Biomedical Engineering Reference
In-Depth Information
significantly [106]. On the other hand, a reduction in particle size
was found to result in a significant decrease in both initial and final
setting times [191, 212, 213], an acceleration of the hardening rate
[191], and hydration kinetics of the hardening cement [213]. Besides,
the crystallite sizes of the final product can be strongly reduced
by increasing the specific surface of the starting powder, which
allows developing calcium orthophosphate cements with tailored
structures at the micro- and nanoscale levels [191]. Unfortunately,
an unclear correlation was found between the particle dimensions
of the initial calcium orthophosphates and mechanical properties of
the hardened cements: namely, a significant increase in compressive
strength and storage modulus was reported for some formulations
[212, 213] but a minor effect on compressive strength was discovered
for other ones [191]. This inconsistence is not surprising because
the manufacturing method used to produce test samples varies from
one author to the other. Therefore, the only remaining fact is that
calcium orthophosphate cements are brittle and hence worthless for
load-bearing applications [197, 198].
Setting process of the most types of apatite cements occurs
according to just one chemical reaction (see chemical equations
(5.1), (5.4-5.6)) and at near the physiological pH. The latter may
additionally contribute to the high biocompatibility observed
for these materials [142-144]. For the classical formulation by
Brown and Chow, the transmission electron microscopy results
suggested the process for early-stage apatite formation as follows:
when TTCP and DCPA powders were mixed in an orthophosphate-
containing solution, TTCP powder quickly dissolved due to its higher
solubility in acidic media. Then the dissolved ions of calcium and
orthophosphate, along with ions already existing in the solution,
were precipitated predominantly onto the surface of DCPA particles.
Few apatite crystals were observed on the surface of TTCP powder.
At a later stage of the reaction, an extensive growth of apatite crystals
or whiskers effectively linked DCPA particles together and bridged
the larger TTCP particles causing the cement setting [214].
However, Norian SRS
®
®
were found to set
according to two chemical reactions: precipitation of DCPD, followed
by precipitation of either CDHA or carbonatapatite:
α-Ca
and Cementek
(PO
)
+ Ca(H
PO
)
·H
O + 7H
O
Æ
4CaHPO
·2H
O (5.7)
3
4
2
2
4
2
2
2
4
2
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