Biomedical Engineering Reference
In-Depth Information
be measured, beyond which it is possible to touch the cement without causing
serious damage. As far as clinical applications are concerned, the proposed
ranges were 4 <
I
< 8 minutes for the initial setting time and 10 <
F
< 15
minutes for the final setting time.
123
In general, setting times of apatite CPCs are too long and several strategies
have to be applied to reach the clinical requirements.
139
Among the parameters
that can be adjusted to accelerate the setting of apatitic cements are (i) the
liquid-to-powder ratio (a smaller amount of liquid reduces the setting time), (ii)
the reduction of the powder size (smaller size, shorter setting time), (iii) the
addition of calcium or phosphate ions either pre-dissolved in the liquid phase
or as highly soluble salt (common ion effect: the higher the concentration,
the shorter the setting time), (iv) the addition of seed materials, which act
as crystal nuclei (the more nuclei, the shorter the setting time).
On the other hand, brushite CPCs tend to set too quickly. The setting
time of brushite CPCs is controlled by the solubility of the basic phase: the
higher the solubility, the faster the setting time.
134
In brushite CPCs, setting
retarders are often used and a common approach to increase the setting time
is the addition of inhibitors of DCPD crystal growth.
140
The cohesion time.
CPCs are designed to be implanted whilst in a paste state.
This means that the paste is in contact with blood or other physiological
fluids. Cohesion can be defined as the capacity of a CPC to set in a fluid
without disintegrating. It has to be clarified that, in fact, several terms have
been used to describe this property, such as non-decay ability, anti-washout,
compliance, swelling or stability, and some studies have been devoted to this
topic.
141-147
In general, this property was evaluated by an immersion test
in water, Ringer's solution or simulated body fluid. The addition of some
water-soluble polymers has been proven to be very effective in enhancing the
cohesion of CPC pastes.
143, 144
However, the approach followed up to now
has been very empirical and there is a need for more understanding of the
underlying mechanisms concerning cohesion. It is indeed an important topic
since if a CPC has no cohesion at all it will not be able to form a solid body
when implanted, or, in the case of poor cohesion, calcium phosphate particles
can be released, which can elicit harmful reactions such as inflammation or
blood clotting.
146, 147
Injectability
. The capability to inject the cement in the surgical site is an
important property, since it can minimize the surgical invasion and permit
complex-shaped defects to be filled adequately. All CPCs are mouldable
materials and in some cases their processing parameters can be adjusted
to obtain injectable CPCs. The injectability of a CPC paste can be defined
as its ability to be extruded through a needle without demixing. Of course,
this will depend on the diameter and length of the needle (2 mm diameter
