Biomedical Engineering Reference
In-Depth Information
Figure 14.8
Change in BMD compared
to baseline for two clinical studies of
the Epoch stem. The percentages
represent bone loss in comparison with
values obtained immediately after the
surgery in the seven Gruen zones. (a)
Median change in bone mineral at 2-
year follow-up examination in 20 hips
with a PAEK and 14 hips with
a titanium stem (Adapted from
K
¨
rrholm, 2002). (b) Mean change in
bone mineral density scans performed
in 16 patients at 24 months. (Adapted
from Ref.
[34]
.)
(a)
(b)
PAEK: -6%
Ti: -23%
PAEK: -15%
Ti: -38%
-11.9%
-27.5%
PAEK: -12%
Ti: -19%
PAEK: -9%
Ti: -22%
-4.3%
-11.7%
PAEK: -6%
Ti: 0%
PAEK: -5%
Ti: 0%
+3.2%
+4.4%
PAEK: -8%
Ti: -6%
-0.8%
receive either an Epoch stem or a hydroxyapatite
e
tricalcium phosphate-coated titanium alloy stem with
a similar geometry (Anatomic, Zimmer). At 2 years
of follow-up, the hips with an Epoch stem had lost
less bone mineral in four (1, 2, 6, and 7) of the seven
Gruen regions than the hips with an anatomic stem.
In the most pronounced regions, the median bone
mineral loss was less than half with the Epoch stem
than the anatomic
[33]
. In a study of 28 patients with
an average age of 51 years, maximal bone loss
occurred in the proximal femur within the first 6
months after implantation. A maximal loss of 27.5%
of bone mineral was noted in Gruen zone 7. This
compared favorably with data reported for other, all-
metal stems
[34]
. The results of these studies,
summarized in
Fig. 14.8
, verify that in addition to
providing for both initial and long-term stability, the
Epoch stem also reduces stress shielding in
comparison to uncemented all-metal stems.
The 510(k) application for the Epoch stem was
cleared by the FDA in July 2002. When BASF dis-
continued UltraPek, Zimmer stockpiled sufficient
quantities of the resin to support the anticipated
future needs of the product line
[41]
. An updated
version of the composite stem was developed using
PEEK as the thermoplastic layer of the design. The
PEEK version of the Epoch stem, branded the Versys
Epoch Fullcoat stem (sometimes referred to as
“Epoch II”), was cleared by the FDA in February
2006 (Fig. 7.3). The PEEK used in the Versys stem
was initially Gatone resin (Gharda, India)
[41]
. After
Gharda Gatone PEEK was withdrawn from the
market, Zimmer shifted to Zeniva PEEK (Solvay) for
the Epoch II.
The Epoch is the first success story for PEEK in
orthopedic implants. Just as the Brantigan cage is the
pioneering application of PEEK in the field of spinal
devices (see Chapter 13), the Epoch stem is the
pioneer of PEEK in orthopedics. Like the Brantigan
cage, the Epoch stem had its inception in the 1980s
and its development history spanned two decades
during which its biocompatibility and mechanical
behavior were characterized, and its pathway to
regulatory approval was successfully navigated. As
with the Brantigan cage, the successful commer-
cialization of the Epoch stem was complicated by the
withdrawal of two PAEK resin suppliers from the
market. In retrospect, the success of the Epoch
established PEEK as a successful biomaterial that
many novel designs of orthopedic implants would
later incorporate.
14.5 Other PAEK Composite Hip
Stems
Many different composite hip stem design
concepts were analyzed and tested in the 1990s,
including the BHC stem discussed previously.
Maharaj and Jamison
[42]
investigated the creep
