Biomedical Engineering Reference
In-Depth Information
Material, fiber
PLA
Minimum
Maximum
Average
PLA, carbon
PLA, inorganic
PLA, PLA
PGA, PGA
POE
Cortical bone
316L Stainless
Nylon 6
UHMW PE
0
2
0
4
0
6
0
8
0
100
120
140
160
180
200
Flexural modulus (GPa)
Fig. 3.2.12-7 Representative flexural moduli of absorbable polymer composites (from Daniels, A. U., Melissa, K. O., and
Andriano, K. P. (1990). Mechanical properties of biodegradable polymers and composites proposed for internal fixation of bone.
J. Appl. Biomater. 1(1): 57-78.).
Their review revealed that unreinforced biodegrad-
able polymers are initially 36% as strong in tension as
annealed stainless steel, and 54% in bending, but only 3%
as stiff in either test mode. With fiber reinforcement,
highest initial strengths exceeded those of stainless steel.
Stiffness reached 62% of stainless steel with non-
degradable carbon fibers, 15% with degradable inorganic
fibers, but only 5% with degradable polymeric fibers.
Most previous work on absorbable composite fracture
fixation has been performed with PLLA polymer. PLLA
possesses three major characteristics that make it a po-
tentially attractive biomaterial:
those of pure polymer plates.
In vivo,
the matrix degraded
and the plates lost rigidity, gradually transferring load to
the healing bone. However, the mechanical properties of
such chopped fiber plates were relatively low; conse-
quently, the plates were only adequate for low-load situ-
ations.
Zimmerman
et al.
(1987)
used composite theory
to determine an optimum fiber layup for a long fiber
composite bone plate. Composite analysis suggested the
mechanical superiority of a 0
/
45
laminae layup. Al-
though the 0
/
45
carbon/PLA composite possessed
adequate initial mechanical properties, water absorption
and subsequent delamination degraded the properties
rapidly in an aqueous environment (
Fig. 3.2.12-8
). The
fibers did not chemically bond to the matrix.
In an attempt to develop a totally absorbable com-
posite material, a calcium-phosphate-based glass fiber has
been used to reinforce PLA. Experiments were pursued
to determine the biocompatibility and in vitro degrada-
tion properties of the composite (Zimmerman
et al.,
1991). These studies showed that the glass fiber-PLA
composite was biocompatible, but its degradation rate
was too high for use as an orthopedic implant.
Shikinami and Okuno (2001)
, have produced mini-
plates, rods, and screws made of HA poly(
L
-lactide).
These composites have been principally applied for in-
dications such as repair of bone fracture in osteosynthesis
and fixation of bony fragments in bone grafting and
osteotomy, exhibiting total resorbability and osteological
1.
It degrades in the body at a rate that can be
controlled.
2.
Its degradation products are nontoxic, biocompatible,
easily excreted entities. PLA undergoes hydrolytic
deesterification to lactic acid, which enters the lactic
acid cycle of metabolites. Ultimately it is metabo-
lized to carbon dioxide and water and is excreted.
3.
Its rate of degradation can be controlled by mixing it
with PGA polymer.
PLLA polymer reinforced with randomly oriented
chopped carbon fiber was used to produce partially de-
gradable bone plates (
Corcoran
et al.
, 1981
). It was
demonstrated that the plates, by virtue of the fiber re-
inforcement, exhibited mechanical properties superior to
