Biomedical Engineering Reference
In-Depth Information
devices
[18,19]
. Brown et al.
[18]
at Case Western
Reserve University in Cleveland became interested
in exploring high-performance thermoplastics for
bone plates in the 1980s. Throughout the 1990s,
researchers at Case Western contributed to the liter-
ature on the biocompatibility of PEEK, as well
as composite hip stem development
[18,20
e
22]
.
Although researchers were curious about these
advanced polymers due to their mechanical perfor-
mance and isoelasticity, they also hoped to take
advantage of the thermoformability of the materials,
so that the fracture fixation plates could be more
precisely fit to patients' bony anatomy in the oper-
ating room, thereby overcoming a previous limitation
of thermosetting polymers, namely epoxy.
Brown et al.
[18]
examined the flexural fatigue and
thermoformability of PS, PBT, and PEEK reinforced
by 30% chopped polyacrylonitrile (PAN) carbon
fibers. Flexural bars were subjected to a variety of
challenges, including repeated steam sterilization,
and presoaking for 3 weeks in saline (0.9% NaCl).
Bars were also tested before and after a thermoform-
ing procedure, which involved heating above the glass
transition temperature in an oven followed by defor-
mation in a surgical plate bending press. In the case of
the PEEK composites, they were heated to 250
C and
bent to an angle of 5.8
. The samples were returned to
their original shape by reverse bending. The
researchers found that not only did PEEK exhibit the
highest fracture toughness and bending fatigue
resistance of the materials studied but also exhibited
properties that were insensitive to preconditioning or
thermoforming. In contrast with PBT and PS, PEEK
exhibited good compatibility with the carbon fibers,
which helped explain its superior fatigue resistance.
Researchers from Finland have recently investi-
gated the mechanical behavior and tissue response of
“optically amorphous” PEEK internal fracture fixa-
tion plates in an ovine model
[3]
. Neat, unfilled
PEEK-OPTIMA LT3 plates were injection molded
and thermally treated to produce “amorphous” and
“crystalline” components, with degrees of crystal-
linity of ~15% and 32
e
34%, respectively, as
measured by differential scanning calorimetry
(DSC). The plates were gamma sterilized (25 kGy
minimum dose), subcutaneously implanted in sheep,
and followed for up to 3 years. Histology showed
minimal inflammatory reactions for both types of
PEEK plates. Although the “amorphous” plates were
found to exhibit about half the strength of
the “crystalline” plates, no substantial changes in
mechanical properties were found over time. The
authors concluded that “both optically amorphous
and crystalline PEEK polymers are suitable materials
for the fixation of fractures and osteotomies”
[3]
.
Rohner et al.
[23]
used an osteotomized tibial sheep
model (n
¼
12) to compare callus stiffness between
fixation with locking compression plates and a 62%
carbon-reinforced PEEK “Snake Plate” (Icotec AG,
Altst¨tten, Switzerland). The study reported that there
were no differences in callus dimensions at any time
point, torsional stiffness, and strength of the healed
tibiae, all of which healed uneventfully (one plate
failure was excluded due to animal management
problems). Furthermore, none of the plates caused any
periosteal vascular disturbances. The study supports
using reinforced PEEK implants for load-bearing
fracture fixation in selected cases.
15.2.4 PEEK in Trauma
The necessary requirements for a polymer implant
material for internal fixation are sufficient strength
and stiffness, long-term stability, the ability to be
sterilized without degradation, and biocompatibility
[24]
. PEEK possesses a number of properties that
make it suitable for certain trauma applications.
PEEK is biocompatible, meaning there are no toxic,
inflammatory, or allergic reactions. Nieminen et al.
[3]
determined the tissue reactions to both optically
amorphous and crystalline (~30%) PEEK to be mild
and similar between groups. No granulomatous
foreign body reactions or clinically significant
inflammation were observed. Furthermore, there is
no expected cytotoxicity, mutagenicity, or immuno-
genicity
[25]
. It has a very high chemical resistance,
being insoluble in common solvents, and is resistant to
attack by all substances except sulfuric acid
[25]
.The
material has a high resistance to aging as it can
maintain its mechanical properties for up to 3 years
[3]
. Another property important to trauma applica-
tions is that PEEK, a thermoplastic, overcomes
limitations of previously used thermosetting polymers
in that it can be molded to patient anatomy in the
operating room
[26]
.
The turning point with the use of PEEK for trauma
applications is captured within its stiffness and
strength properties. Although unmodified PEEK has
a flexural modulus of only 4 GPa and a strength of
93 MPa, its stiffness and strength can be modified
through annealing and CFR filling, hydroxyapatite
(HA) filling, and HA coating
[27]
. Although there are
