Biomedical Engineering Reference
In-Depth Information
Table 3.2.12-2 Typical mechanical properties of polymer-carbon
composites (three-point bending)
Polymer
Ultimate strength (MPa)
Modulus (GPa)
P
A
L
M
PMMA
772
55
Polysufone
938
76
Polysulfone
Epoxy
±
Directional
braid
Stycast
535
30
Uni-directional
core
Hysol
207
24
Polyurethane
289
18
A
stress state. Femoral stem stiffness has been indicated as
an important determinant of cortical bone remodeling
( Cheal et al. , 1992 ). Composite materials technology
offers the ability to alter the elastic characteristics of an
implant and provide a better mechanical match with the
host bone, potentially leading to a more favorable bone
remodeling response.
Using different polymer matrices reinforced with
carbon fiber, a large range of mechanical properties is
possible. St. John (1983) reported properties for 15
laminated test specimens ( Table 3.2.12-2 ) with moduli
ranging from 18 to 76 GPa. However, the best reported
study involved a novel press-fit device constructed of
carbon fiber/polysulfone composite ( Magee et al. , 1988 ).
The femoral component designed and used in this study
utilized composite materials with documented biologic
profiles. These materials demonstrated strength com-
mensurate with a totally unsupported implant region and
elastic properties commensurate with a fully bone-
supported implant region. These properties were
designed to produce constructive bone remodeling. The
component contained a core of unidirectional carbon/
polysufone composite enveloped with a bidirectional
braided layer composed of carbon/polysulfone composite
covering the core. These regions were encased in an outer
coating of pure polysulfone ( Fig. 3.2.12-9 ). Finite-ele-
ment stress analysis predicted that this construction
would cause minimal disruption of the normal stresses in
the intact cortical bone. Canine studies carried out to
4 years showed a favorable bone remodeling response. The
authors proposed that implants fabricated from carbon/
polysulfone composites should have the potential for use
in load-bearing applications. An implant with appropriate
elastic properties provides the opportunity for the natural
bone remodeling response to enhance implant stability.
Adam et al. (2002) reported on the revision of 51
epoxy resin/carbon fiber composite press fit-hip pros-
theses implanted in humans. Their result showed that
within 6 years 92% of the prostheses displayed aseptic
loosening, i.e., did not induce bone ongrowth. Authors
L
M
P
Fig. 3.2.12-9 Construction details of a femoral stem of a
composite total hip prosthesis. (From Magee, F. P., Weinstein, A.
M., Longo, J. A., Koeneman, J. B., and Yapp, R. A. (1988). A
canine composite femoralstem. Clin. Orthop. Rel. Res. 235: 237.)
attributed the failure to the smoothness of the stem
surface. No osteolysis or wear or inflammatory reaction
were, however, observed.
Different fibers matrices and fabrication technologies
have been proposed for the fabrication of hip prostheses.
Reviews of materials and methods are in Ramakrishna et al.
(2001) and in de Oliveira Simopes and Marques (2001).
Conclusions
Biomedical composites have demanding properties that
allow few, if any, ''off the shelf'' materials to be used. The
designer must almost start from scratch. Consequently,
few biomedical composites are yet in general clinical use.
Those that have been developed to date have been fab-
ricated from fairly primitive materials with simple de-
signs. They are simple laminates, chopped fiber, or
particulate reinforced systems with no attempts made to
react or bond the phases together. Such bonding may be
accomplished by altering the surface texture of the filler
or by the introduction of coupling agents: molecules that
can react with both filler and matrix. However, concerns
about the biocompatibility of coupling agents and the
high development costs of surface texture alteration
procedures have curtailed major developments in this
area. It is also possible to provide three-dimensional re-
inforcement with complex fiber weaving and impregna-
tion procedures now regularly used in high-performance
aerospace composites. Unfortunately, the high de-
velopment costs associated with these techniques have
restricted their application to biomedical composites.
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