Biomedical Engineering Reference
In-Depth Information
around the surfaces of the particles in response
to the mechanical stresses that promulgate
growth and remodeling processes [
t he surface, as wel l as in t he core of t he ceramic.
The formation of microcrystals with Ca/P
ratios similar to those of bone apatite crystals
occurs simultaneously. The abundance of these
crystals is directly related to the HA/
]. In
these situations, the total particulate surface
area partially replaces the role of macropores.
Bioresorption is the process by which the
ceramic material dissolves into its ionic com-
ponents in the physiological fl uid. The remod-
eling process occurs simultaneously, generating
new bone-ceramic interfaces that control the
rate of formation of new bone. If the resorption
activity of osteoclasts and the associated osteo-
blastic activity are balanced by the proper
choice of a ceramic, the remodeling process
appears to be at its maximum effectiveness.
Therefore, after complete resorption of the
ceramic, the remodeled bone is stronger than
new bone/ceramic mix formed with nonresorb-
ing biomaterial [
30
,
56
-TCP
ratio in the BCP. This indicates that it is possi-
ble to regulate the kinetics of dissolution and
precipitation and therefore the bioactivity.
“The coalescing interfacial zone of biological
apatite and residual crystals provides a scaffold
for bone cell adhesion and further bone
ingrowth” [
β
]. The “bioactive concept” is
thus based on the assertion that the dissolu-
tion/transformation processes are applicable to
bulk, coating and injectable ceramic-based bio-
materials, and that the events at the calcium
phosphate biomaterial/bone interfaces repre-
sent a dynamic process that ultimately contrib-
utes to the unique strength of these interfaces
[
20
,
63
]. The remod-
eling process of the host bone and the resorption
of the ceramic are affected by the phagocytos-
ing cells of the host and are a function of the
microporosity and chemical makeup of the
ceramic [
8
,
20
,
40
,
88
,
102
].
The above-mentioned work has led to a
variety of commercially available BCP prod-
ucts for bone graft or bone substitutes in ortho-
pedics and dentistry [
20
]. These processes are modulated
by the osteoclast-induced degradation of a
calcium phosphate ceramic [
8
,
20
]. In vivo studies of a
number of injectable bone substitutes (IBSs)
have been carried out [
21
]. The
attachment of osteoclasts to the ceramic surface
is mediated largely by extracellular matrix
(ECM) proteins. In addition, the capability of
the osteoclasts to resorb calcium phosphate
ceramic appears to be related to the solubility
of the ceramic [
40
,
88
,
102
30
]. This work utilized
a
weight ratio mineral phase of BCP
granules, (
50
:
50
40
to
80
µ
m or
80
to
200
µ
m in diam-
eter) dispersed in a
% aqueous solution of
hydroxypropylmethylcellulose (HPMC). The
injectability and properties of an IBS (with
3
40
-
]. After attachment to the
ceramic surface, the osteoclasts create a sealed
extracellular microenvironment into which
calcium ions are released. As resorption pro-
ceeds, [Ca
2
+
]
i
increases in the fl uid inside the
podosome membrane. Beyond a certain [Ca
2
+
]
i
value, resorption ceases and the osteoclast
migrates away. A highly soluble ceramic such
as
88
to
m particles) were compared with those
of a calcium phosphate cement composed of
∼
10
µ
80
-
µ
m calcium phosphate granules. Calcium
phosphate cement was found to be more readily
injectable than the IBS when needles with
inside diameters of
0
.
84
or
0
.
61
mm were used;
when needles of
mm were used,
however, calcium phosphate cement was not
injectable, whereas the IBS material remained
injectable.
In vivo studies on the two IBSs were com-
pared with those performed on calcium phos-
phate cement [
0
.
51
or
0
.
41
-TCP leads to ineffective remodelling of the
defect site. Therefore mixtures of low-solubility
hydroxyapatite (HA) with high solubility
β
β
-
tricalcium phosphate (
-TCP) have been used
to tailor the osteoclastic process to the re-
quirements of a particular scaffold applica-
tion [
β
]. Ten New Zealand white
rabbits were injected with bone substitutes
implanted into critical-sized defects at the
distal end of the femur. Three weeks after
implantation, scanning electron microscopy
(SEM) indicated that newly formed bone devel-
oped throughout the defect volume and in the
intergranular spaces that surrounded the BCP
granules in the IBS. With the calcium phos-
phate cement, on the other hand, newly formed
bone only developed on the surface of the
30
].
The “bioactive concept” for biphasic calcium
phosphate ceramics (BCP) is based on an
optimum balance of the more stable phase of
HA and the more soluble TCP [
8
,
102
]. The
biodegradation of implanted particles or blocks
of BCP results in an increase in the HA/
20
,
22
,
36
-TCP
ratio, a decrease in the average size of the BCP
crystals, and an increase in macroporosity of
β
