Biomedical Engineering Reference
In-Depth Information
changing the calcium orthophosphate components. Furthermore,
such formulations can also be modulated by adjoining different
additives as setting retardants, polymeric adjuvants, visco-enhancing
agents, suspension stabilizers, osteoinductive agents, radio-opaque
fillers or macropore-forming agents [327]. Nevertheless, this
approach is limited to acid-base cement formulations only [34].
Before recent publications [590, 592-594], no study had been
available on a possibility of the premixed brushite formulations
at ambient temperatures. Thus, researchers developed a way to
overcome this problem at low temperatures by creation of pre-
mixed frozen calcium orthophosphate “slabs” [155]. By freezing,
the setting reaction of a cement was slowed down or even inhibited.
When the cement had to be applied, the “slab” was softened at
room temperature and molded by hands. Three different pre-mixed
brushite formulations were prepared by freezing the cement pastes
following combination of the powder and liquid components. When
frozen and stored at t = -80
C or less, significant degradation in
compression strength did not occur for the duration of the study
(28 days). Interestingly, in the case of the brushite cement formed
from a combination of β-TCP with 2 M H
°
solution, freezing
the paste had the effect of increasing mean compressive strength
fivefold (from 4 to 20 MPa), which was accompanied by a reduction
in the setting rate of the cement. This strength improvement was
attributed to a modification of crystal morphology and a reduction
in damage caused to the cement matrix during manipulation [155].
A lack of macropores is a substantial disadvantage of many
current self-setting calcium orthophosphate formulations [273]. As
a result, biodegradation takes place layer-by-layer on the surface,
from outside to inside. To solve this problem, various types of
porogens are used [296-318]. Using a hydrophobic liquid instead
of soluble particles could be an alternative. At the turn of the
millennium, an open macroporous structure was obtained using
a mixture of oil and a cement paste [595]; however, since than no
research papers on this subject have been published. Besides, by
means of surfactants, air bubbles might be incorporated into the
bulk of the cements [290]. Unfortunately, the mechanical strength
and porosity are conflicting requirements. As porosity of calcium
PO
3
4
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