Biomedical Engineering Reference
In-Depth Information
One should note, that among the existing calcium orthophosphates
(Table 1.1), only certain compounds are useful for biomedical
applications, because those having the Ca/P ionic ratio less than
one are not suitable for implantation due to their high solubility and
acidity. Furthermore, due to its basicity, TTCP alone is not suitable
either. However, to be used in medicine, these “unsuitable” calcium
orthophosphates might successfully be combined with either other
calcium orthophosphates or other chemicals.
4.5.1
Cements and Concretes
The need of bioceramics for minimal invasive surgery has induced
the development of a concept of self-setting formulations consisting
of only calcium orthophosphates to be applied as injectable and/
or mouldable bone substitutes [156, 400, 401, 418, 475, 514-522].
In addition, there are reinforced formulations, which, in a certain
sense, might be defined as calcium orthophosphate concretes [516].
Furthermore, porous formulations of both the cements and the
concretes are available [401, 418, 515-520].
Calcium orthophosphate cements and concretes belong to a
low temperature bioceramics. They are divided into two major
groups. The first one is a dry mixture of two different calcium
orthophosphates (a basic one and an acidic one), in which, after
being wetted, the setting reaction occurs according to an acid-base
reaction. The second group of the cements contains only one calcium
orthophosphate. Typical examples include ACP with Ca/P molar ratio
within 1.50-1.67 and α-TCP: they form CDHA upon contact with an
aqueous solution [156, 515, 516]. The setting reaction (= hardening,
curing) of these materials is initiated by mixing the initial powder(s)
with an aqueous solution. Chemically, hardening is due to the
succession of dissolution and precipitation reactions. Mechanically,
hardening results from crystal entanglement and intergrowth (Fig.
4.9) [2]. Setting of calcium orthophosphate cements and concretes
occurs mostly within the initial ~6 h, yielding a ~80% conversion
to the final products and a compressive strength of 40-60 MPa. A
hardening rate is strongly influenced by a powder to liquid ratio,
as well as by addition of other chemicals [156, 514-522]. Despite a
large number of formulations, all calcium orthophosphate cements
can only form two different end products: CDHA and DCPD [156,
515, 516].
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