Biomedical Engineering Reference
In-Depth Information
Brushite calcium phosphate bone cements
Some CPCs have been designed that give brushite (DCPD) as the end-
product. brushite is an acidic calcium phosphate, which is metastable under
physiological conditions.
114
For this reason, brushite CPCs are much more
quickly resorbable than apatite CPCs, although it has been shown that
in
vivo
DCPD tends to convert into PHA.
133
all brushite CPCs are obtained as a result of an acid-base reaction.
Several compositions have been proposed for brushite cements, e.g. b-TCP
+ MCPM,
134
b-TCP + H
3
PO
4
,
135, 136
and TTCP + MCPM + CaO.
133
in the
first of them (b-TCP + MCPM), the reaction responsible for the setting of
the cement is reported in equation [10.4].
b-Ca
3
(PO
4
)
2
+ Ca(H
2
PO
4
)
2
·H
2
O + 7H
2
O Æ 4CaHPO
4
·2H
2
O
[10.4]
The paste of brushite CPCs is acidic during setting because brushite can only
precipitate at a pH lower than 6.
114
After setting, the pH of the cement paste
slowly changes towards the equilibrium pH, which depends on the singular
points of the phases present in the cement.
137
10.4.2 Processing parameters and setting properties of
calcium phosphate bone cements
Processing parameters
as mentioned for the case of abCs, the properties of a cement system depend
on, and therefore can be tailored by, several processing parameters, which
are summarized in Table 10.3. The most important are the following:
∑ The composition of the solid phase: This can vary with the setting
reactions desired. normally they are chosen between the orthophosphates,
listed in Table 10.4. On the other hand, those calcium orthophosphates
containing biocompatible components like Na
+
, K
+
, Mg
2+
, Zn
2+
, CO
2-
,
SO
2-
or Cl
-
are also suitable as constituents of CPC powders and calcium
carbonate is also added in some formulations.
∑ The particle size of the starting powder plays an important role in the
setting and final properties of the cement. As mentioned before, the setting
reaction occurs through a dissolution-precipitation process. Hence, the
fineness of the powder will increase the rate of hardening since smaller
particles will dissolve faster than bigger particles and the precipitation
of a new phase will begin earlier.
126
∑ The addition of crystal nucleators in the powder phase: Another strategy
that can be used to produce cement with good setting characteristics
is the use of seed crystals to act as a 'nucleator' for the precipitation
reaction. Several parameters can be modified, such as the amount of seed
