Biomedical Engineering Reference
In-Depth Information
Absorbable matrix composites
ranges from 17 to 24 GPa, depending upon age and lo-
cation of the specimen, while the commonly used alloys
have moduli ranging from 110 GPa (titanium alloys) to
210 GPa (316L steel). This large difference in stiffness
can result in disproportionate load sharing, relative
motion between the implant and bone upon loading, as
well
Absorbable matrix composites have been used in situa-
tions where absorption of the matrix is desired. Matrix
absorption may be desired to expose surfaces to tissue
or to release admixed materials such as antibiotics or
growth factors (drug release) (
Yasko
et al.
, 1992
).
However, the most common reasons for the use of this
class of matrices for composites has been to accomplish
time-varying mechanical properties and assure complete
dissolution of the implant, eliminating long-term bio-
compatibility concerns. A typical clinical example is
fracture fixation (
Daniels
et al.
, 1990; Tormala, 1992
).
as
high
stress
concentrations
at
bone-implant
junctions.
Another potential problem is that the alloys currently
used corrode to some degree. Ions so released have been
reported to cause adverse local tissue reactions as well as
allogenic responses, which in turn raises questions of ad-
verse effects on bone mineralization as well as adverse
systemic responses such as local tumor formation (Martin
et al.,
1988). Consequently, it is usually recommended
that a second operation be performed to remove hardware.
The advantages of absorbable devices are thus two-
fold. First, the devices degrade mechanically with time,
reducing stress protection and the accompanying osteo-
porosis. Second, there is no need for secondary surgical
procedures to remove absorbable devices. The state of
stress at the fracture site gradually returns to normal,
allowing normal bone remodeling.
Absorbable fracture fixation devices have been produced
from PLLA polymer, PGA polymer, and PDS. An excel-
lent review of the mechanical properties of biodegradable
polymers was prepared by Daniels and co-workers
(
Daniels
et al.
, 1990
;see
Figs. 3.2.12-6
and
3.2.12-7
).
Fracture fixation
Rigid internal fixation of fractures has conventionally
been accomplished with metallic plates, screws, and
rods. During the early stages of fracture healing, rigid
internal fixation maintains alignment and promotes pri-
mary osseous union by stabilization and compression.
Unfortunately, as healing progresses, or after healing is
complete, rigid fixation may cause bone to undergo stress
protection atrophy. This can result in significant loss of
bone mass and osteoporosis. Additionally, there may be
a basic mechanical incompatibility between the metal
implants and bone. The elastic modulus of cortical bone
Material, fiber
PLA
PLA, carbon
PLA, inorganic
PLA, PLA
PGA, PGA
PGA/PLA
PGA/PLA, carbon
PGA/PLA, PGA/PLA
POE
Minimum
Maximum
Average
Cortical bone
316L Stainless
Nylon 6
0
5
0
100
150
200
250
300
350
400
450
Flexural strength (MPa)
Fig. 3.2.12-6 Representative flexural strengths 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.).