Biomedical Engineering Reference
In-Depth Information
In 1940, the available level of knowledge on basic calcium
orthophosphates (TCP, TTCP and apatites) was summarized into a big
review [108], which might be considered as the first comprehensive
review on the subject. Interestingly, that the authors of this review
have written about HA that “Existence as a unique stoichiometric
compound doubtful” [108, p. 259, Table). Furthermore, according
to this review, α- and β-modifications of TCP (see Table 1.1) have
been differentiated in 1931-1933 by Schneiderhöhn and Bredig
et
al
., who also created the initial versions of the high-temperature
diagram for the binary system CaO-P
. That time, the existence
of OA was uncertain [108]. Besides, in 1940, a fundamental study on
the equilibrium in the system CaO-P
O
2
5
O was published [109].
To finalize this chemical section, one should mention that by
1928 it was clearly known that TCP could not be prepared by wet-
precipitation. Let me cite: “
O
-H
2
5
2
We have been unable to find any evidence
of the existence of a molecular species with the formula Ca
3
(PO
4
)
2
.
Precipitates of this “substance” rarely have the correct empirical
composition and they cannot be purified by recrystallization. The
evidence is such as to lead us to suspect that there may be no such
chemical entity as Ca
. No one has succeeded in preparing it by
precipitation (12, 35-48), bearing out the theoretical objections to
such a reaction on the grounds that fifth order reactions do not occur
(49). We do not take the position that there can be no compound
with this formula. It may be found possible to synthesize it by
other methods, but so far no one has succeeded in preparing it by
precipitation.” [110, p. 128]. Similar conclusions were made by other
researchers [111]. Nevertheless, this knowledge was not common
yet, since, in 1935, a report was published that “Tricalcium phosphate
monohydrate was prepared by the slow addition of calcium chloride
solution to a constantly agitated alkaline solution of disodium
phosphate, maintained at 65° to 70°C." [112]. This controversy has
been explained in Ref. [108] by a matter of definitions: “The terms
“tricalcium phosphate” and “hydroxyapatite” are very widely and
very loosely used. For example, some authors use the former for
any precipitate more basic than dicalcium phosphate, although such
precipitates have been frequently shown to have an apatite lattice
or to be mixtures of dicalcium phosphate and an apatite. Others
confine the use of the term to those precipitates with P
(PO
)
3
4
2
O
:CaO ratios
2
5
approaching that of Ca
(PO
)
.” (p. 265).
3
4
2
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