BONE INGROWTH CONDITIONS ARE SENSITIVE TO THE PRESENCE OF
TRAPPED PERIACETABULAR FLUID
Introduction: Interfacial gaps between periacetabular bone and
acetabular components in total hip arthroplasty have been observed
experimentally and clinically, even immediately after implantation5,6.
These gaps inhibit bone ingrowth2 and may communicate with the joint
space to allow joint fluid to penetrate the interface, bringing in wear
debris and soluble factors that can induce bone resorption8. Joint fluid
that remains trapped along the interface may further prevent direct boneimplant contact and worsen ingrowth conditions. In addition, joint
loading pressurizes the trapped fluid, which could lead to significant
pressure fluctuations in the periprosthetic joint space. Pressure
fluctuations may induce bone trauma, affect the vascularity and
oxygenation of periacetabular bone, and lead to the formation of
osteolytic lesions 1,8,9. The load transfer between the cup and bone will
also be affected because load will be transferred through the trapped
periprosthetic fluid film as well as through regions of bone-implant
contact. Such changes in load distribution could have substantial effects
on periprosthetic bone remodeling. The objective of this study was to
examine the mechanical effects of trapped periprosthetic fluid on boneimplant ingrowth conditions.
Methods: The geometry and non-uniform element material properties
for a 3D finite element (FE) model of a cadaveric pelvis7 were obtained
from CT scan data (10,808 elements). This FE model (NF – no fluid)
and a similar FE model, which included a periprosthetic fluid cavity
(WF – with fluid), were used to examine the mechanical response due to
the fluid-filled cavities at the bone -cup interface following surgical
insertion and post-operative joint loading of an oversized hemispherical
cup. Bone was modeled as elastic-perfectly plastic (yield strain of 1.0%)
to account for possible bone damage. The clinically realistic shape of the
acetabulum was ellipsoidal 5 (56.6 mm equatorial dia. and 28 mm polar
radius). The initial fluid-filled cavity was bounded by hydrostatic fluid
surface elements (n=1,144) on the periacetabular surface and on the cup
surface, whose center was initially 10 mm away from the acetabular
center. Thus, the deformation of the pelvis was coupled to the pressure
exerted by the confined fluid in the WF model. The cavity was initially
set at a rest pressure 3 of 5.9 kPa. Subsequently, NF and WF models
followed similar loading protocols. The surgical insertion of the 57 mm
dia. hemispherical cup was modeled by the quasi-static displacement of
a rigid analytical surface along the acetabular polar axis. The cup was
inserted till its face was flush with the edge of the acetabulum. The cup
was unloaded, and then reloaded to three cycles of peak gait joint
loading and unloading (3329 N at a polar angle of 33.6 deg.). During
gait loading, the cavity was ‘sealed’ with its current fluid volume, and
fluid pressure was allowed to vary. However, during gait unloading,
fluid was allowed to enter and leave the cavity via a fluid link element to
maintain the initial rest pressure.
Finite sliding and fully nonlinear frictional contact conditions
(ABAQUS 6.1) with coefficient of friction of 0.5 (to represent a porous
coating) were used to evaluate the ingrowth conditions in both models,
which were quantified by the total bone -implant contact area, the
interfacial gap volume, and the maximum interfacial gap size. The
average periacetabular Von Mises (VM) strains in both models and the
fluid pressure fluctuations in the WF model were also computed. The
above measures were evaluated for the third gait load-unload cycle.
Table 1: Comparison of ingrowth conditions and mechanical response
between NF (no fluid) and WF (with fluid) models during 3 rd gait cycle.
Measures
Gait cycle
NF
WF
Total contact area
load
65.3
58.2
(% cup surface)
unload
40.2
39.4
Interfacial gap
load
17.6
64.9
volume (mm3)
unload
35.5
77.8
Max. interfacial gap
load
0.27
1.86
size (mm)
unload
0.25
1.84
Average Von Mises
load
6.1
7.2
strain (%)
unload
5.9
6.9
Results: The NF model had slightly greater total contact areas than the
WF model (Table 1). The WF model had 269% and 119% greater
interfacial gap volumes during gait load and unload, respectively.
Furthermore, the interfacial gaps in the WF model were greater (Fig. 1)
with the maximum gap sizes about 7 times those of the NF model (Table
1). The average periacetabular VM strains in the WF model were about
18% greater than in the NF model. The pressure fluctuation in the
periprosthetic fluid cavity during the third gait cycle was 75.4 kPa.
SUP
NF
ANT
POS
INF
SUP
WF
ANT
POS
INF
0 5e-4 1e-3 5e-3 1e-2 5e-2 1e-1 5e-1 1.0 mm
Figure 1: Location and size of periacetabular interfacial gaps in no fluid
(NF; left) and with fluid (WF; right) models [polar view into
acetabulum]. [ANT:anterior; POS:posterior; INF:inferior; SUP:superior]
Discussion: Our results suggest that trapped periprosthetic fluid may
inhibit bone ingrowth by increasing interfacial gap volume and gap
sizes. During gait loading, the periacetabular bone deformed more to
contain the sealed incompressible fluid, thus increasing the gap volume
and gap sizes. The contact areas decreased slightly in the WF model.
The trapped fluid altered the direct bone-implant loading via regions of
contact to a more uniform loading through the pressurized interfacial
fluid. The fluid redistributed loads to gap regions that would otherwise
not be loaded directly, increasing the average VM strains by 18%.
Pressure fluctuations of 75 kPa occurred in the interfacial fluid
cavity during gait, exceeding a previous finding of 26 kPa in a lytic
lesion in the femur 1 based on passive hip flexion and extension. Pressure
fluctuations would most likely be greater during joint load application,
consistent with our results. The calculated pressures are also consistent
with the largest previously measured intra-capsular hip pressures for gait
of 69 kPa in patients with prosthetic hips 4. The somewhat larger
calculated pressures may be due to the more confined space in which
fluid is trapped in a squeeze film environment at the bone -implant interface compared to the less confined joint capsule. It should be noted that
the calculated pressures would have decreased if fluid permeability into
the periacetabular bone had been permitted or if the fluid cavity was not
sealed during loading and fluid was allowed to escape. The calculated
pressures are also within the range of measured intra-capsular pressures
during other daily activities4 such as stair climbing (max. 103 kPa).
This study has shown that bone ingrowth conditions and the
mechanical response of the bone-implant structure are sensitive to the
presence of trapped periprosthetic fluid. Therefore, in addition to
compromising bone ingrowth conditions and thus stability, the trapped
fluid may also result in interfacial pressures during extreme loading that
could affect the vascularity and oxygenation of bone as well as pump in
wear debris along the interface; the outcome of which could be
osteolytic lesion formation1,8,9. These effects might be modulated
because the trapped fluid redistributes the load between the cup and
periacetabular bone, increasing bone strain.
49th Annual Meeting of the Orthopaedic Research Society
Poster #1359
OK02640607.pdf