Biomedical Engineering Reference
In-Depth Information
many variations to CFR-PEEK, and alterations to
mechanical properties can be quite complex, the
virgin material can be modified to have a flexural
modulus of up to 21 GPa and a tensile strength of
225 MPa. There is some anisotropy that is introduced
through the fiber reinforcement. When compared
with other high-performance thermoplastic materials
of PBTand PS, Brown et al.
[18]
found PEEK to have
the highest fracture toughness and bending fatigue
resistance, properties they found to be insensitive to
preconditioning and thermoforming. Its superior
fatigue resistance is understood to be derived from
its intimate bond with carbon fibers, and the
matrix
e
fiber interfacial bond is enhanced by the
creep resistance of PEEK.
Another significant advantage of PEEK is that it is
radiolucent. Traditional biomedical metals are radio-
opaque and create artifacts in X-ray radiography,
computed tomography (CT), or magnetic resonance
imaging (MRI)
[3,10]
. This can make it difficult to
monitor the status of the healing bone in the vicinity
of the plate. PEEK creates no such artifacts, allowing
good visualization and evaluation of bone healing by
radiography. This is an especially useful property in
anatomic regions where the prosthesis covers a large
amount of the bone surface, such as a typical bone
plate for distal radius fractures.
Finally, PEEK is thermally stable at sterilization
temperatures and will not degrade during either
electron beam or gamma irradiation
[24]
. PEEK will
absorb water at only 0.5% of its volume under
ISOR262A conditions and will not hydrolyze
[23,25]
. Furthermore, the degree of water uptake can
be reduced by increasing the volume fraction of the
fiber-reinforced material as carbon fibers do not
absorb water and do not degrade under physiological
conditions
[23]
. If sterility is to be achieved through
gamma radiation, neither carbon fibers nor virgin
PEEK degrade under gamma irradiation. In general,
carbon-reinforced PEEK is resistant to all common
sterilization techniques
[23]
.
plates. Difficulties in manufacturing not only affect
performance characteristics but also increase the costs
of the polymer devices in what has traditionally been
a comparatively cost-sensitive device market.
Although fracture micromotion is now accepted as
beneficial, simply creating more elastic fixation is not
the answer to the stress-shielding problem
[9]
.
Micromotion must be limited to the axial direction and
should be resisted using adequate stiffness in bending,
torsional, and shear directions
[9]
. Each directional
stiffness of the bone
e
implant complex, for example
the bending stiffness, is the product of both the elastic
modulus and the moment of inertia. Because of this,
the stress-shielding arguments of the 1980s are not
currently regarded as credibly advantageous, because
the bending stiffness, for example, of an implant can
be altered by changing its moment of inertia, as well as
by modifying its elastic modulus
[31]
. Due to the
limited space between the soft tissues and fractured
bones, a small profile or small volume plate is gener-
ally preferred as long as the strength requirements of
the system are met. It may, however, be beneficial to
use PEEK implants on a selective basis. In its current
form, PEEKmay be suitable for low andmedium load-
bearing applications. Another option may be to design
optimized PEEK structures, specific to the biome-
chanics of a particular fracture site. This techniquewas
most recently used by Lovald et al.
[32
e
34]
for tita-
nium and bioabsorbable polymers. The result of this
work could lead to low-profile PEEK structures that
minimize instability in selective directions.
In addition to the basic material stiffness and
strength limitations with PEEK for trauma applica-
tions, there is also a significant notch-sensitive factor
with PEEK materials
[27]
. Metallic bone plates are
often notched to minimize the overall volume of the
plate and to facilitate bending in various directions.
Notched specimens of PEEK samples have shown
brittle behavior (i.e., little to no yielding) via an
increased reduction in maximum axial strength with
increasing notch severity
[27]
. Notching is reported
to decrease the maximum axial true stress by
approximately 35%
[27]
. Given this limitation,
PEEK bone plates will likely have a different profile
compared with their notched metallic counterparts.
15.2.5 Limitations
Over the past decade, research has continued on the
development of braided, laminated, and unidirectional
fiber-oriented PEEK fracture fixation plates, as well as
extruded PEEK screws
[8,10,28
e
30]
. These articles
describe perceived limitations in the manufactur-
ability of the complex composites as historical
barriers to the use of CFR-PEEK as fracture fixation
15.2.6 Current Landscape for
PEEK Trauma Implants
Both epoxy and PEEK composite fracture fixation
plates are currently commercially available. Epoxy
