Biomedical Engineering Reference
In-Depth Information
of living bone with normal bone marrow and osteocytes in lacunae
could be seen in an implanted cement. At 8 weeks, the cement
was almost totally surrounded by mature bone. At this stage, no
resorption of the cement was observed [369]. Only ~ 30% decrease
of the implanted amount of Norian SRS
®
was reported after 24
months in a rabbit femur [370]. Moreover, several differences could
be expected depending on the cement type. For example, as the end
product of BoneSource
TM
and Cementek
®
is a crystalline CDHA,
TM
®
BoneSource
are expected to resorb slower than
other apatite cements. Indeed no resorption of BoneSource
and Cementek
TM
was
observed after several years implantation; though some resorption
of Biobone
®
was detected. However, porosity appears to be the main
biodegradability factor at play: a more porous (for cells) hardened
cement degrades faster than a less porous one [371]. For example,
as Biobone
®
TM
, the discovered
diversity could be due to the differences in the cement porosity
[197, 198]. The latter conclusion is confirmed by the results of other
studies: a positive influence of the cement porosity on the resorption
rate was found [273]. The interested readers are referred to the
study on the suitability of porous calcium orthophosphate cements
as scaffold material for bone regeneration, using a rabbit model
[372].
The resorption properties of bioceramics are generally believed
to relate to the solubility of their constitutive phases. The implanted
calcium orthophosphates might be resorbed by two possible
mechanisms, namely: an active resorption, mediated by the cellular
activity of macrophages, osteoclasts and other types of living cells
(so-called phagocytosis or literally “cell-eating”) [373-375] and
a passive resorption due to either chemical dissolution [11] or
chemical hydrolysis (brushite cements only) [230] in the body fluids.
Unfortunately, the factors concerning the biodegradation of calcium
orthophosphate biomaterials have not been completely elucidated
yet. The chemical composition, physical characteristics and crystal
structures certainly play an important role in the biological
behavior of calcium orthophosphates. In addition, biodegradation
may be influenced by the conditions, such as experimental models,
implantation sites and animal species [374].
The data are available that macrophages and giant cells
decompose quickly resorbed calcium orthophosphates (e.g.,
is more porous than BoneSource
brushite
cements) [226], while slowly (from months to years) resorbed apatite
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