Biomedical Engineering Reference
In-Depth Information
phase (from β-TCP to α-TCP and further to ACP) and with a smaller
particle size [175]. Nominally, it might be stated that formation of
apatites through a cementation reaction is a sort of a biomimetic
process because it occurs in physiological environment and at body
temperature [37]; however, both the crystallization kinetics and a
driving force are very far away from the biomimeticity. A unique
feature of the hardened apatite cements is that the force linking the
newly formed crystals (of both CDHA and carbonatapatite) is weak;
therefore, crystals can be easily detached from the bulk of hardened
cement, especially after dissolution has partly occurred. When this
happens, osteoclasts and other cells can easily ingest the apatite
crystals [209].
Immediately after implantation, any cement becomes exposed to
blood and other tissue fluids that delays the setting time. Intrinsic
setting time for apatite cements has been extensively studied and it
appear to be rather long. For example, for the original formulation
by Brown and Chow it ranges from 15 to 22 min [14, 15]. This may
result in procedural complications. To remedy this, the amount
of liquid might be reduced to a possible minimum. Therefore, all
apatite cements look like viscous and easily moldable pastes, which
tend to be difficult to inject. Besides playing with the P/L ratio, the
setting time can also be reduced by using additives to the liquid
phase (which is distilled water in the Brown-Chow formulation [14,
15]). The list of additives includes phosphoric acid, MCPM, and other
soluble orthophosphates. These additives promote dissolution of
the initial solids by lowering the solution pH. In such cases, a setting
time in the range of 10-15 min can be obtained [169, 171-177,
210]. The influence of soluble orthophosphates (e.g.,
Na
HPO
or
2
4
NaH
) on the setting time of apatite cements is explained by the
fact that dissolution of DCPA and formation of CDHA during setting
occur in a linear fashion, thus avoiding early formation of CDHA.
This is important because too early formation of CDHA might engulf
un-reacted DCPA, which slows down DCPA dissolution and thus
the setting kinetics becomes slower, while the presence of sodium
orthophosphates prevents DCPA particles from being isolated [211].
Particle size [191, 212, 213], temperature of the liquid phase, and
initial presence of HA as a seed in the solid phase are other factors
that influence the setting time [14, 15, 37, 207, 208]; however,
PO
2
4
in
vitro
studies demonstrated that these parameters did not affect
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