Biomedical Engineering Reference
In-Depth Information
(Sintbone Slurry Gel®) was inserted. Seven
rabbits (
hydrogel-based scaffold may require external
support or a composite scaffold structure that
can support external stresses, yet retain the
advantages of the osmotic environment pro-
vided by the hydrogel. A creative use of com-
posite technology and an understanding of the
viscoelastic properties of the material during
injection and in the environment at the defect
site are necessary to achieve this objective.
5
+
2
controls) were sacrifi ced after
1
month and after
months. Histological, histo-
morphometric, and high-resolution x-ray inves-
tigations were carried out on sections
3
200
±
10
µ
m in thickness cut at different depths
between the defect surface and bottom. The
results were compared with those obtained
from empty defects and from defects fi lled with
the control material. Histological sections of
the untreated and treated defects at
4
weeks are
7.5 Outlook
shown in Fig.
. The formation of new bone
(NB) in the untreated cavities of control speci-
mens re-mained restricted to the edge of the
defects (Fig.
7
.
3
A). Newly formed bone was gen-
erated radially inward from the defect surfaces
in both the synthetic polymer-treated control
(Fig.
7
.
3
Injectable scaffolds for the regeneration of bone
fall into two main categories: fl owable ceramic-
water mixtures that set in situ as either com-
pacted, porous scaffolds or porous particulate
mixtures in a polymeric carrier; and natural or
synthetic hydrogels with high water content
that encapsulate and carry compounds and
cells to the injection site.
Differences in composition and physical
properties notwithstanding, these two catego-
ries of scaffolds can promote bone tissue regen-
eration and can replace invasive surgery in
many situations. They do this, however, by two
very different mechanisms. With ceramic-
based scaffolds, solubilization and resorption
of the scaffold furnish the appropriate mineral
environment to guide the regeneration process.
With hydrogel scaffolds, bone regeneration
involves a self-regulating process that is guided
by the cells and mineral components in the
aqueous phase of the scaffold.
In materials that are calcium phosphate-
based, osteoclasts adhere to the external sur-
faces and to accessible pore surfaces, i.e., those
larger than
B) and the fi broin hydrogel-treated
experimental animals (Figs.
7
.
3
C and D). Thin
and dense new trabeculae (NB in Fig.
7
.
3
) grew
radially from the old bone (OB) surface of the
synthetic polymer-treated defect, but with a
noticeable interphase between the NB and the
OB (Fig.
7
.
3
B). Thin and dense new trabeculae
(NB) also grew radially f rom the OB surface of
the fi broin hydrogel-treated defect. In this case,
however, there was no noticeable interface
between the NB and the OB (Fig.
7
.
3
C). One of
the fi ve rabbits treated with the fi broin hydrogel
exhibited full recovery after
7
.
3
4
weeks (Fig.
7
.
3
D).
At
weeks, the regrown cancellous bone in the
fi broin hydrogel-treated defects was signifi -
cantly thicker and denser than either normal
bone or bone grown in the synthetic polymer-
treated defects. Twelve weeks after surgery,
however, the bone grown in the fi broin hydro-
gel-treated defects had changed appearance
completely. It appeared more similar to normal
bone than bone in the synthetic polymer-treated
defects. At
4
m, generating an extra-
cellular matrix that contains calcium ions from
the ceramic. The osteoclasts initiate new bone
growth at the ceramic surface and migrate at
the receding bone-ceramic interfaces at a rate
that is determined by the rate of dissolution of
the ceramic. In the case of the highly swollen
hydrogels, cells and dissolved mineral compo-
nents from the host penetrate the dilute space
of the scaffold, and it is the movement of cells,
ionic mineral components, and polymer chains
of the hydrogel skeletal network in the dilute
environment of the body fl uids that determines
the rate of bone regeneration. Ideally, the
freedom of movement of cells in the physiologi-
cal environment of the hydrogel is much like
200
to
300
µ
weeks all six rabbits (fi ve fi broin-
treated and one polyester-based control) showed
full recovery, but the fi broin hydrogel acceler-
ated bone remodeling. The distal femur areas
were occupied by trabecular bone with the
spatial orientation, shape, and size seen in
healthy cancellous bone.
To f u nc t ion a s a n i nje c t a ble s c a f fold , a hyd ro -
gel must be injected as a solution or dispersion
that can be cross-linked in vivo. Alternatively,
it must be injected as a reversible gel and
restructured in the defect cavity. Either scaf-
fold will have poor tensile and shear properties
because of high water content. Where load
bearing is needed during repair, an injectable,
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