Biomedical Engineering Reference
In-Depth Information
are less organized in structure but have higher
DNA content than corresponding tendons [
5
,
31
].
4.3 Applications of
Biodegradable
Orthopedic Implants
In designing scaffolds for orthopedic implants,
the envisioned fi nal application must be a
primary concern from the beginning. Scaffolds
may be used as internal fi xation devices to
support the defect site. Alternatively, scaffolds
may be implanted to induce cell migration and
proliferation to aid in tissue repair. Another
potential strategy is the use of scaffolds to
provide localized delivery of bioactive mole-
cules, cells, or a combination to enhance defect
healing.
4.3.1 Systems for
Mechanical Support
Figure 4.1.
Photographs of a biodegradable fixation plate
and an interbody fusion spacer fabricated by the use of trans-
parent silicone molds. (A) 1.5-mm, eight-hole adaptation plate
manufactured with 70 : 30 P(L/DL-LA) (left) and PPF/PPF-DA
(double-bond ratio of 0.5) (right). (B) Plastic model (left) and
PPF/PPF-DA (double-bond ratio of 0.5) replicate (right) of a 5-
mm lordotic anterior cervical fusion (ACF) spacer. Reproduced
with permission from Timmer et al. [98]. Copyright 2003, with
permission from Elsevier.
In many cases, biodegradable orthopedic mate-
rials have been applied during the healing
process in the form of fi xation implants such as
screws, staples, pins, rods, and suture anchors
to support areas weakened by bone fracture,
sports injury, or osteoporosis [
]. High
mechanical strength and stiffness are extremely
important in designing biodegradable devices
for orthopedic procedures in which high loads
are applied after the devices have been
implanted. Long degradation times for the
biomaterials are also often desired for these
applications [
14
,
37
,
98
4.3.2 Systems for Delivery of Cells
or Bioactive Factors
4.3.2.1 Bioactive Factors
In addition to providing physical support, scaf-
folds have been employed to introduce bioac-
tive molecules at the defect site [
]. A study comparing a bio-
degradable interference screw made of poly(L-
lactide) with a titanium interference screw in
the porcine anterior cruciate ligament demon-
strated that the poly(L-lactide) screw could
provide a promising alternative in terms of
primary fi xation strength [
17
,
20
]. In one
strategy, scaffolds can be used to control the
release of bioactive molecules, thus accelerat-
ing the healing process [
39
,
66
]. A mixture
of poly(propylene fumarate) (PPF) and
poly(propylene fumarate)-diacrylate (PPF-DA)
has been molded into a biodegradable fi xation
plates (Fig.
84
]. In other cases, the
effectiveness of less stable drugs may be
extended by encapsulating them inside a matrix
[
41
A) and a bone allograft interbody
fusion spacer (Fig.
4
.
1
B) with acceptable
mechanical properties for use in these applica-
tions [
4
.
1
]. Several delivery systems have been devel-
oped, including nano- or microparticles and
hydrogel-based implants.
50
98
].
