Biomedical Engineering Reference
In-Depth Information
The structure assumed by any solid is such that, on an
atomic level, the configuration of the constituents is of the
lowest possible energy. In phosphates, this energy re-
quirement results in the formation of discrete subunits
within the structure and the PO 3 4 group forms a regular
tetrahedron with a central P 5 þ ion and O 2 ions at the four
corners. In a similar manner, the (OH) groups are also
ionically bonded. In terms of the volume occupied, the
oxygen ions exceed all other elements in phosphates. Any
other elements present may therefore be considered as
filling the interstices, with the exact position being de-
termined by atomic radius and charge (See Fig. 3.2.10-11 ).
The HA lattice contains two kinds of calcium posi-
tions; columnar and hexagonal. There is a net total of four
''columnar calcium'' ions that occupy the [1/3, 2/3, 0]
and [1/3, 2/3, 1/2] lattice points. The ''hexagonal cal-
cium'' ions are located on planes parallel to the basal
plane at c ¼ 1/4 and c ¼ 3/4 and the six PO 4 tetrahedra
are also located on these planes. The (OH) groups are
located in columns parallel to the c axis, at the corners of
the unit cell, which may be viewed as passing through the
centers of the triangles formed by the ''hexagonal cal-
cium'' ions. Successive hexagonal calcium triangles are
rotated through 60 (See Fig. 3.2.10-12 ).
Defects and impurities in HA may be identified as
either substitutional or as discrete, extraneous crystalline
phases (as discussed above). Methods of detection of
impurities include X-ray diffraction, IR spectroscopy,
and spectrochemical analysis. It is important to make
a full spectrochemical analysis of HA since contact with
any metal ions during production can lead to high levels
of impurties in the product. Typical data for one com-
mercial HA powder are shown in Table 3.2.10-8 .
Ions that may be incorporated into the HA structure,
either intentionally or unintentionally, include carbonate
ions (substituting for hydroxyl or phosphate groups),
fluoride ions (substituting for hydroxyl groups), silicon,
Fig. 3.2.10-12 Theoretical positions of the ionic species within
the unit cell of HA (Hing, 1995).
or silicate ions (substituting for phosphorus or phosphate
groups) and magnesium ions substituting for calcium,
e.g., Newsley, 1963; Le Geros, 1965; Jha et al. , 1997;
Gibson et al. , 1999).
The presence of carbonate may be observed directly,
using IR spectroscopy, in the form of weak peaks
at between 870 and 880 cm 1 and a stronger doublet
between 1460 and 1530 cm 1 , and also through alterations
in the HA lattice parameters from X-ray diffraction
Table 3.2.10-8 Trace elements in a commercial HA
Trace element
ppm
Al
600
Cu
1
Fe
1000
Ge
100
Mg
2000
Mn
300
Na
3000
Pb
4
Si
500
Fig. 3.2.10-11 Structure of HA projected on the x,y Plane,
adapted from Kay, M. I., Young, R. A., and Posner, A. S. (1964).
Crystal structure of hydroxyapatite. Nature 12: 1050. (Hing, 1995).
Ti
30
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