Biomedical Engineering Reference
In-Depth Information
postulated for this formulation [135]. A similar suggestion on the
intermediate formation of OCP was made for the setting mechanism
of Brown-Chow classical cement formulation [101, 106]; however, a
reliable evidence for its presence is still lacking [172, 215]. In both
cases, OCP was suggested to appear as an intermediate because it
was a faster forming phase than CDHA. This hypothesis is based
upon the classical studies performed by Prof. W. E. Brown et al.
about the precursor phase formation during chemical crystallization
of apatites in aqueous solutions [216-218].
Solubility of the hardened apatite cements in aqueous solutions is
expected to be rather similar to that of bone mineral. This means that
they are relatively insoluble at neutral pH and increasingly soluble
as pH drops down; this is an important characteristic of normal bone
mineral that facilitates controlled dissolution by osteoclasts [206].
To conclude this part, one should mention, that in 2000 the US
bone-substitute market for Norian SRS
®
accounted for ~15% of
TM
®
the total sales, followed by BoneSource
at ~13%, and α-BSM
at
~8.5% [137].
5.3.2
Brushite Cements
As indicated by its name, DCPD is the major end product of the
setting reaction of brushite cements (chemical equations (5.2) and
(5.3)). Mirtchi and Lemaître [150] and independently Bajpai et al.
[151] introduced this type of the cements in 1987. Up to now, several
formulations have been already proposed, e.g.,
β-TCP + MCPM [150,
[151, 153, 154], and TTCP + MCPM + CaO [219].
All brushite cements are set by the acid-base interaction only. As
DCPD can only precipitate at the solution pH < 6, the paste of brushite
cement is always acidic during setting [153, 220]. For example,
during setting of a β-TCP + MCPM formulation, the cement pH varies
from very acidic pH values of ~2.5, to almost neutral pH values of
~6.0 [153]. Replacing MCPM by orthophosphoric acid renders the
cement paste very acidic for the initial ~30 s but then the pH profile
follows that obtained with MCPM. It is important to notice that
β-TCP + H
152], β-TCP + H
PO
3
4
formulations have several advantages over β-TCP +
MCPM formulations, namely: (i) easier and faster preparation, (ii)
a better control of the chemical composition and reactivity, (iii)
improved physico-chemical properties, such as longer setting times
PO
3
4
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