Biomedical Engineering Reference
In-Depth Information
more in the superior part of the reconstructed
acetabulum, demonstrating that a prosthetic acetab-
ulum cannot deform in a manner similar to the intact
acetabulum. Nonphysiologic polar contact around
the acetabular fossa has been observed in unce-
mented hemispherical cups
[64]
, which has been
hypothesized to decrease implant stability. Further
evidence of dome loading has also been demon-
strated in computational models of the pelvis
[63]
.
Retrieved metal-backed acetabular components also
exhibit inconsistent bone ingrowth around the cup
periphery compared to that at the polar region,
indicating more load distribution through the cup
dome
[73]
.
Periprosthetic bone changes around acetabular
components have not been the attention of much
research, despite bone loss being a significant
problem associated with socket loosening. Adverse
bone remodeling and bone loss may contribute to
fixation failure or allow wear debris to penetrate the
interfacial joint space
[74]
, as well as possibly
complicate reconstruction in the event of a revision.
The long-term clinical consequences of pelvic
stress shielding remain unknown, as most clinical
follow-up has been limited to 5 years or less
[75
e
87]
.
The relative lack of densitometric studies of the
pelvis compared to the femur is also complicated by
inconsistent protocols, making comparisons between
these studies challenging. In addition to the choice
of DEXA
[75,77
e
81,84,86,88,89]
or computed
tomography (CT)
[76,82,83,85,87,89]
imaging tech-
niques, these approaches have relied on different
regions of interest for measuring bone density,
namely serial slices
[76,85,87]
, DeLee
e
Charnley
zones
[75,79,80,88]
, or nonstandard sites
[77,78,
81
e
84,86,89]
. Furthermore, only few studies have
been able to distinguish cortical and cancellous bone
changes separately
[82,83,85,87,89]
.
From these studies, one may conclude that the
bone loss around cementless cups is generally greater
than that around cemented prostheses
[75
e
78,80,
82,83,85,87]
. Cancellous bone density has been
reported to decline progressively by as much as 30%
cranially, 49% anteriorly, and 66% posteriorly at 3
years postsurgery in uncemented hips
[83]
. Concur-
rently, cortical bone density remained relatively
stable superior to the cup, increasing by 7%, but bone
loss was progressive in the anterior (
12%) and
posterior (
22%) regions. Korovessis et al.
[80]
also
measured no significant bone changes in DeLee
e
Charnley zone 1 for 33 uncemented Zweymueller
hips after a mean follow-up of 25 months. However,
there was a combined cortical and cancellous BMD
reduction of 19% and 24% in the medial (zone 2) and
caudal (zone 3) regions compared to the contralateral
hip. In a study of 50 patients undergoing bilateral
uncemented total hip arthroplasty (THA) with an
alumina liner on one side and polyethylene (PE) liner
on the other side, Kim et al.
[75]
observed significant
regional changes in BMD in all zones for both liner
groups after 5 years postoperatively. BMD increased
in both groups by 20% in DeLee
e
Charnley zone 1
(
p
¼
0.003), but decreased by 24% and 25% in zone 2
(
p
¼
0.001) for the alumina and PE groups, respec-
tively. Although both groups exhibited progressive
bone loss in zone 3 in the first 3 years (24%: alumina;
17%: PE), these were restored to baseline levels at
final follow-up. Interestingly, the study also
concluded that the change in stiffness of the liner was
insufficient to affect density changes, which indicates
that the choice of liner material could not overcome
the stiffness of the metal backing. Similar lack of
influence of soft and hard liners has also been
reported by others
[87]
.
The presence of a cementless cup appears to shift the
proximal stress distribution from the central cancellous
bone to the peripheral cortical bone
[85,87]
.Although
an increase in proximal cortical bone density may
result, an overall decrease in combined cancellous and
cortical bone density has been observed
[85]
,which
indicates an altered stress pattern due to cup implan-
tation. Wright et al.
[76]
also reported similar bone
remodeling phenomena in 26 patients who underwent
hybrid THA with uncemented hemispherical Ti alloy
cup. These patients experienced decreases in cancel-
lous bone mineral density of 20
e
33%, superior to the
cup, at 1.3 years postoperatively.
Over time, peripheral load transfer to the pelvic
cortices may cause stress shielding and reduced bone
mineral density in the retroacetabular cancellous
bone, especially between the acetabular dome and
the medial wall of the pelvis (
Fig. 14.10
). Thus, it
appears that the overall structural stiffness of
a cementless hemispherical cup is so great in
comparison with periprosthetic cancellous bone that
substantial changes in the modulus of the liner or
shell have only relatively modest influence on the
overall load transfer mechanisms of the reconstructed
pelvis.
In the late 1980s, a flexible, horseshoe-shaped
acetabular component, known as the Cambridge Cup
(not approved for use in the United States by the FDA)
