Biomedical Engineering Reference
In-Depth Information
in the indicated range. Unfortunately, cement viscosity is not a
constant value, which, after a decrease in the first seconds after
mixing, increases considerably during curing, eventually leading to
hardening. Furthermore, viscosity should be high enough to prevent
extravasation; therefore, it is very important to define an adequate
injection window [277].
5.4.4
Properties Improving
As written above, properties of the existing self-setting calcium
orthophosphate formulations are not ideal. Several ways can be
adopted to improve them. The first approach consists of injectability
improvement. There are two options for this. Firstly, the injection
device can be modified. For example, shorter cannulas with a larger
diameter, as well as smaller injection rates favor a good injectability.
The last option is not so straightforward: for example, Habib et
al. have shown that large injection rates are not detrimental to
injectability because of the shear-thinning behavior of many calcium
orthophosphate cements [266]. Secondly, the cement composition can
also be adapted. Namely, a decrease of the particle size, the P/L ratio
and the plastic limit was found to contribute to a better injectability
[263, 267]. For example, injectability was found to be unaffected by
P/L ratio within the range of 3.85-4.50 g/ml but drops by nearly 100%
between P/L ratio of 4.50 and 5.00 g/ml [23]. However, a decrease
in P/L ratio leads to a decrease in the mechanical properties of the
cements and cohesion might be destroyed. Furthermore, both the
initial and final setting times decreased markedly with the P/L ratio
increasing [227, 278]. Therefore, variations in the P/L ratio appear
to be valid to a certain extent only. That is why the manufacturer of
Biopex
®
suggests using a P/L ratio of 2.8 or 3.3 g/ml.
Particle size decreasing of calcium orthophosphate crystals is
the second approach for the injectability improvement. For example,
α-BSM
®
is well injectable because it consists of small particles.
Even though small particles require a larger amount of mixing
liquid to obtain a paste, injectability and cohesion of the cements
are generally very good [197, 198]. An indirect approach is to add
calcium orthophosphate particles those act as spacers between
other particles. For example, DCPA is added to the formulation of
Biocement D
®
to improve injectability [197, 198]. Similarly, there is
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