Biomedical Engineering Reference
In-Depth Information
cements are decomposed by osteoclast-type cells [30, 209, 376].
Clearly, a fast resorption of brushite cements can only be achieved
if the resorption occurs before conversion DCPD to CDHA according
to Eq. (5.14) [48]. Both types of the resorption mechanisms (active
+ passive) might occur almost simultaneously, if a hardened cement
consists of two different calcium orthophosphates, e.g.,
from DCPD
and β-TCP. For example, the biphasic brushite cement ChronOS™
Inject was found to resorb by dissolution with cement disintegration
and particle formation followed by the phagocytosis of the cement
particles through macrophages [377]. A similar cement was found
to be degraded through a dissolution process associated with a
cellular process. The observations suggested that cell activities
could be influenced by a small particle size, without close correlation
between the particle size and the cell activities but with a correlation
between particle concentration and the cell activities [374]. To get
further details on this topic, the interested readers are referred to
a very interesting review on the cellular mechanisms of calcium
orthophosphate ceramic degradation [378].
The summary of studies on brushite cement implantation in
various animal models and defect locations is available in literature
[230]. Generally, in the same animal model, a degradation rate
decreases with a sample size increases, as does DCPD to CDHA
conversion time. The compositional changes of a brushite cement
after implantation in sheep is well described elsewhere [350, 379].
The kinetics of passive resorption depends on porosity of the
samples, ionic substitutions in calcium orthophosphates (when
applicable), crystallinity and pH of the cement-tissue interface. The
active resorption is due to cellular activity; however, it is also related
to the passive one. Namely, the solution pH near macrophages and
osteoclasts can drop to ~5 by excretion of lactic acid, which increases
the solubility (Fig. 1.6), whereas near osteoblasts (bone forming
cells) solution pH can become as high as 8.5 by excretion of ammonia
[29]. Dissolution chemistry of CDHA (therefore, of hardened apatite
cements) in acidic media (calcium orthophosphates are almost
insoluble in alkaline solutions (Fig. 1.6)) might be described as a
slightly modified sequence of four successive chemical equations
[380, 381] (see section
7.2.5. Chemical model
for details):
Ca
(HPO
)
(PO
)
(OH)
+ (2-
x
)H
+
Æ
10−-
x
4
x
4
6−
x
2−
x
(2−
x
)+
Ca
(HPO
)
(PO
)
(H
O)
(5.15)
10−
x
4
x
4
6−
x
2
2−
x
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