Biomedical Engineering Reference
In-Depth Information
Fig. 3.2.12-8 Scanning electron micrograph of laminae buckling and delamination (D) between lamina in a carbon fiber-reinforced
PLA fracture fixation plate (from Zimmerman, M. C., Parsons, J. R., and Alexander, H. (1987). The design and analysis of a laminated
partially degradable composite bone plate for fracture fixation. J. Biomed. Mater. Res. Appl. Biomater. 21A(3): 345.).
bioactivity while retaining sufficient stiffness high stiff-
ness retainable for a long period of time to achieve bony
union. These plates are commercialized with the name of
Fixsorb-MX.
Furukawa
et al.
(2000b)
have investigated the
in vivo
biodegradation behavior of HA/poly(
L
-lactide) composite
rods implanted
sub cutem
and in the intramedullary
cavities of rabbits, showing that after 25 weeks of
sub
cutem
implantation rods maintained a bending strength
higher than 200 MPa. Their conclusion was that such
a strength was sufficient for application of the rods in the
fixation of human bone fractures.
By using a sintering technique,
Tormala
et al.
(1988)
have produced self-reinforced PGA (SR-PGA) rods that
have been used in the treatment of fractures and
osteotomies. Afterwards, by using the same technique,
self-reinforced PLLA (SR-PLLA) pins and screws
have been produced. The higher initial mechanical
properties of SR-PLGA are counterbalanced by their
faster decrease with respect to the SR-PLLA material,
which has a slower degradation rate and is reabsorbed in
12-16 months. These products are commercially available.
and chopped-fiber reinforcement has been used in bone
cements and bearing surfaces to stiffen and strengthen
these structures.
For fracture fixation, reduced-stiffness carbon-fiber-
reinforced epoxy bone plates to reduce stress-protection
osteoporosis have been made. These plates have also been
entered into clinical use, but were found to not be as reliable
or biocompatible as stainless steel plates. Consequently,
they have not generally been accepted in clinical use. By far
the most studied, and potentially most valuable use of
nonabsorable composites has been in total joint replacement.
Total joint replacement
Bone resorption in the proximal femur leading to aseptic
loosening is an all-too-common occurrence associated
with the implantation of metallic femoral hip re-
placement components. It has been suggested that
proximal bone loss may be related to the state of stress
and strain in the femoral cortex. It has long been recog-
nized that bone adapts to functional stress by remodeling
to reestablish a stable mechanical environment. When
applied to the phenomenon of bone loss around implants,
one can postulate that the relative stiffness of the me-
tallic component is depriving bone of its accustomed
load. Clinical and experimental results have shown the
significant role that implant elastic characteristics play in
allowing the femur to attain a physiologically acceptable
Nonabsorbable matrix composites
Nonabsorbable matrix composites are generally used as
biomaterials to provide specific mechanical properties
unattainable with homogeneous materials. Particulate
