
two OH
and the plane of CO
3
2
triangles is almost parallel to the
c
-axis [
81
].
This parallel orientation creates much less steric interference than the perpendicular
orientation. When CO
3
2
has replaced OH
, it is defined as A-type CO
3
;whenit
has replaced PO
4
3
, it is B-type CO
3
; and when it has replaced OH
and PO
4
3
,it
is AB-type CO
3
.
In developing enamel HAP, not only the CO
3
2
content, but also the ratio
of the replacement sites, i.e., PO
4
3
or OH
, changes during maturation [
74
].
In young and immature enamel HAP, substitution for PO
4
3
is favored, and then, as
maturation progresses, substitution for OH
is increasingly favored. Incorporation
of CO
3
2
in HAP increases with an increase in the CO
3
2
concentration of the
surrounding fluid, and substitution for PO
4
3
is favored under neutral pH conditions
[
84
]. Such diversity in substitution and site occupancy of CO
3
2
in enamel crystal
has been ascribed to changes in the enamel fluid composition and the pH conditions
during the formation process.
5.3.3
Crystal Structure of HAP
The crystal structure of synthesized H-Ap was described initially as an F-Ap type,
e.g., hexagonal,
P
6
3
/m, with the O of the OH
at (0,0,0.25) [
85
]. Later analysis
using mineral H-Ap (Holly Spring, United States) by both X-ray and neutron
diffraction technique [
86
] showed that the O is at (0,0,0.201) and that the H is at
