Biomedical Engineering Reference
In-Depth Information
to get more workable and less viscous pastes of brushite cements
[23, 227-230]. Similar effect might be achieved by addition of
chondroitin 4-sulfate [231] and glycolic acid [232]. For the cement
formulations with orthophosphoric acid as the initial reactant (see
chemical equation (5.3)), acid-deficient formulations were also
found to improve the workability. In this case, the setting reaction
might be described by the following chemical equation [230]:
3.7β-Ca
(PO
)
+ H
PO
+ 27.8H
O
Æ
3
4
2
3
4
2
O (5.13)
Although several studies revealed that too much of DCPD in
a given volume was not detrimental to the biological properties
of brushite cements [35, 206, 219], occasionally, when large
quantities of brushite cements were used, a certain degree of tissue
inflammation during the first weeks of
3CaHPO
·2H
O + 2.7β-Ca
(PO
)
+ 21H
4
2
3
4
2
2
implantation were
reported [226, 230, 233]. Further investigations indicated that the
inflammatory could be due to a partial transformation of DCPD into
CDHA with release of orthophosphoric acid [234]:
(10−
in vivo
x
)CaHPO
·2H
O
Æ
Ca
(HPO
)
(PO
)
(OH)
4
2
10−
x
4
x
4
6−
x
2−
x
O (5.14)
Transformation of DCPD into CDHA occurs via two successive
processes: dissolution and precipitation [235] and can be retarded
by adding magnesium ions to the cement paste, thus reducing the
possibility of inflammation [197, 198]. The aforementioned case of
acid-deficient formulations of brushite cements (chemical equation
(5.13)) is an alternative, because it reduces the amount of unreacted
acid in the cement [230] with an option to consume liberating in
chemical equation (5.14) orthophosphoric acid by the excess of
β-TCP. Implantation of previously set brushite cement might be
the third option, because a solid material was found to be better
tolerated than paste implants. Besides, more bone was formed at the
solid implant contact and the solid material degraded not so rapidly
[236]. For brushite cements, a linear degradation rate of 0.25 mm
per week was reported [237]. This rapid degradation rate might
lead to formation of an immature bone. Adding β-TCP granules to
the cement paste could solve this problem because β-TCP granules
might act as bone anchors and encourage formation of a mature
bone [237, 238].
+ (4−
x
)H
PO
+ (18−
x
)H
3
4
2
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