Biomedical Engineering Reference
In-Depth Information
Besides the aforementioned, it is important to mention on
concretes, after hardening consisting of calcium orthophosphates
only [237, 238, 377, 451-454]. The first biphasic composition
consisting of a hardened DCPD matrix filled with β-TCP granules was
introduced in 1992 [452]. Further development of this formulation
is well described in other papers [237, 377]; unfortunately, neither
mechanical nor rheological properties of this concrete have been
disclosed. At physiological pH, the
solubility of DCPD is
approximately 100 times higher than that of β-TCP (Table 1.1 and
Fig. 1.6); roughly, the same order of magnitude applies for the
in vitro
in
vivo
resorption kinetics of these calcium orthophosphates. Thus, a
new bone is formed in the space left after resorption of the DCPD
matrix, while β-TCP granules act as guiding structures. This feature
of the cement can be considered an inverse scaffolding effect
[455]. Another group of investigators invented a formulation that
incorporated as major powder components α-TCP, ACP and BCP (HA
+ β-TCP in various HA/β-TCP ratios) [413]. It was believed that after
setting such a formulation could provide a porous ceramics
in vivo
due to preferential dissolution of a better soluble ACP component
compared to the other calcium orthophosphates in the matrix.
Further, this combination was extended to a multiphase concrete
composition consisting of 70% w/w settable matrix (mixture
of 45% α-TCP, 5% MCPM and 25% ACP [456]) with the average
particle dimensions of 15 µm and 30% BCP granules (ranging
between 80 and 200 µm) as a filler [451]. The role of BCP granules
is quite interesting: after implantation of a formulation without
BCP granules, the quality of newly formed bone was not identical to
the host bone, while implantation of a concrete with BCP granules
resulted in formation of a new bone identical to the host bone. The
reason of this phenomenon is not clear yet; but, perhaps, it correlates
with similar results for β-TCP granules, which act as bone anchors
and encourage formation of a mature bone [237, 238]. Other ACP-
containing cement formulations were elaborated as well [457].
Effects of added α-TCP and β-TCP were investigated to shed light
on the setting reaction of apatite cement consisting of TTCP and
DCPA [454]. Added β-TCP showed no reactivity, and thus resulted
in extended setting time and decreased mechanical strength. In
contrast, α-TCP dissolved to supply calcium and orthophosphate
ions after initial apatite crystal formation by the chemical reaction
(5.1). Although setting time was delayed because α-TCP was involved
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