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A kinematic comparison of fixed
-

and mobile
-
bearing
knee replacements

[Knee]

Delport, H. P.; Banks, S. A.; De Schepper, J.; Bellemans, J.

From the AZ Waasland Hospital, St. Niklaas, Belgium

H. P. Delport, MD, Orthopaedic Surgeon; J. De Schepper, MD,
Orthop
aedic Surgeon; 60 Regentiestraat, B
-
9100 Sint Niklaas,
Belgium.

S. A. Banks, PhD, Assistant Professor; Department of Mechanical &
Aerospace Engineering; University of Florida, Gainesville, Florida
32611
-
6250, USA.

J. Bellemans, MD, PhD, Professor; Universi
ty Hospital Kuleuvem, U. Z.
Pellenberg, Weligerveldt 1, B
-
3212 Lubbeek, Belgium.

Correspondence should be sent to Dr H. P. Delport; e
-
mail:
hendrik.delport@pandora.be

Received 14 December 2005; Accepted after revision 25 April 2006

Abstract

Mobile
-
bearing posterior
-
stabilised knee replacements have been
developed as an alternative to the s
tandard fixed
-

and mobile
-
bearing
designs. However, little is known about the
in vivo

kinematics of this
new group of implants. We investigated 31 patients who had
undergone a total knee replacement with a similar prosthetic design
but with three different

options: fixed
-
bearing posterior cruciate
ligament
-
retaining, fixed
-
bearing posterior
-
stabilised and mobile
-
bearing posterior
-
stabilised. To do this we used a three
-
dimensional to
two
-
dimensional model registration technique. Both the fixed
-

and
mobile
-
be
aring posterior
-
stabilised configurations used the same
femoral component. We found that fixed
-
bearing posterior stabilised
and mobile
-
bearing posterior
-
stabilised knee replacements
demonstrated similar kinematic patterns, with consistent femoral roll
-
back

during flexion. Mobile
-
bearing posterior
-
stabilised knee
replacements demonstrated greater and more natural internal rotation
of the tibia during flexion than fixed
-
bearing posterior
-
stabilised
designs. Such rotation occurred at the interface between the
insert
and tibial tray for mobile
-
bearing posterior
-
stabilised designs.
However, for fixed
-
bearing posterior
-
stabilised designs, rotation
occurred at the proximal surface of the bearing. Posterior cruciate
ligament
-
retaining knee replacements demonstrated
paradoxical
sliding forward of the femur.

We conclude that mobile
-
bearing posterior
-
stabilised knee
replacements reproduce internal rotation of the tibia more closely
during flexion than fixed
-
bearing posterior
-
stabilised designs.
Furthermore, mobile
-
beari
ng posterior
-
stabilised knee replacements
demonstrate a unidirectional movement which occurs at the upper and
lower sides of the mobile insert. The femur moves in an
anteroposterior direction on the upper surface of the insert, whereas
the movement at the
lower surface is pure rotation. Such
unidirectional movement may lead to less wear when compared with
the multidirectional movement seen in fixed
-
bearing posterior
-
stabilised knee replacements, and should be associated with more
evenly applied cam
-
post str
esses.




Recent studies have improved the understanding of the kinematic
behaviour of the modern total knee replacement.
1
-
3

It is now clear
that the majority of posterior cruciate liga
ment (PCL)
-
retaining knee
replacements demonstrate aberrant kinematics, with forward slide of
the femoral component, whereas posterior
-
stabilised knee
replacements (PCL
-
sacrificing) usually demonstrate more consistent
but symmetrical posterior translation
of the femoral condyles during
flexion.
4
-
8

Mobile
-
bearing knee replacements may theoretically mimic the
asymmetrical posterior translation of the femoral condyles which
mainly occurs on
the lateral side of the natural knee.
9
-
13

It has also become clear that classic mobile
-
bearing PCL
-
retaining
knee replacements demonstrate the same aberrant kinematics as
fixed
-
bearing P
CL
-
retaining designs.
1,2,14
-
16

In an attempt to improve
this, mobile
-
bearing posterior
-
stabilised knee replacements have been
introduced on the basis that the cam
-
post system might induc
e
femoral roll
-
back during flexion in the same way as for fixed
-
bearing
posterior
-
stabilised designs. Rotational mobility of the bearing surface
could also allow a better reproduction of tibial internal rotation during
flexion.

However, there is little or
no information to substantiate this. Although
some cadaver simulation studies have been published, very few
in vivo

studies exist where fixed and mobile posterior
-
stabilised versions of an
identical design have been compared.
17
-
20

We have, therefore, analysed the
in vivo

kinematic patterns of three
bearing options (fixed
-
bearing posterior
-
stabilised, mobile
-
bearing
posterior
-
stabilised, fixed
-
bearing PCL
-
retaining) for the same design
of
knee replacement (Performance, Biomet
-
Spain, Valencia, Spain).
Our aim was to investigate whether mobile
-
bearing posterior
-
stabilised
knee replacements would demonstrate more favourable kinematics
when compared with fixed
-
bearing posterior
-
stabilised and f
ixed
-
bearing PCL
-
retaining designs.

Patients and Methods

We analysed 31 patients with

well
-
functioning total knee replacements
(Performance; Biomet) using a three
-
dimensional (3
-
D) to two
-
dimensional (2
-
D) model registration technique implant superimposed
during four basic movements. All patients had been operated on by the
same surgeon (H
PD), using the same knee replacement system but
using three different configurations; fixed PCL
-
retaining, fixed
posterior
-
stabilised and mobile posterior
-
stabilised. The Performance
(Biomet) design allows the use of two different femoral components;
one f
or the PCL
-
retaining configuration and the other for the fixed or
mobile posterior
-
stabilised configurations. Both the PCL
-
retaining and
posterior
-
stabilised femoral components, and their corresponding
bearings, have the same sagittal radius, but differ in

the coronal plane.
The posterior
-
stabilised coronal plane condylar radius is smaller,
resulting in a more rounded femoral component, while the mobile
posterior
-
stabilised bearing is more concave in both the sagittal and
coronal planes.

The distribution of

the three different configurations of tibial bearing
was investigated. These configurations were: fixed
-
bearing PCL
-
retaining (11 patients), fixed
-
bearing posterior
-
stabilised (10), and
mobile
-
bearing posterior
-
stabilised (10). For the fixed
-
bearing PCL
-
r
etaining knee replacements, a posterior
-
lipped insert was used.
Fixedand mobile
-
bearing posterior
-
stabilised knee replacements had
identical cam
-
post mechanisms. Comparing equal sizes of component
in the femur and tibia, the contact point between a posteri
or
-
stabilised
femoral component and insert theoretically occurs at 30° of flexion in
the fixed and 40° in the mobile configuration. This is the result of the
interactive effect of both the anteroposterior position of the post and
the design of the bearing
in the sagittal plane. Both PCL
-
retaining and
posterior
-
stabilised fixed
-
bearings are secured to the tibial tray
through an anterior slot, a peripheral rim and a central locking screw,
whereas a posterior
-
stabilised mobile insert can freely rotate around a

central peg on a polished tibial tray. The underside of the tray, and the
tibial stem, are identical in both the fixed and mobile designs of tibial
tray.

Only patients with a pre
-
operative diagnosis of end
-
stage
osteoarthritis of the knee with well
-
functi
oning total knee
replacements were included in the study at a minimum follow
-
up of 12
months (mean 16; 12 to 24).

We evaluated the three
-
dimensional femorotibial kinematics during a
weight
-
bearing extension stance, kneeling at 90° of flexion, kneeling
at f
ull flexion, and an active lunge on a 25 cm step. Radiological
images obtained during these activities were subsequently analysed
using a 3
-
D to 2
-
D model registration technique as previously
described,
3

and allowing three
-
dimensional quantification of
femorotibial kinematics and relative contact positions in the sagittal
and coronal planes. The rotations of the mobile
-
bearing components
were determined by using a three
-
dimensional model
of the metal pins
in the polyethylene bearing, constructed following the manufacturer's
specifications. This model was then positioned to coincide with the
shape
-
matching solution for the tibial tray. The model was next
rotated about the post on the tibial

tray until the projection of the pins
matched their projections on the radiological image. The precision of
the internal
-
external rotation measurement was 0.5°. Relative
movements between the femoral component and the bearing were
computed using the three
-
dimensional positions of the two parts. No
assumptions were made about the pattern of movement.

Three
-
dimensional positions and orientations of the femoral, tibial and
mobile
-
bearing components were determined using shape
-
matching
techniques
(Fig. 1)
. Statistical analysis was performed using analysis of
variance (ANOVA) with results considered significant at p < 0.05.




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Fig. 1 Three
-
dimensional positions and
orientations of the femoral, tibial and mobile
-
bearing compone
nts were determined using
shape
-
matching techniques. The upper row of
images shows the outlines of the
superimposed three
-
dimensional to two
-
dimensional model registration technique for
the femur and tibial tray (left) and femur and
tibial insert (right).
The lower row of images
show the three
-
dimensional to two
-
dimensional model registration technique
projected onto the radiograph.


Results

Sagittal plane kinematics.

In extension, PCL
-
retaining knee
replacements demonstrated a tibiofemoral contact position close to the
anteroposterior tibial midline, with paradoxical forward slide of th
e
femoral component during early flexion in 83% (10 of 12 knees) of
cases. With deeper flexion, particularly when > 90°, all PCL
-
retaining
knee replacements demonstrated some posterior translation of the
femoral contact point. This was a mean of 3.9 mm (
-
1
.1 to +8.4) on
the medial side and 4.8 mm (+0.4 to +9.4) on the lateral side
between 90° and full flexion
(Figs 2 and 3)
. All PCL
-
retaining knees
exhibited posterior translation of the

lateral contact point, and ten of
the 12 showed posterior translation of the medial contact point. Both
fixed
-

and mobile
-
bearing posterior
-
stabilised knee replacements
demonstrated similar tibiofemoral contact positions in extension when
compared with PC
L
-
retaining designs. None of these posterior
-
stabilised knee replacements demonstrated paradoxical forward slide
of the femur during flexion.




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Fig. 2 Medial tibiofemoral contact positions
from extension to flexion.





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viewing]


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Fig. 3 Lateral tibiofemoral contact positions from
extension to flexion.


Between extension and maxim
al flexion there was a mean posterior
translation of 6.4 mm (
-
1.5 to +10.6) on the medial side and 6.5 mm
(
-
5.0 to +10.1) on the lateral side for the fixed
-
bearing posterior
-
stabilised knee replacements, compared with 7.2 mm (+0.6 to +16.4)
medially and 7.
9 mm (+3.5 to +17.2) laterally for the mobile
-
bearing
posterior
-
stabilised knee replacements (p > 0.1). This was significantly
more than for PCL
-
retaining designs, where there was a mean
posterior translation of the femoral contact point of only 0.4 mm (
-
6
.2
to +14.6) on the medial side, and 1.4 mm (
-
5.1 to +8.1) on the lateral
side (p < 0.001).

Flexion during maximum lunge was clearly above the degree of flexion
at which cam
-
post engagement occurs (40° for mobile
-
bearing
posterior
-
stabilised and 30° for fi
xed
-
bearing posterior
-
stabilised;
Biomet)
(Table I)
.




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Table I. Flexion (°) in maximum lunge


Tibiofemoral rotation.

Internal rotation of the tibia during flexion
was measured for each of the three designs tested
(Fig. 4)
. The mean
tibial internal rotation when kneeling fully was 5.89° (1.49° to 16.91°)
for PCL
-
retaining designs, 2.36° (1.29° to 7.26°) for fixed
-
bearing
posterior
-
stabilised knees,

and 7.46° (2.05° to 15.78°) for mobile
-
bearing posterior
-
stabilised knees (p < 0.05)
(Figs 5 and 6)
. In mobile
-
bearing posterior
-
stabilised knees, almost all rotation occurred at the
distal surface between the tibial insert and the tibial tray (mean 7.63°,
range 1.50° to 16.51°) whereas almost no rotation occurred at the
proximal surface between femur and insert (mean 1.09°, range 2.06°
to 3.23°;
Figs 7 and 8
).




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Fig. 4 Tibial rotation during flexion.





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Fig. 5 Tibiofemoral contact in the cruciate
-
retained design.





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Fig. 6 Tiobiofemoral contact in the fixed
-
bearing posterior
-
stabilised design.





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Fig. 7 Tibiofemoral contact in the mobile
-
bearing posterior
-
stabilised design.





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Fig. 8 Mobile insert
-
tibial tray contact in the
mobile
-
bearing posterior
-
stabilised design.


Dis
cussion

To our knowledge, this is the first study where the
in vivo

kinematic
perform
ance of both fixed
-

and mobile
-
bearing posterior
-
stabilised
implants, with the same femoral component, have been compared with
the PCL
-
retaining version of the same system. Our results showed that
both fixed
-

and mobile
-
bearing posterior
-
stabilised knee re
placements
demonstrated similar tibiofemoral kinematic patterns with respect to
femorotibial translation and axial rotation during flexion, although
mobile
-
bearing posterior
-
stabilised knee replacements more closely
replicated the asymmetrical posterior co
ndylar translation of the
normal knee.

Fixed
-
bearing posterior
-
stabilised knee replacements demonstrated a
relatively symmetrical posterior femoral translation during flexion with
a mean of 6.4 mm for the medial condyle and 6.5 mm for the lateral,
which re
sulted in an overall tibial internal rotation of only 3.0°, which
is considerably less than the 10° to 15° of tibial internal rotation which
occurs in the normal knee during deep flexion.
20

In contrast, mobile
-
bearing posterior
-
stabilised knee replacements demonstrated more
asymmetrical posterior femoral translation during flexion, with a mean
of 7.2 mm for the medial condyle and 7.9 mm for the lateral, resulting
in a more natural tibial
internal rotation of 8.0° (p < 0.05).

In our study, both fixed
-

and mobile
-
bearing posterior
-
stabilised knee
replacements only partially restored the posterior femoral translation
which occurs in the normal knee during flexion. Although no clear
consensus
exists on the exact data, recent work by several groups has
identified that the normal knee demonstrates a negligible posterior
femoral translation on the medial side, as opposed to between 15 mm
and 25 mm on the lateral side, when the knee is analysed fro
m
extension to full flexion. Their findings also depend on factors such as
the measurement method, muscular tone, foot position, and size of
the knee.
9
-
13,20,21

It is clear that in our p
atients, posterior translation
of this magnitude was not achieved, certainly not on the lateral side.

Previous work by other authors seems to confirm this observation.
8,20

Most et al
20

have investigated the kinematics of both the fixed
-

and
mobile
-
bearing versions of the posterior
-
stabilised LPS knee (Zimmer
Inc., Warsaw, Indiana) in a cadaver model and using a robotic

testing
system (Orthopaedics Biomechanics Laboratory, Harvard Medical
School, Boston, Massachusetts). There was no difference between the
two versions. Both fixed
-

and mobile
-
bearing posterior
-
stabilised
knees only partially restored the posterior femoral

translation and axial
rotation of the normal knee. Dennis et al
1

recently reported a
multicentre videofluoroscopic kinematic analysis of 733 cases,
including 103 mobile
-
bearing posteri
or
-
stabilised knees. Only limited
posterior femoral translation was found, with a mean of 3.8 mm for
the lateral condyle, and
-
0.7 mm for the medial.
1

Tibial internal
rotation was a mean

of 3.9° from full extension to full flexion.
1

Similar values were found by Ranawat et al
18

in a kinematic study
wh
ich compared fixed
-

and mobile
-
bearing versions of the posterior
-
stabilised Sigma knee (Depuy, Warsaw, Indiana). They demonstrated
greater axial rotation (4.1°
vs

7.3°) and less condylar lift
-
off (60%
vs

45%) in the fixed
-

vs

mobile
-
bearing posterior
-
stabi
lised knee
replacements, respectively, although the mean lateral posterior roll
-
back during flexion was limited for both designs (3.7 mm and 3.6 mm,
respectively).

Compared with what is known from the literature, our data confirm
that both fixed
-

and mobil
e
-
bearing posterior
-
stabilised knee
replacements replicate axial rotation and posterior condylar translation
of the natural knee during flexion, but to a lesser degree. Our patients
demonstrated more posterior translation than has been reported in the
lite
rature, and greater restoration of tibial internal rotation for mobile,
compared with fixed
-
bearing, posterior
-
stabilised knee replacements.

Both mobile
-

and fixed
-
bearing posterior
-
stabilised knee replacements
demonstrated similar tibiofemoral contact pos
itions in extension when
compared with PCL
-
retaining knees. None of the posterior
-
stabilised
knee replacements demonstrated paradoxical forward slide of the
femur during flexion. This may be explained by the fact that we did not
make any observations in ea
rly
-

to mid
-
flexion; our study was limited
to a straight
-
leg stance and the more deeply
-
flexed positions.

We also analysed the pattern of tibiofemoral rotation. In the mobile
-
bearing posterior
-
stabilised knee replacements, almost all rotation
occurred at t
he distal surface of the bearing where it articulates with
the tibial baseplate.
Figure 7

shows parallel lines which are different
from any of the other figures and from the findings d
escribed
elsewhere in the literature.
1
-
25

Only minimal rotation appears to occur
on the proximal surface of the mobile
-
bearing posterior
-
stabilised knee
replacements. Theoretically, it i
s possible that rotation occurs at both
the superior and inferior surfaces of the bearing, perhaps more likely
in the unloaded than loaded state.

Compared with the fixed
-
bearing posterior
-
stabilised knee
replacements, where complex translation and rotation
al movements
occur on the proximal bearing surface, the decoupling of translation
and rotation may have important theoretical benefits with regard to
polyethylene wear.
21
-
24

McEwen et al

23
-
25

have reported in a number
of publications that ultra
-
high
-
molecular
-
weight polyethylene
(UHMWPE) molecules become orientated in the principal direction of
slide (anteroposterior),

thereby producing a hardening effect which
increases wear resistance. Consequently, the polyethylene softens
along the axis perpendicular to the sliding movement and exhibits less
wear resistance in that direction. Introducing a movement such as
internal
-
external rotation produces a friction force in a direction which
is transverse to the sliding movement and, therefore, increases the
wear rate. This concept has been confirmed in knee simulator studies,
comparing the
in vitro

wear between the rotating plat
form LCS knee
replacement (DePuy, Warsaw, Indiana) and the fixed
-
bearing Sigma
knee replacement (DePuy).
25

These demonstrated that the decoupling
of knee movements, by allowing unidirect
ional rotation to occur at the
insert
-
tibial tray articulation and unidirectional translation at the
femur
-
tibial insert surface, leads to reduced UHMWPE wear as a result
of altered molecular orientation and decreasing cross
-
shear on the
polyethylene.
23,24

Our study demonstrates that such decoupling
indeed occurs
in vivo

in patients with mobile
-
bearing posterior
-
stabilised knee replacements.

The fact that mobile
-
bearing posterior
-
stabilis
ed knee replacements
demonstrate almost pure translation at the femur
-
insert interface has
another theoretical advantage, since it implies that the cam
-
post
stress remains evenly applied because there is no rotation occurring.
Such rotation does occur in f
ixed
-
bearing posterior
-
stabilised knee
replacements which leads to disturbed cam
-
post parallelism and more
concentrated stress at the cam
-
post interface.

In conclusion, we believe that this
in vivo

kinematic study shows that
when compared with fixed
-
bearin
g posterior
-
stabilised and fixed
-
bearing PCL
-
retaining knee replacements, mobile
-
bearing posterior
-
stabilised designs demonstrate similar posterior condylar translation,
but slightly more natural tibial rotation during flexion. This rotation
occurs at the
distal bearing surface, which is decoupled from the
unidirectional translation at the proximal bearing surface. However, as
highlighted by Victor et al,
8

the pattern of axial rotation of

a specific
device is highly dependent on the surface geometry of the design
used. For this reason it remains impossible to draw generalised
conclusions about fixed
-
bearing posterior
-
stabilised and mobile
-
bearing posterior
-
stabilised knee replacements.

We
would like to thank Dr Y. Nackaerts, MD and her team from the
Radiology Department AZ Waasland, Sint Niklaas, Belgium, for their
cooperation in their free time and Ms I. Meyskens, our scientific nurse,
for her enthusiastic commitment.

No benefits in any fo
rm have been received from any commercial party
related directly or indirectly to the subject of this article.

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