Biomedical Engineering Reference
In-Depth Information
Apatitic CPCs can form either precipitated HA (PHA) or CDHA through
a precipitation reaction. Their fabrication process allows the incorporation
of different ions in the lattice depending on the composition of the starting
materials. The formation of hydroxyapatite through a cement-type reaction
takes place at body temperature and in a physiological environment, as happens
when bone is formed or remodelled. This can account for the fact that the
hydroxyapatite formed in the setting of CPCs is much more like biological
apatites than those obtained when high temperature sintering processes are
applied to fabricate ceramic hydroxyapatite.
The CPCs leading to the formation of PHA or CDHA can contain either
a single compound or a mixture of reactants in the powder phase. The only
cement system that contains a single calcium phosphate was first reported
by Monma
et al
.
119, 120
and was further optimized and characterized by
Ginebra
el al
.
121-126
This system is based on the hydrolysis of a-TCP to
CDHA according to equation [10.1]:
3a-Ca
3
(PO
4
)
2
+ H
2
O Æ Ca
9
(HPO
4
)(PO
4
)
5
(OH)
[10.1]
Since the Ca/P ratio of the initial and final calcium phosphates is the same,
no acid or base is released as by-products.
CPCs can also be formed by two reactants, an acidic calcium phosphate
and a basic calcium phosphate, which set following an acid-base reaction.
The basic component is normally TTCP, since it is the only calcium phosphate
that has a Ca/P ratio higher than PHA. From a theoretical point of view,
any calcium phosphate more acidic than PHA can react directly with TTCP
to form PHA or CDHA. The most widely studied combinations are the
TTCP+DCPD and TTCP+DCP mixtures, which were first developed by
brown and Chow
8,127
and have been the object of extensive research.
117,
118, 128-131
These mixtures produce cements that set at body temperature in
a pH range around neutral, according to equations [10.2] and [10.3]:
Ca
4
(PO
4
)
2
O + CaHPO
4
Æ Ca
5
(PO
4
)
3
OH
[10.2]
Ca
4
(PO
4
)
2
O + CaHPO
4
·2H
2
O Æ Ca
5
(PO
4
)
3
OH + 2H
2
O
[10.3]
Other salts can also be added as reactants in CPCs, such as calcium or strontium
carbonate or magnesium phosphates. An example is the product developed
by norian Corporation (norian SrSTM, Skeletal repair System),
132
where
mixtures of calcium phosphates with a Ca/P ratio lower than PHA are used
and CaCO
3
is added as an additional source of calcium ions. Specifically
this system is formed by using a mixture of a-TCP, MCPM and CaCO
3
.
The initial setting process involves the formation of DCPD, while the final
setting product is dahllite, a carbonated hydroxyapatite with a Ca/P ratio
between 1.67 and 1.69, and with a carbonate ion content similar to bone
mineral.
132
