Biomedical Engineering Reference
In-Depth Information
However, in practice, the strength of the cements is lower than that
of bones, teeth or sintered calcium orthophosphate bioceramics
[144].
Having the ceramic origin, the set products of all calcium
orthophosphate cements are brittle, have both a low impact
resistance and a low tensile strength (within 1 to 10 MPa), whereas
the compression strength varies within 10 to 100 MPa [141, 195,
196]. The latter value exceeds the maximum compression strength
of human trabecular bones. Furthermore, at 12 weeks after
implantation the compressive strength of these cements was found
to be still significantly higher (60 to 70 MPa) than that of normal
bone [35]. In general, hardened brushite cements are slightly weaker
than hardened apatite cements. A tensile strength of ~ 10 MPa and
a compressive strength of ~ 60 MPa were obtained for brushite
cements [397]. In comparison, apatite cements can reach a tensile
strength of ~ 16 MPa [398] and a compressive strength of ~ 83 MPa
[399]. However, due to the inherent brittleness of ceramics, these
values are close to be meaningless. Namely, the indication of a mean
compressive strength of, say, 50 MPa measured on well-prepared
(e.g., under vibrations and pressure) and perfectly shaped samples
does not inform the readers with which probability this cement
will fail
under a cyclic load of e.g., 10 MPa. Furthermore, a
comparison of the compressive strength of hardened cements with
that of cancellous bone is not very helpful either because cancellous
bone is much less brittle than the ceramic cements [125].
Moreover, the mechanical properties of hardened calcium
orthophosphate cements are not narrowly distributed around a
mean value (as for metals), but widespread over a very large range
of values, which strongly reduces their clinical application [400].
In vivo
in situ
, the difference between the hardened apatite and brushite
cements boosts: namely, the mechanical properties of apatite
cements were found to increase [356], whereas those of brushite
cements decreased [35]. This is attributed to a higher solubility
of DCPD when compared with that of CDHA (Table 1.1). However,
the mechanical properties of the hardened cements may vary with
implantation time. For example, animal studies indicated that
the mechanical properties of apatite cements tended to increase
continually [356], in contrast to those of brushite cements, which
initially decreased and again increased when bone was growing [35].
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