Original Article
Pre-Clinical Assessment of Total Knee Replacement Anterior-
Posterior Constraint
Halewood, C.a*, Athwal, K.K.a*, Amis, A.A.a,b
aDepartment of Mechanical Engineering, Imperial College London, South Kensington
Campus, London SW7 2AZ, UK
and b Musculoskeletal Surgery group, Department of Surgery and Cancer, Imperial
College London School of Medicine, Charing Cross Hospital, London, W6 8RF, UK
* Credited as co-first authors, as equal contribution to the manuscript
Corresponding author:
Dr Kiron Athwal,
Department of Mechanical Engineering,
Imperial College London,
South Kensington Campus,
London SW7 2AZ, UK
Tel: +44 (0)20 7594 1986
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Keywords: TKR, TKA, Constraint, Laxity
Word count (Intro-Discussion): 3420
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Abstract
Pre-clinical, bench-top assessment of Total Knee Replacements (TKR) can provide
information about the inherent constraint provided by a TKR, which does not depend on
the condition of the patient undergoing the arthroplasty. However little guidance is given by
the ASTM standard on test configurations such as medial-lateral (M:L) loading distribution,
flexion angle or restriction of secondary motions. Using a purpose built rig for a materials
testing machine, four TKRs currently in widespread clinical use, including medial-pivot and
symmetrical condyle types, were tested for anterior-posterior translational constraint.
Compressive joint loads from 710 to 2000 N, and a range of medial-lateral (M:L) load
distributions, from 70:30% to 30:70% M:L, were applied at different flexion angles with
secondary motions unconstrained. It was found that TKA constraint was significantly less
at 60 and 90° flexion than at 0°, whilst increasing the compressive joint load increased the
force required to translate the tibia to limits of AP constraint at all flexion angles tested.
Additionally when M:L load distribution was shifted medially, a coupled internal rotation
was observed with anterior translation and external rotation with posterior translation. This
paper includes some recommendations for future development of pre-clinical testing
methods.
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1. Introduction
According to the UK National Joint Registry (NJR, 2016), 59 different designs of Total
Knee Replacement (TKR) were implanted into patients in 2015. Post-operative data may
be available for established replacements but there is a lack of information about how a
new device is likely to perform clinically (Liow and Murray, 1997). The ASTM standard
tests F1223 (ASTM-F1223, 2014) measure the inherent constraint of the TKR prosthesis
itself, that which is independent of the patient’s physiological condition or the surgical
implantation process. The ASTM standard describes test guidelines for determining
constraint in anterior-posterior (AP) drawer, medial-lateral shear, internal-external and
varus-valgus rotations, and in distraction. This information may help the surgeon in
choosing the most appropriate TKR for each patient, depending on factors such as the
intrinsic stability of the native knee which is affected by the condition of the soft tissues
surrounding it (Kakarlapudi and Bickerstaff, 2000). The ASTM-F1223 (2014) standard
aims to: “provide a database of product functionality capabilities …... that is hoped will aid
the physician in making a more informed total knee replacement (TKR) selection”. In the
European Union, this testing is mandatory for all new TKRs before they are marketed and
used clinically if CE marking is required by the manufacturer (European Parliament, 2007).
Haider and Walker (2005) used the test methods outlined in the 2005 version of the
standard to assess the constraint of three designs of TKR. Moran et al. (2008) assessed
one TKR device experimentally in order to validate a computer simulation of the ASTM test
methods. These studies looked at TKR AP constraint, but did not consider the effect on
constraint of the medial: lateral (M:L) tibiofemoral loading distribution, which varies
depending on subject and activity (Mündermann et al., 2008; Varadarajan et al., 2008;
Zhao et al., 2007). The ASTM standard itself does not include guidance on this M:L
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loading distribution, therefore experiments attempting to replicate this standard would likely
assume a 50:50 M:L axial load.
Haider and Walker (2005) explored whether keeping secondary motions restricted during
translation tests, as suggested by the ASTM standard, led to anomalous results. They
concluded that, other than flexion angle and the degree of freedom (DoF) being measured,
all the other motions should be left unrestricted in order to obtain reliable results. Heim et
al. (1996 and 2001) looked at AP constraint of mobile bearing and posterior stabilised
TKRs but restricted all the DoF of motion other than the one being measured. That
restriction could be expected to lead to unrealistic edge-loading conditions when there is
displacement between the components of an asymmetrical TKR, for example. In a
symmetrical TKR with equal M:L loading, we would expect minimal coupled rotation with
AP drawer. However a shift of the resultant force either medially or laterally would create
friction-induced constraint on the more-loaded compartment and more displacement on
the less-loaded compartment, which overall manifests as a coupled internal-external
rotation.
The objective of this study was to assess how AP displacement outcomes from the ASTM-
F1223 standard for measuring AP constraint in TKRs were affected by unrestricting
coupled rotations, and varying the M:L loading distribution, axial load and flexion angle. It
was expected that altering the load distribution medially would cause a coupled internal
rotation and external rotation of the tibia when displaced anteriorly and posteriorly
respectively, with the opposite occurring with a more lateral load distribution. When
considering different flexion angles, it was hypothesised that most AP constraint would be
shown at full extension, with an increase in laxity exhibited with flexion. Furthermore,
increasing the compressive joint load was hypothesised to increase the displacing AP
force required to reach the translation limits of TKR.
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2. Materials and Methods
The tests described for this study follow the general standard as set out in ASTM-F1223
(2014). Any changes to this standard have been highlighted in the methods.
2.1 Test rig set-up
A single-axis, screw-driven Instron model 5565 materials testing machine was employed
for the constraint tests. A test rig was designed and constructed, which could
accommodate the femoral and tibial components of a TKR (Figure 1). The femoral
component was mounted using polymethylmethacrylate (PMMA) bone cement onto an
aluminium alloy cross-bar shaped to match the component’s internal geometry, similar to
the shape of the distal femur as prepared during surgery. The flexion angle could be
adjusted by rotating and then fixing the cross-bar into position. A pivoting frame was used
so that the femoral component was free to rotate in varus-valgus, about an anterior-
posterior axis at the level of the flexion axis, not far from the joint line. The pivot point could
be adjusted medially-laterally, in order to vary the load distribution between the medial and
lateral compartments of the TKR, across the range 30:70% to 70:30% M:L. The pivot
frame was mounted on linear bearings, which allowed it to translate proximally-distally. A
calibrated pneumatic cylinder forced the femoral mounting distally, against the tibial
component, thus providing the compressive joint force. Being a pneumatic cylinder, it did
not prevent secondary proximal translations occurring when the TKR was tested.
The tibial components were mounted into the end of a freely-rotating shaft, which allowed
internal-external rotation. The posterior slopes of the tibial components were set at 0° in
this study. This assembly was mounted onto a linear bearing which allowed free medial-
lateral translation. The whole tibial assembly was then mounted on another linear bearing,
which allowed anterior-posterior translation. This was attached directly to the load cell on
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the moving cross-head of the Instron, which provided the AP motion and measured both
force (N) and translation (mm).
Thus, the Instron machine imposed AP translation of the TKR at a chosen angle of flexion
and M:L load distribution, while all other degrees-of-freedom were unrestricted. This varies
from ASTM-F1223, which restricts movement other than AP translation and does not
suggest M:L variation. ASTM-F2083 (2012) recommends testing implants at 0°, 15°, 90°
and maximum flexion; in this study 0°, 30°, 60° and 90° flexion were chosen to fully
explore extension to deep flexion at equal increments and a ‘true maximal flexion’ was not
be tested because of the difficulty of relating it to the clinical situation.
2.2 Implants and tests:
Four TKRs were tested. AP constraint tests were conducted on two MatOrtho TKRs
(MatOrtho, Leatherhead, UK): the Medial Rotation Knee (MRK) which had been in clinical
use for over twenty years; and a newer design, the Saiph Knee. Both of these devices
were medial-sphere, highly congruent posterior cruciate ligament (PCL)-sacrificing type
TKRs, with asymmetrical condylar geometry. AP constraint tests, at a range of
compressive loads and ML loading distributions, were also conducted using the
conventionally designed, PCL-retaining Stryker Triathlon (Stryker (UK) Ltd, Newbury, UK)
and Smith & Nephew Legion (Smith & Nephew, Memphis, TN, USA), which both had
symmetrical condylar geometries. The schedule of tests conducted is shown in Table 1.
2.3 Test method:
ASTM-F1223 determines the neutral position by either ‘applying a compressive force of
100 N and allowing the implant to settle or by measuring the vertical position of the
movable component with respect to the stationary and using the low point of the
component as the neutral point’. Pilot testing deemed this not to be a repeatable method
with which to find a neutral position with differing geometries, therefore a different method
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at a higher load was proposed. The femoral component was fixed at the desired flexion
angle and required M:L load distribution, then AP drawer was imposed by the Instron. To
first approximate the neutral AP position, a 350 N axial compressive load was applied and
the AP position was adjusted until the femoral components sat in the lowest compressive
point on the concave tibial surface. Small AP translations of ±3 mm were applied to the
tibia and the neutral AP position was adjusted until the hysteresis loop of the force versus
displacement graph was symmetrical above and below the zero load axis (Figure 2).
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Once the AP position was adjusted, a 710 N compressive axial load was then applied and
the tibial component was anteriorly translated at a speed of 0.1 mm/sec until, as the ASTM
stipulates, ‘dislocation of the components is imminent… or if a dangerous or unrealistic
situation is about to occur’. In this experiment, the anterior limit was defined as the point at
which the force-displacement graph started to plateau (Figure 2); this suitable limit was
chosen by the authors to avoid permanent deformation of the edge of the UHMWPE
bearing, which would have affected the results of future tests using the bearing. This
displacement limit was recorded and the process was then repeated in the posterior
direction using the same procedure. The TKR was returned to the neutral position,
lubricated with water, reloaded to 710 N and cycled between the translation limits found
previously at a speed of 1 mm/sec (ASTM-F1223 states not to exceed 10 mm/sec). Three
“pre-conditioning” cycles were completed and data were collected on the fourth cycle. For
both the Triathlon and Legion implants, the axial load was increased to 2000 N and four
further cycles were performed at the same AP limits as the 710 N test (this higher load is
not included in ASTM-F1223).
2.4 Statistical Analysis:
For the Legion TKR, four nominally identical samples were tested. The following statistical
analyses were performed for the Legion TKR in SPSS 24 (IBM SPSS Statistics, version
22, Armonk, NY):
1. One-way repeated-measures analysis of variance (RM-ANOVA) at each M:L load
distribution to compare AP translations across different flexion angles.
2. Two-way RM-ANOVA to compare AP force at the translation limits across different axial
loads (710 N or 2000 N) and across different flexion angles.
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Post-hoc paired t-tests with Bonferroni correction were applied when differences were
found, with significance level set at p < 0.05.
3. Results
The AP constraint characteristics varied considerably between the four different TKR
designs. The AP force-displacement graphs for the tests at full extension (here defined as
0° flexion) for 50:50 M:L loading distribution are shown in Figure 3. Constraint was
calculated as displacing force (N) per mm of translation per Newton of compressive load
(Haider and Walker, 2005), which was made after taking into account the frictional forces
present in the rig and implant articulation, as evident in the hysteresis curves.
The results were consistent with the geometry of the tibial bearings and the congruence of
the devices. The relatively incongruent Triathlon and Legion, which had shallow tibial
bearing concavities, were less constrained in AP translation than the MRK and Saiph, with
a shallow sloped force-displacement curve.
At full extension and 50:50 M:L loading distribution, total AP laxity ranged from 8.5 mm
with the MRK to 15 mm with the Triathlon (Table 2). At 90° flexion, the MRK and Saiph
again showed similar constraint characteristics to each other, while the Triathlon and
Legion allowed larger ranges of AP laxity, 22 mm and 24 ± 1 mm (average ± 95%
confidence interval) respectively. Constraint at full extension varied from 0.01 mm-1 with
the Triathlon to a maximum of 0.19 mm-1 with the MRK design in anterior tibial drawer. In
posterior drawer, the two medial-sphere type TKRs showed similar high levels of
constraint, while the Triathlon and Legion exhibited much less (Table 2).
3.1 Varying flexion angle
The total AP laxity of the Legion implant (at 50:50 M:L distribution and 710 N axial load)
ranged from 13.6 ± 1.0 mm at full extension to 24.3 ± 0.8 mm at 90° (Figure 4). The load-
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displacement graphs at 30° and 60° were similar, and at 90° the most posterior translation
was demonstrated. An overall observation was that the articulations of the TKRs were
usually most-congruent near full extension, and progressively disengaged with flexion, and
this was reflected by reduced constraint as the knee flexed. When the M:L load distribution
was 60:40, anterior and posterior translations at 60° and 90° were significantly larger than
at 0° (all with p<0.05). At 50:50, increase in AP translation at 60° and 90° were also found,
with an additional significance found in posterior translation at 30° (p=0.008).
3.2 Varying Axial load
Two compressive loads were tested: 710 N and 2000 N. At all flexion angles tested with
the Legion TKR, increasing the axial load resulted in a higher AP force reached at the
displacement limits (all with p<0.05). For example at 0° flexion the displacing force
required for 6.7 ± 1.4 mm anterior translation of the Legion implant increased from 248 ±
66 N to 472 ± 87 N when axially loaded to 710 N and 2000 N respectively (p<0.01, Figure
5). For 6.9 ± 2.4 mm posterior drawer, the displacing forces increased from 320 ± 83 N to
635 ± 137 N (p<0.01). Similarly with the Triathlon implant, the displacing force for given AP
translation at 0° and 90° increased significantly (p<0.05) with axial load.
3.3 Varying M:L load distribution
Changing the M:L loading distribution from 50:50 to 60:40 increased the total AP laxity for
both the MRK and Saiph devices, particularly at 90° flexion. This increase in total laxity
was due to increased tibial rotation.
The medial-sphere type TKRs exhibited more coupled tibial IE rotation in AP translation at
the 50:50 M:L loading distribution than the symmetrical Triathlon and Legion, which
displayed almost 0° rotation for translations in both directions at full extension and 90°
flexion. Total tibial rotation at both 0° and 90° flexion for both the Saiph and MRK was 11°
at the 50:50 M:L loading condition. When shifted towards the medial side to give a 60:40
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loading condition, an increased amount of tibial rotation was observed with both the MRK
and the Saiph at 90° flexion, with total rotation increasing to 15° and 19° respectively. This
secondary tibial rotation in the MRK resulted from greater movement in the less
constrained lateral compartment. At full extension, rotation only increased for the Saiph
TKR, this time to 18°. It should also be noted that the direction of the majority of the
rotation varied between the two devices. For the Saiph, most of the coupled rotation
occurred internally during anterior tibial drawer but with the MRK, it was external during
posterior tibial drawer.
At different M:L loading distributions from 30:70 to 70:30, both the Triathlon and Legion
implants exhibited differences in both magnitude and direction in associated tibial rotation
(Figure 6 and 7). As the loading was shifted further towards the medial condyle, there was
an increase in internal rotation coupled with anterior translation and external rotation
coupled with posterior translation. Conversely, with increasing lateral loading distribution
there was an increase in external rotation with anterior translation and internal rotation with
posterior translation.
3.4 Reproducibility of results
Four samples of the Legion knees were tested by the same investigator in order to assess
intra-rater repeatability of the results between nominally identical devices. At 60:40 M:L
loading the 95% confidence intervals (CI) from the total AP translation results at 0° and
90° flexion were 0.9 mm and 0.9 mm respectively; at 50:50 M:L loading this was 1.6 mm
and 1.3 mm respectively. The constraint, in both anterior and posterior translation, had a
95% CI of ± 0.01 mm-1.
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4. Discussion
Four TKR designs were tested for AP constraint at different flexion angles, M:L load
distributions and compressive loads. Increasing the axial compressive load on the TKRs
increased the friction between femoral and tibial components, and thus increased AP
displacing forces as hypothesised. Maximal AP constraint occurred at full extension, with
increasing translations at larger flexion angles. The coupled tibial IE rotations, in response
to AP translations, were found to be sensitive to the M:L load distribution in the
symmetrical TKR designs: as the load was shifted medially there was a coupled internal
rotation and external rotation with anterior and posterior translations respectively, which
reversed with increasing lateral load distribution. With increasing load shift in either medial
or lateral directions, the magnitude of coupled rotation increased. Mündermann et al.
(2008) have shown that the resultant joint force may oscillate between the medial and
lateral compartments during daily activities. Taken together with our results, it implies that
secondary rotational instabilities may occur.
The focus of the study was not to compare AP constraints of TKRs from different
manufacturers in order to evaluate the best performance, but rather to assess the
sensitivity of the ASTM standard to different variations and unrestricted coupled rotations.
However, by testing different TKRs designed to produce distinct kinematic behaviour (such
as asymmetrical designs), it was demonstrated that these tests are an intrinsic measure of
the device without the influences of soft tissues. The ASTM standard testing is mandatory
for all new TKRs in order for the device to be CE marked; therefore every manufacturer
has these constraint data. Unfortunately there is almost no data publicly available in peer-
reviewed journal articles. Therefore this is a lost opportunity to learn about the effect of the
constraint characteristics on patient outcome.
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These results can be used by a surgeon alongside patient specifics, such as the condition
of the PCL, in selecting the correct device. If these results are considered in isolation and
compared to data from normal knees (Amis, 1989; Shino et al., 1987) the Saiph was
closest to physiological, the MRK was more constrained and the Triathlon was less
constrained, which is in agreement with the designs of the devices and the way in which
they are intended to be used clinically.
Constraint results such as these may also be viewed with potential implant loosening in
mind; they suggest that a more constrained implant has a greater ability to transfer shear
forces to the underlying bone, and this may have implications for the efficacy of the fixation
of the device. It is difficult to scrutinise this theory any further from large datasets such as
joint registries, because although they contain failure mechanisms for all primary TKRs
that need revising, and revision rates for individual TKR designs, they do not combine the
two.
These bench-top tests could provide research benefits outside of pre-clinical use. The
tests can be used to validate computer modelling approaches (Rullkoetter et al., 2017),
although fidelity of the soft-tissue and knee geometry is still debated. The current rig
design can test implanted cadaveric specimens under the same loading regimes; thus the
data presented here may provide comparative information on how much stability is
provided by the inherent implant geometries as opposed to soft-tissues. The data may also
be used to establish protocols for robotic testing systems that could apply more complex
and clinically relevant 6 DoF movements and loads.
Bench-top tests are relatively quick and cheap to perform and producing easy to interpret
data. There was inherent variation in implant cementing and alignment and determining
the neutral position and subluxation points of the TKRs; despite this the repeat tests with
Legion implants demonstrated good intra-rater reproducibility within the same implant
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design. However there are limitations with this approach to pre-clinical testing. The test
simulates a very specific motion of the tibial component relative to the femur which,
although representing a clinical laxity assessment (Daniel et al., 1985), is unlikely to be
representative in everyday movements. The constraint test described here should continue
to be used as a “common sense” check for new designs of implant, but care must be taken
when making functional inferences from constraint testing results. A limitation of the set-up
was that water was used for lubrication, as specified by the ASTM: it may not represent
the frictional behaviour of synovial fluid, but is convenient and reproducible.
4.1 Recommendations:
The authors recommend the following on the basis of the findings in this study:
1) The test protocol defined by ASTM F1223 should stipulate the variation of the M:L
loading distribution. It has been shown to vary between patients and activities
(Mündermann et al., 2008; Varadarajan et al., 2008; Zhao et al., 2007) and the AP
constraint and associated secondary motions have been shown to be very sensitive to
this distribution, particularly with a less-congruent device such as the Triathlon.
2) The secondary motions observed during AP constraint tests should be measured and
recorded as part of the ASTM standard, as these can provide more information about
the device’s stability characteristics.
3) The tests could be extended to include a higher axial load such as 2000 N
(Mündermann et al., 2008), approximately three times body weight, in order to
investigate coupled rotations and M:L distribution effects whilst under normal walking
gait loads.
4) The tests could be extended to include in-vitro cadaveric assessment to establish
whether or not the soft tissues interact with the TKR in the way that is intended.
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Acknowledgements
This work was funded by MatOrtho Ltd. Part of the study design was in collaboration with
MatOrtho, but the sponsors had no part in the work or the preparation and submission of
this paper. The Instron machine was provided by the Arthritis Research UK charity. The
authors would like to thank Peter Barnado, Tamsin Bromley, Jennifer Lenz and Matthew
Taylor for designing and manufacturing the rig.
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Tables
Table 1 Test Schedule (cross indicates a completed test)
Implant Flexion Angles (°)
Axial Load (N) Loading (Medial:Lateral)
70:30 60:40 50:5
040:60 30:70
MRK0 710 X X
90 710 X X
Saiph0 710 X X
90 710 X X
Triathlon
0710 X X X X X
2000 X X X X X
90710 X X X X X
2000 X X X X X
Legion
0710 X X X X X
2000 X X X X X
30710 X X X X X
2000 X X X X X
60710 X X X X X
2000 X X X X X
90710 X X X X X
2000 X X X X X
Table 2 Laxity (mm) and constraint (mm-1) for 50:50 medial: lateral load distribution.
For the Legion, the values given are the average ± 95% confidence interval (n = 4).
Flexion Angle (°) Measurement
Implant Type
MRK Saiph Triathlon Legion
0 Total AP laxity (mm) 8.5 10.1 15.0 13.6 ± 1.6
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Anterior constraint (mm-1) 0.19 0.07 0.01 0.05 ± 0.01
Posterior constraint (mm-1) 0.13 0.12 0.05 0.07 ± 0.01
90 Total AP laxity (mm) 10.5 8.7 22.0 24.3 ± 1.3
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Figure legends
Figure 1 The anterior-posterior translation testing rig. Inset: a close up image of the
implants in the testing rig (at 30° flexion).
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Figure 2 Graphs showing the constraint of the Legion implant (90° flexion, 50:50 medial:
lateral load distribution, 710 N axial load). a) An approximate neutral position was first
found, and a ± 3mm cyclic draw showed that the hysteresis loop was not symmetrical
above and below the zero load axis. b) The neutral position of the tibia was moved 0.5 mm
anteriorly, and a new ± 3mm draw showed that the hysteresis loop was now symmetrical.
c) The tibia was translated anteriorly and posteriorly until the force-displacement graph
peaked and started to drop. d) The implant was cyclically translated between these two
limits four times.
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Figure 3 Constraint of different implants (0° flexion, 50:50 medial: lateral load distribution,
710 N axial load).
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Figure 4 Constraint of the Legion implant at different angles of flexion (50:50 medial:
lateral load distribution, 710N axial load).
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Figure 5 Constraint of the Legion implant under 710N and 2000N axial loads (50:50
medial: lateral load distribution, 0° and 90° flexion angles).
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Figure 6 Coupled tibial rotation during anterior-posterior translation of the Triathlon implant
at different medial: lateral load distributions (710 N axial load, 90° flexion).
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Figure 7 Peak coupled tibial rotations of the Legion implant during anterior (left) and
posterior (right) translation at different medial: lateral load distributions and flexion angles
(710 N axial load).
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