Biomedical Engineering Reference
In-Depth Information
1.2.4
Apatite in the Bone
The bone tissue is a mineralized CT. The bones consist of inorganic constituents,
calcium hydroxyphosphate, Ca
5
(PO
4
)
3
OH, also known as the
mineral apatite
(or
hydroxyapatite, abbreviated sometimes as HA or HAP), and calcium carbonate
CaCO
3
, and an organic constituent, collagen, which is a protein. Nature has
evolved sophisticated strategies for developing hard tissues through the interaction
of cells and, ultimately, proteins with inorganic mineral phases.
Hydroxyapatite (or hydroxylapatite - according to the nomenclature accepted
by the International Mineralogical Association) is a naturally occurring form
of calcium apatite with the formula Ca
10
(PO
4
)
6
(OH)
2
tiswr teninthis
form to denote that the crystal unit cell comprises two entities). We have
Ca
10
(PO
4
)
6
(OH)
2
10Ca
2
+
+
6PO
3
−
4
2OH
−
. It has relatively high compres-
sive strength but low tensile strength of the order of 100 MPa. It has a specific
gravity of 3.08 and is 5 on Mohs hardness scale. It crystallizes in the hexagonal
system.
Pure hydroxylapatite powder is white. Naturally occurring apatites can, however,
also have brown, yellow, or green colorations, comparable to the discolorations
of dental fluorosis. It is estimated that a modified form of the inorganic mineral
hydroxylapatite (known as
bone mineral
) accounts for about 50% of the dry weight
of bone.
A calcium phosphate mineral found in the bone is similar in composition and
structure to minerals within the apatite group. It belongs to biominerals - minerals
produced by living organisms. Apatites are widely distributed as accessory minerals
in different rocks and are important for the study of geological thermal history
[78-81].
Apatites have the general formula Ca
10
(PO
4
)
6
X
2
where X denotes F (fluorapatite,
abbreviated as FAp), OH (hydroxyapatite, abbreviated OHAp), or Cl (chlorapatite,
ClAp). The apatite lattice is tolerant of substitutions, vacancies, and solid solutions;
for example, X can be replaced by 1
↔
+
2O;CabySr,Ba,Pb,Na,or
vacancies; and PO
4
by HPO
4
,AsO
4
,VO
4
,SiO
4
,orCO
3
.
The mineral of bones and teeth is an impure form of OHAp, the major departures
in composition being a variable Ca/P mol ratio (1.6- 1.7, OHAp is 1.66), and a
few percent CO
3
and water. The mineral is microcrystalline. The crystals are
approximately 15 nm wide by 40 nm long in bone and dentine, and 40 nm wide
by 100 nm to 5
/
2CO
3
or 1
/
m or more long in dental enamel. They are much thinner
compared to their width. The mineral in the bone comprises crystals that are
smaller than those in dental enamel, so that many of the constituent ions occupy
surface, or near-surface, positions. The result is that there are greater uncertainties
about the crystal structure of bone mineral, compared with that of dental enamel.
Apatite OHAp is also used as a biomaterial, for bone replacement, and for coating
metal prostheses to improve their biocompatibility. The osseous tissue without
collagen would be hard and brittle, and its fairly large elasticity is contributed
by collagen. Biological apatites present in the natural bone, dentin, and enamel
µ
