Biomedical Engineering Reference
In-Depth Information
could jeopardize the interface stability. Analysis of
two Epoch stems retrieved at autopsy after 13 and 48
months after clinical implantation demonstrated
mean bone ingrowth along the entire length of the
stem to be 49.62% and 73.57% and the mean bone
ongrowth to be 54.18% and 80.92%
[34]
. These
results demonstrate the potential for bony fixation of
the Epoch stem.
Zimmer initiated a prospective, multicenter trial
of the Epoch stem in 1994 as part of an Investiga-
tional Device Exemption (IDE) study for the Food
and Drug Administration (FDA). This study was
two-armed with 171 hips implanted at 11 different
centers in the United States and an additional 230
total hips in 10 countries outside of the United States
[32,37]
. The mean ages of the populations were 57
years in the American study and 56 years in the
international study. At a minimum follow-up of 2
years, all the implants had achieved ingrowth. At
a mean follow-up of 8 years, there were no radio-
graphically loose femoral components and none
required revision for mechanical failure
[37]
.
Hartzband et al. have reported on the survivorship of
106 implants in 102 patients from the original IDE
study at 10 years. Secure implant fixation and
maintenance of periprosthetic bone were noted
radiographically with retention of medial and lateral
endocortical-implant contact in all cases (97/106),
where it was observed after the initial procedure
[38]
. There were four reoperations in this cohort;
three reoperations were related to polyethylene wear
and one was for a femoral fracture. The results of
these clinical studies demonstrate that the implant
design has fulfilled its design objectives of providing
a stable implant in both the short and intermediate
term.
Having established that the Epoch stem was
capable of fulfilling the requirements for an effica-
cious uncemented stem, it remains to examine
whether it fulfilled its primary design goal of
reducing stress shielding of the proximal femur. Two-
dimensional FE models of isoelastic stems have
predicted reduced bone loss with isoelastic stems
compared with metal stems
[7,8]
and these results
have been further verified using more complex three-
dimensional FE models
[39]
. Adaptation of the peri-
prosthetic bone has been evaluated clinically by
evaluating bone mineral density using Dual-Energy
X-ray Absorptiometry (DEXA)
[32
e
34,37,40]
.
Karrholm et al. compared periprosthetic bone
mineral density in 68 hips that were randomized to
(a)
(b)
(c)
Figure 14.7
Production images of the Epoch stem: (a)
preassembly of the core and mesh (upper stem) before
(upper) and after (lower) injection molding; (b) removing
the stem from the injection-molding press; (c) tray of
molded stems, prior to trimming PAEK sprues and
flashing. Images courtesy of Michael Hawkins, Zimmer
Inc.
the stiffness demonstrated reduced bony fixation of
the composite stem
[31]
. The authors noted that this
study confirmed that excessive flexibility (even in the
presence of a favorable surface for bony ingrowth)
