Biomedical Engineering Reference
In-Depth Information
bone graft materials, implant coatings, or therapies in bone disease such as osteo-
porosis [4,39,47,49,56,71,85,95,105,111,114].
Different types of calcium phosphates (for example,
- TCP, ACP,
tetracalcium phosphate, TTCP) are used in the preparation of calcium phosphate
cements [11,74,101]; other calcium phosphates (HA, BCP, CHA) are used in
polymer/calcium phosphate composites [8,43,93].
This chapter provides a brief review of hydroxyapatite (HA), substituted
apatites, and related calcium phosphates, and their properties and applications as
biomaterials. Particular emphasis has been made on carbonate apatites since
bone and tooth mineral consists of carbonate apatite. This chapter covers a brief
description of the fundamentals of hydroxyapatite (structure and properties of
unsubstituted and substituted apatites) and related calcium phosphates; calcium
phosphates occurring in biologic systems (normal and pathologic calcifi cations);
and calcium phosphate based biomaterials for medical and dental applications.
α
- TCP,
β
2.3 FUNDAMENTALS OF HYDROXYAPATITE
2.3.1 Structure and Properties of Hydroxyapatite, HA and
Calcium-Defi cient Apatite, CDA
The name “ Apatite ” (from the Greek word, “ apatit ” , meaning to deceive) was
given to mineral apatites because they were often mistaken for precious gems
like topaz, aquamarine, amethyst [17]. The name 'apatite' describes a family of
compounds having similar structure (hexagonal system, with space group, P6
3
/m)
in spite of a wide range of composition. The structures of calcium hydroxyapatite
(HA), Ca
10
(PO
4
)
6
(OH)
2
; calcium fl uor - apatite (FA), Ca
10
(PO
4
)
6
F
2
; and calcium
chlor - apatite (ClA), Ca
10
(PO
4
)
6
Cl
2
; are well established from analyses of mineral
FA apatite single crystals [5] and synthetic apatite single crystals [50,124]. The
similarity of the x-ray diffraction (XRD) patterns of enamel, dentin and bone to
those of mineral apatites together with chemical analyses showing calcium and
phosphate as principal constituents led to the conclusion as early as 1926 that the
inorganic phases of bones and teeth are basically a calcium hydroxyapatite, HA,
idealized as Ca
10
(PO
4
)
6
(OH)
2
[19]. Association of carbonate in biologic apatite
led to speculation of the similarity between bone mineral and mineral carbonate-
containing apatites (that is, dahllite) [98], Combined studies on mineral, biologic
and synthetic apatites provided experimental evidence that established that bio-
logic apatites are carbonate apatites [57,59,107].
Crystallographically, HA and FA belong to the hexagonal system while ClA
belongs to the monoclinic system. The apatite hexagonal system, space group
P6
3
/m, is characterized by a six-fold
a
-axis perpendicular to three equivalent
a
-
axes (a1, a2, a3) at angles 120 ° to each other. The unit cell, the smallest building
unit containing a complete representation of the apatite crystal, contains ten cal-
cium (Ca), six phosphate (PO
4
) and two hydroxyl (OH) groups closely packed
together in an arrangement shown in Figure 2.1 [124]. The ten calcium atoms are
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