Biomedical Engineering Reference
In-Depth Information
(2−
x
)+
Ca
(HPO
)
(PO
)
(H
O)
Æ
10−
x
4
x
4
6−
x
2
2−
x
2+
3Ca
(PO
)
+ (1-
x
)Ca
+ (2-
x
)H
O (5.16)
3
4
2
2
Ca
(PO
)
+ 2H
+
Æ
Ca
2+
+ 2CaHPO
(5.17)
3
4
2
4
+
Æ
2+
4
−
CaHPO
(5.18)
Obviously, the dissolution chemistry of DCPD (therefore, of
hardened brushite cements) in acidic media is described by Eq.
(5.18). One should stress that in Eq. (5.18) water is omitted for
simplicity. Therefore, dissolution of DCPA is written instead.
Themechanismofbonehealingcausedbycalciumorthophosphate
cements is very multifactorial because the surface of the cements is
rapidly colonized by cells. Several types of these cells degrade calcium
orthophosphates by either phagocytotic mechanisms (fibroblasts,
osteoblasts, monocytes/macrophages) or an acidic mechanism
with a proton pump to reduce the pH of the microenvironment and
resorb the hardened bioceramics (osteoclasts) [378, 382]. Various
mesenchymal cells located at the implantation sites can induce
solubilization of calcium orthophosphates. Upon the cells arrival,
various active enzymes, such as acid phosphatase, are secreted that
causes dissolution of the hardened cements [383-385]. Much more
biology, than chemistry and material science altogether, is involved
into this very complex process and many specific details still remain
unknown. See section
+ H
Ca
+ H
PO
4
2
for further details.
It is well known that various polypeptides and growth factors
present in bone matrix might be adsorbed onto HA [386-388]
and modulate the local milieu of cells. This is supported by many
purification protocols of growth factors and bone morphogenetic
proteins/osteogenins involving HA chromatography [389, 390].
However, osteoblasts are not found in direct contact with calcium
orthophosphates. A complex proteinaceous layer, usually osteoid,
directly contacts the osteoblasts. After implantation of calcium
orthophosphate cements, mitogenic events could occur either
during the initial mesenchyma1 cell contact or after osteoid
degradation by osteoblast collagenase. In a dense, mineralized
material such as calcium orthophosphate cements, which provides a
barrier to the free diffusion of circulating hormones, growth factors,
and cytokines, it is questionable whether the local responses at the
periphery of the material regulate osteoconduction [22]. The tissue
response to injectable calcium orthophosphate cements is well
4.6.4. Bioactivity
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