Biomedical Engineering Reference
In-Depth Information
(e)
Figure 14.6
Schematic representation of the Epoch I
stem (Zimmer, Warsaw, IN). Image courtesy of Clare
Rimnac, Ph.D., Case Western Reserve University.
PAEK polymer was then injected between the core
and mesh. After the macrocomposite stem was
removed from the mold (
Fig. 14.7b and c
), the sprues
and any flashing were trimmed prior to final cleaning,
packaging, and sterilization.
The addition of titanium fiber mesh to the surface
of the Epoch stem offers advantages for both short-
term and long-term stability of the stem. Mechanical
testing of this material has demonstrated that the
coefficient of friction between fiber mesh and bone is
approximately 0.64
[36]
. It has been demonstrated
using finite element (FE) modeling that increasing
the coefficient of friction at the stem
e
bone interface
greatly reduced the expected micromotion and
normal stresses
[7]
. This has also been validated
clinically. Initial stability has been verified by using
X-ray radiostereometry to compare migration of the
Epoch stem to a forged CoCr stem with similar
geometry at 1 week, 3 months, 1 year, and 2 years
after implantation. It was determined that proximal
distal migration of the stem center was close to 0 mm
in both the groups
[33]
.
In addition to providing favorable conditions for
initial stability, titanium fiber mesh promotes long-
term fixation through bony ingrowth. Preclinical
testing of fiber mesh-coated cobalt chrome alloy and
PAEK composite stems in a canine model demon-
strated increased bone ingrowth proximally and
distally with greater ingrowth in the PEEK composite
device proximally and the cobalt chrome device
distally
[30]
. A further canine study comparing
a titanium and PAEK composite stem with 1/3 to 1/5
Figure 14.4
Continued.
Figure 14.5
When polysulfone was evaluated for hip
stem applications, the amorphous polymer was
observed to craze and fail when exposed to a lipid envi-
ronment. Image courtesy of Michael Hawkins, Zimmer
Inc.
