Biomedical Engineering Reference
In-Depth Information
an apatite cement containing spherical particles of TTCP to improve
injectability [279].
Using various additives (this approach transforms cements into
composites—see Chapter 6 of this topic) is the second way to improve
the physical properties of calcium orthophosphate cements [280]. For
example, water demand of calcium orthophosphate cements can be
reduced by ionically modifying the liquid component, e.g.,
by adding
nontoxic sodium salts of α-hydroxy di- and tri-acids [281, 282]. A list
of additives, that have been already studied, includes fluidificants,
air-entraining agents, porogens, workability improvement agents,
setting time controllers, and reinforcing additives [162, 203, 283].
Besides, various radiopacifiers might be used to simplify an un-
invasive
monitoring of the implanted cements [284-287]. The
main role of fluidificants is to reduce a mixing time of the cement.
Citric acid is an example of this reagent; it retards the dissolution-
precipitation reactions in cements, decreases the compressive
strength during initial setting, but increases its strength in the
final stages of the cement hardening [228]. Furthermore, data are
available that citric acid decreases the setting time and improves
the mechanical properties of the hardened cements [288]. Adding of
surfactants to calcium orthophosphate cements was found to have
two different meanings: they might act as both air-entraining agents
by lowering the surface tension [289, 290] and interaction modifiers
by shifting the isoelectric point [291].
Besides, studies are available in which calcium orthophosphate
cements have been modified by various bioorganic compounds in
attempts to influence the bone-healing process [292-295]. For example, in
a recent study, a calcium orthophosphate cement was set in the presence
of cocarboxylase, glucuronic acid, tartaric acid, α-glucose-1-phosphate,
L-arginine, L-aspartic acid, and L-lysine, respectively, with the aim to
influence formation and growth of CDHA crystals through the functional
groups of these biomolecules [295]. Except for glucuronic acid, all these
modifications were found to result in the formation of smaller and more
agglomerated CDHA particles, which had a positive impact on the biological
performance indicated by first experiments with the human osteoblast cell
line hFOB 1.19. Moreover, initial adhesion of human bone marrow-derived
mesenchymal stem cells was improved on the cement modifications with
cocarboxylase, arginine, and aspartic acid. Furthermore, cell proliferation
was enhanced on the cement formulations modified with cocarboxylase
in vivo
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