FUNDAMENTALS OF HYDROXYAPATITE AND RELATED CALCIUM PHOSPHATES - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 38

in biologic systems. Pure -TCP can only be prepared by solid state reactions or

by sintering calcium-defi cient apatite (Ca/P molar ratio, about 1.5) at tempera-
tures 800 ° C and above.
The presence of Mg 2= ions in solution allows the formation of biologic and
synthetic Mg-substituted tricalcium phosphate (

β

- TCMP or Mg - TCP) [57,64,77,
113]. Other ions that allows the formation of substituted TCP from solution in-
cludes: zinc, nickel, cobalt [91].

β

-TCMP can form at low or high pH (pH 5 or 9)
and temperatures 25 °C to 95 °C. Larger crystals are favored at low pH [64,71].

β

-TCMP or Mg-TCP heated at the same temperature as that used in the
preparation of unsubstituted

β

- TCP [38,64,71] .
Zn - substituted TCP (Zn - TCP) is less soluble than Zn - free TCP [42] .

β -TCP is less soluble than the pure β

2.4.5 Tetracalcium Phosphates (TTCP)
TTCP is prepared by solid state reactions (e.g., CaHPO 4 + CaCO 3 ) at high
temperatures. TTCP is very reactive and can rapidly convert to apatite in a wet
atmosphere.

2.5 BIOLOGIC APATITES AND RELATED CALCIUM PHOSPHATES

Biologic apatites (mineral phases of calcifi ed tissues), idealized as calcium hy-
droxyapatite, Ca 10 (PO 4 ) 6 (OH) 2 [50], are associated with minor but important
components such as CO 3 and Mg and are more accurately described as carbonate
apatite with Type B carbonate substitution (CO 3 - for - PO 4 coupled with Na - for -
Ca) [57,59,64,107]. Biologic apatites differ from synthetic ceramic HA used as
bone graft materials in composition, in crystal size (Figure 2.13), in lattice param-
eters (Table 2.2) and in dissolution properties [6,59,64]. The larger a - axis dimen-
sion of human enamel apatite compared to pure HA was fi rst believed to be due
to the CO 3 -for-OH substitution [21]. However, since it has been demonstrated
that carbonate substitution in biologic apatite, like human enamel apatite, is pre-
dominantly CO 3 - for - PO 4 coupled with Na - for - Ca substitution [57,59] , the larger
a -axis of human enamel compared to pure HA may be attributed to other types
of substitution, such as HPO 4 - for - PO 4 , partial Cl - for - OH, or partial substitution
for Ca of larger cations (e.g., Sr 2+ ) [57,59,64] .
Biologic apatites differ in crystallite size (enamel >> dentin or bone) and also
differ in the concentrations of Mg 2+ and CO 2− ions (bone dentin >> enamel)

[6,57,59,64,89]. In synthetic systems, presence of Mg 2+ or CO 2− ions causes the
formation of smaller and more soluble crystals and when simultaneously, present,
exert a synergistic effect on the properties of the apatite crystals [57,59,64,
73,89,92,103]. The larger size and lower solubility of enamel apatite compared to
either dentin or bone apatite may be attributed to the difference in their composi-
tion [57,59,64]. Biologic apatites are usually calcium defi cient (Ca/P molar ratio

>

< 1.67). Upon ignition at 700 °C and above, Mg-TCP and HA are obtained

[57,59,64] .