Biomedical Engineering Reference
In-Depth Information
Owing to the intrinsic porosity of CPCs, their strength is lower than that of
calcium phosphate ceramics.
115
The liquid-to-powder ratio, the particle size
of the reactants, the crystallinity and amount of seed and the use of liquid
accelerators are factors that affect the strength of the CPC. A wide range
of values can be found in the literature, depending on the composition and
processing parameters, and it is difficult to make comparisons between them
owing to a lack of consistency in specimen dimensions, testing protocols
and sample pre-treatments. As indicative values, the compressive strength
of apatite cements normally ranges between 20 and 50 MPa
121, 126, 132, 142-145
although lower and higher values for some formulations have also been
reported.
159
Brushite CPCs are in general weaker than apatite CPCs and
compressive strengths of 25 MPa have been reported.
160
The reduction of porosity in CPCs has been explored as a way to increase
their strength. Reducing the amount of added water and improving particle
packaging can reduce the porosity of the cements. Compaction of the cement
paste during setting has been demonstrated to increase the compressive strength
of apatitic CPCs,
161-163
and values as high as 118 MPa have been reported.
163
The addition of water-reducing or liquefying agents, such as sodium citrate,
allows for a further densification of the paste and values of 180 MPa in wet
conditions have been reported.
163
However, it has to be considered that this
compaction cannot be applied if the cement is implanted or injected within
the bone tissue and therefore at present it can be used only to fabricate pre-set
substrates or scaffolds for bone regeneration. A two-step protocol, including
pre-compaction of a paste followed by a conventional application has been
suggested as an alternative for potential clinical use.
163
The evolution of the CPC strength after implantation has also been studied.
The mechanical properties of apatite CPCs are reported to increase,
164
whilst
those of brushite CPCs tend to decrease,
165
owing to the higher solubility of
DCPD compared with that of PHA. Only after a few weeks of implantation,
when bone growth is significant, the mechanical properties of brushite CPCs
increase.
165
10.4.4 Biological performance of calcium phosphate
bone cements: present and future strategies
As mentioned previously, CPCs are highly biocompatible osteoconductive
materials and can stimulate tissue regeneration.
14, 132, 133, 166-175
The
biodegradability of apatite CPCs is larger than that of sintered hydroxyapatite,
but it is still slow. it has been shown, for instance, that some CPCs could
remain as long as 78 weeks when implanted in dog femurs.
171
Most of the
apatitic cements are resorbed via cell-mediated processes. in these processes
osteoclastic cells degrade the materials layer by layer, starting at the bone-
cement interface throughout its inner core.
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