Biomedical Engineering Reference
In-Depth Information
Further follow-up of the RMI stem confirms this
conclusion. A survival rate of 70% was reported after
15 years follow-up of 149 hips in 135 patients
with revision for any reason as the endpoint. Revi-
sions included four hips with revision of the acetab-
ular component only, 15 hips with revision of the
femoral component only, and 50 with revision of both
components. Of the 64 revised patients, 58 were for
confirmed loosening. However, the authors noted that,
in their experience, use of the RM cup together with
other types of stems resulted in better performance
[14]
. Nagi et al. reported on 10- to 15-year follow-up
of 120 hips in 102 patients with average age of 42.5
years at the time of surgery. Although eight stems
required revision and 10 stems were radiographically
loose, none of the cups showed any evidence of
loosening. The authors concluded that “provision of
a titanium coating on the isoelastic stem, comparable
to that on the RM cup, would presumably have
improved its long-term fixation by encouraging bony
ingrowth”
[15]
.
In the 1970s and 1980s, surgeons also experi-
mented with metallic, noncemented hip implants that
were coated with a thin layer of Proplast, a porous
polymer composite of vitreous carbon and poly-
tetrafluorethylene
[16
e
21]
. The low-modulus,
porous surface layer was employed in the hopes that
it would promote cementless fixation
[16]
. Whether
employed on hemiarthroplasties
[18]
or, with later
incarnations, in total hip replacement femoral stems
[17,22]
, the polymer coating led to unacceptably high
loosening rates and was ultimately abandoned in the
1990s
[21]
, but unfortunately not before it had been
implanted in hundreds of patients.
The BHC stem provides an example of a polymer
composite, isoelastic hip stem that survived preclinical
evaluation but resulted in unanticipated failures clini-
cally. In 1985, Biomet (Warsaw, IN) formed BHC
Laboratories Inc., a joint venture with Hercules Inc., to
produce high-performance composite orthopedic
products. BHC developed a composite stem that was
fabricated from laminated, carbon-fiber-reinforced
polysulfone and did not contain a metal core. An
animal study, in which a CF/polysulfone hemi-
arthroplasty was implanted in 17 greyhounds,
demonstrated encouraging bone remodeling around
the stem and a “benign host-tissue response”
[23]
.The
BHC stem was clinically implanted in Europe. Short-
term fractures of the stem occurred at the neck
e
body
junction, and the report of one such casewas published
[24]
. The joint venture was dissolved in May 1995.
Despite initial setbacks in the development of
polymer
e
metal composite cementless femoral
stems, the ideal of an isoelastic prosthesis has
remained conceptually appealing to researchers in
the orthopedic community. Several high-
performance polymer composites, including PEEK,
were recognized in the late 1980s as possible
candidates for new isoelastic hip stem designs
[25]
.
A variety of polymers, including polysulfones and
polyaryletherketone (PAEK) materials, were consid-
ered as candidate biomaterials during this period by
the orthopedic community
[23,25
e
27]
. In 1988,
Skinner wrote that “despite the lack of evidence
demonstrating a clear need for a hip prosthesis with
a lower modulus and stiffness, composite prosthetic
hips are being intensively studied by the orthopedic
implant industry”
[25]
. Notwithstanding its concep-
tual attraction, even in biomechanical engineering
circles, the clinical importance of stress shielding in
the early 1990s was a topic of considerable debate
and uncertain significance
[28]
. Concern over stress
shielding in femoral stems was totally eclipsed by
ultrahigh-molecular-weight polyethylene (UHMWPE)
wear-debris-induced osteolysis as the defining ortho-
pedic research issue of the 1990s
[29]
.
14.4 The Epoch Hip Stem
Although many different isoelastic stem designs
have been conceptualized over the years, the Epoch
was the first successful pioneer of PAEK polymers
for total hip arthroplasty. Created by engineers at
Zimmer Inc. (Warsaw, IN), the Epoch stem has the
longest and most extensive track record of composite
femoral implants published in the literature,
including animal studies
[30,31]
, as well as human
clinical trials
[32
e
34]
(
Fig. 14.3
). The development
history of the Epoch stem, recounted by Michael
Hawkins, Ph.D., from Zimmer at the 2010 MD&D
Minneapolis conference
[35]
, illustrates the chal-
lenges facing an engineering team when trying to
incorporate what was, at the time, a new polymer into
a new implant design.
An engineering development team at Zimmer
embarked on developing a polymer composite stem
in the mid-1980s
[35]
. A wide range of polymer
composites was considered, including polysulfones
and ultimately macrocomposites fabricated from
PAEK polymers and metal (
Fig. 14.4
). Polysulfone
was one of the first high-performance thermoplastics
