Biomedical Engineering Reference
In-Depth Information
apparatus in each case limits their applicability to the
clinical situation. The results of unidirectional tests
cannot be compared directly with the multidirec-
tional pin-on-plate studies. These studies do however
illustrate the growing interest in screening PEEK
biomaterials using simplified in vitro experiments. It
must be emphasized that for hip-bearing applica-
tions, multidirectional kinematics are a prerequisite
for a valid screening test.
Overall, the basic science studies summarized in
this section provide further confirmation of the low
wear rates that can be achieved with CFR-PEEK
orthopedic bearings under very specific and simpli-
fied biotribological test conditions. These studies
also provide a technical basis for easy comparison of
the wear resistance of a wide range of potential
bearing couples involving PEEK-based biomaterials,
CoCr alloys, and ceramics. For example, the perfor-
mance of PEEK-on-PEEK generally resulted in
higher wear rates than tests involving CFR-PEEK.
Pin-on-disc studies most likely have the greatest
relevance to low-load or low-stress joint applications,
including hip resurfacing, mobile bearing knees, and
finger joints. However, extrapolating the results of
pin-on-disc experiments to the more complex loads
and stress states of actual orthopedic and spinal
devices remains problematic. As a result, the pin-on-
disc results reported here should be considered as the
findings from a battery of biomaterials screening
tests. The ultimate goal of these pin-on-disc tests is to
help guide and accelerate early discovery and mate-
rial selection for the most promising bearing surface
combinations. Once identified, these candidate
bearing surface combinations must be evaluated
under more clinically relevant device tests, as have
been performed for hip resurfacing.
second-generation hip resurfacing designs that do not
employ an MOM-bearing couple.
Contemporary acetabular components are hemi-
spherical, resulting in nonphysiologic stress distribu-
tions in the acetabulum
[52]
. A novel horseshoe-shaped
acetabular component was developed by Mr. Richard
Field and Professor Neil Rushton at Cambridge
University (Cambridge, UK) with the rationale of
duplicating the physiologic load distribution in the
natural acetabulum, thereby reducing stress shielding,
which can compromise fixation and facilitate
wear debris migration into the periprosthetic bone
[18,52
e
56]
. The acetabular component was fabricated
with an outer shell of CFR polybutylene terephthalate
and an inner liner of UHMWPE. The Cambridge Cup
was implanted by Field and Rushton in 50 elderly
female patients. Of the 50 implants, 24 were coated
with hydroxyapatite and the remaining 26 cups were
uncoated. The HA-coated implants did not exhibit
significant wear or migration, but the uncoated
implants migrated and three of the 26 cups had to
be revised at 2 years
[53]
. In a DEXA study of Cam-
bridge Cup patients, after 2 years, researchers did not
find a significant decrease in periprosthetic bone
mineral density, as compared with unimplanted
controls
[54]
. Consequently, the flexible, horseshoe-
shaped cup design appeared to afford reduced stress
shielding in clinical use, as anticipated by the surgeon
designers.
Based on the encouraging clinical history of
the Cambridge Cup, a new flexible horseshoe-shaped,
CFR-PEEK cup design was recently developed for hip
resurfacing applications (MITCH PCR: Stryker SA,
Montreux, Switzerland). The MITCH PCR horseshoe
cup is injection molded using a similar formulation as
in the ABG clinical trial: 30% pitch fibers blended
with PEEK-OPTIMA LT3 resin. To accommodate the
large ceramic head sizes associated with a resurfacing
arthroplasty, the total thickness of the composite
acetabular component is approximately 3 mm, which
is far below the 6 mm minimum thickness of
UHMWPE currently used in traditional hip replace-
ments
[57]
. COC andMOMbearings, while satisfying
the wear resistance requirements for a resurfacing
arthroplasty, are not sufficiently flexible to be incor-
porated into a horseshoe-shaped cup design.
The back surface of the MITCH cup has fins for
initial stability
[56]
. For bone apposition and long-
term stability, the back surface of the CFR-PEEK cup
is also initially plasma sprayed with Ti and then
plasma sprayed a second time with a layer of
16.4 Hip Resurfacing
A promising application for CFR-PEEK bearings
is in hip resurfacing, which has emerged in the past
decade as an alternate treatment for degenerative
arthritis for the young active patient population.
First-generation hip resurfacing designs employed
a large-diameter, MOM articulation, combined with
a minimally invasive femoral head prosthesis and
a short, cemented stabilization pin. Due to long-
standing concerns about metal hypersensitivity and
long-term exposure to metal ions by young patient
candidates for this procedure, there is great interest in
