Flexing and downsizing the femoral component is not detrimental to patellofemoral biomechanics in posterior-referencing cruciate-retaining total knee arthroplasty

Abstract

Purpose:
When downsizing the femoral component to prevent mediolateral overhang, notching of the anterior femoral cortex may occur, which could be solved by flexing the femoral component. In this study, we investigated the effect of flexion of the femoral component on patellar tendon moment arm, patellofemoral forces and kinematics in posterior-referencing CR-TKA. Our hypothesis was that flexion of the femoral component increases the patellar tendon moment arm, reduces the patellofemoral forces and provides stable kinematics.

Methods:
A validated musculoskeletal model of CR-TKA was used. The flexion of the femoral component was increased in four steps (0°, 3°, 6°, 9°) using posterior referencing, and different alignments were analysed in combination with three implant sizes (3, 4, 5). A chair-rising trial was analysed using the model, while simultaneously estimating quadriceps muscle force, patellofemoral contact force, tibiofemoral and patellofemoral kinematics.

Results:
Compared to the reference case (size 4 and 0° flexion), for every 3° of increase in flexion of the femoral component the patellar tendon moment arm increased by 1% at knee extension. The peak quadriceps muscle force and patellofemoral contact force decreased by 2%, the patella shifted 0.8 mm more anteriorly and the remaining kinematics remained stable, with knee flexion. With the smaller size, the patellar tendon moment arm decreased by 6%, the quadriceps muscle force and patellofemoral contact force increased by 8 and 12%, and the patellar shifted 5 mm more posteriorly. Opposite trends were found with the bigger size.

Conclusion:
Flexing the femoral component with posterior referencing reduced the patellofemoral contact forces during a simulated chair-rising trial with a patient-specific musculoskeletal model of CR-TKA. There seems to be little risk when flexing and downsizing the femoral component, compared to when using a bigger size and neutral alignment. These findings provide relevant information to surgeons who wish to prevent anterior notching when downsizing the femoral component.

Full text

Vol.:(0123456789)
1 3
Knee Surgery, Sports Traumatology, Arthroscopy
https://doi.org/10.1007/s00167-018-4900-z
KNEE
Flexing anddownsizing thefemoral component isnotdetrimental
topatellofemoral biomechanics inposterior-referencing cruciate-
retaining total knee arthroplasty
MarcoA.Marra1· MartaStrzelczak1· PetraJ.C.Heesterbeek2· SebastiaanA.W.vandeGroes3· DennisJanssen1·
BartF.J.M.Koopman4· NicoVerdonschot1,4· AteB.Wymenga5
Received: 28 September 2017 / Accepted: 16 March 2018
© The Author(s) 2018
Abstract
Purpose When downsizing the femoral component to prevent mediolateral overhang, notching of the anterior femoral cor-
tex may occur, which could be solved by flexing the femoral component. In this study, we investigated the effect of flexion
of the femoral component on patellar tendon moment arm, patellofemoral forces and kinematics in posterior-referencing
CR-TKA. Our hypothesis was that flexion of the femoral component increases the patellar tendon moment arm, reduces the
patellofemoral forces and provides stable kinematics.
Methods A validated musculoskeletal model of CR-TKA was used. The flexion of the femoral component was increased
in four steps (0°, 3°, 6°, 9°) using posterior referencing, and different alignments were analysed in combination with three
implant sizes (3, 4, 5). A chair-rising trial was analysed using the model, while simultaneously estimating quadriceps muscle
force, patellofemoral contact force, tibiofemoral and patellofemoral kinematics.
Results Compared to the reference case (size 4 and 0° flexion), for every 3° of increase in flexion of the femoral component
the patellar tendon moment arm increased by 1% at knee extension. The peak quadriceps muscle force and patellofemoral
contact force decreased by 2%, the patella shifted 0.8mm more anteriorly and the remaining kinematics remained stable,
with knee flexion. With the smaller size, the patellar tendon moment arm decreased by 6%, the quadriceps muscle force and
patellofemoral contact force increased by 8 and 12%, and the patellar shifted 5mm more posteriorly. Opposite trends were
found with the bigger size.
Conclusion Flexing the femoral component with posterior referencing reduced the patellofemoral contact forces during a
simulated chair-rising trial with a patient-specific musculoskeletal model of CR-TKA. There seems to be little risk when
flexing and downsizing the femoral component, compared to when using a bigger size and neutral alignment. These findings
provide relevant information to surgeons who wish to prevent anterior notching when downsizing the femoral component.
Keywords Flexion· Femoral· Component· Sagittal· Alignment· Musculoskeletal· Model· CR· TKA· Biomechanics·
Patellofemoral· Quadriceps· Force· Chair· Rising· Total knee arthroplasty· Total knee replacement· Posterior-
referencing
Introduction
Implant alignment in total knee arthroplasty (TKA) is a
key factor to restore natural knee kinematics and physi-
ological loads in the tibiofemoral (TF) and patellofemoral
(PF) joints, yet sagittal plane alignment of the femoral
component has received relatively little attention with
respect to function and outcome [13]. Previous studies
recommended that the flexion of the femoral compo-
nent (FFC) should be within 0°–3°, to reduce the risk of
implant failure [17] and to limit the incidence of flexion
Electronic supplementary material The online version of this
article (https ://doi.org/10.1007/s0016 7-018-4900-z) contains
supplementary material, which is available to authorized users.
* Marco A. Marra
mamarra@outlook.com
Extended author information available on the last page of the article

Knee Surgery, Sports Traumatology, Arthroscopy
1 3
contracture [19]. However, these studies addressed poste-
rior-stabilised (PS) TKA only.
Sagittal alignment is also related to the size of the
femoral component, as implants aligned in flexion have
typically smaller sizes [7]. This interplay often resides in
the attempt to prevent mediolateral overhang. Sometimes,
the femoral component is too wide in the mediolateral
dimension, which irritates the surrounding soft tissues
[4]. In this situation, the surgeon typically resorts to a
smaller size. However, a smaller size, in turn, increases
the chance of notching of the anterior femoral cortex in
non-gender specific implants. Therefore, additional flexion
of the femoral component is necessary to prevent notching,
when using a smaller size.
In adjusting the flexion of the femoral component, the
outcome may be different depending on implant design
and the surgical technique utilised. With anterior refer-
encing, the anterior femoral cortex serves as a reference
for the anterior distal femur resection, thus notching is
avoided. However, this technique has the disadvantage
of producing variable resection of the posterior femoral
condyles with subsequent difficult balancing of the flex-
ion space [11], and the outcome may be influenced by the
type of implant chosen (single- or multi-radius design).
Furthermore, because the posterior condylar offset (PCO)
is not controlled, subtle increments in FFC can tighten
the flexion gap substantially, as a result of over-stretching
of the posterior cruciate ligament (PCL) [21]. Therefore,
controlling the PCO appears essential to achieve a good
TF stability. This can be achieved using posterior-refer-
encing technique, in which the posterior femoral condyles
serve as reference for the posterior resection. However, the
anterior resection becomes more variable and subject to
notching [11].
Flexing and downsizing the femoral component could
be a solution to prevent anterior notching, alternative to a
larger size. However, the effect of FFC on PF joint forces
and kinematics remains largely unclear. Previous cadaver
and clinical studies could not separate the effect of FFC
from that of other possible confounding variables (e.g.
PCO), and have shown contrasting results [5, 6, 23, 25].
The present study examines the effect of FFC and
implant size on quadriceps moment arm, PF contact forces
and kinematics in posterior-referenced CR-TKA, using a
highly-controlled study design, in which all variables are
controlled for, thus overcoming the limitations of previ-
ous cadaver studies and clinical trials. The hypothesis was
that flexing and downsizing the femoral component would
result in similar PF contact forces and equally stable kin-
ematics as with neutrally-aligned upper-size implant. If
this hypothesis was confirmed, then FFC could represent
a viable surgical option to reconstruct the knee extensor
mechanism.
Materials andmethods
For this study, a validated patient-specific musculoskeletal
model was used. The creation and validation processes are
described elsewhere [20]. Briefly, the model was devel-
oped using the AnyBody Modeling System (AMS, ver-
sion 6, AnyBody Technology A/S, Aalborg, Denmark),
it was constructed based on medical images of a patient
with a telemetric CR-TKA implant, and it was validated
against experimental measurements of TF contact forces
and sagittal plane kinematics. In the present study, spe-
cific changes to the original model were made, which are
detailed in a separate additional file [see Additional file1].
Geometries of pre- and post-operative bones, and of the
TKA implant, were obtained from an open-access dataset
[12]. The femoral component was the size 4 of the Natu-
ral Knee CR-TKA system (Zimmer Biomet, Warsaw, IN,
U.S.). The femoral component had a J-curved multi-radius
design. The patella was resurfaced. Based on the post-
operative model reconstruction in the AMS, the FFC angle
was measured as the angle between the vertical axis of the
femoral component and the mechanical axis of the femur.
The vertical axis of the femoral component was the line
perpendicular to the distal flat inner facet of the implant,
and the mechanical axis of the femur was the line pass-
ing through the centre of the hip joint and the midpoint
between the medial and lateral femoral epicondyles. The
post-operative FFC angle was equal to 0° (neutral align-
ment) and represented our reference case.
One smaller size (size 3) and one bigger size (size 5)
and three more FFC cases (+ 3°, + 6°, + 9°) were created,
based on the reference model. These will be referred to as
the custom post-operative cases. Geometrical models for
size 3 and 5 of the femoral component were made available
to us by courtesy of Zimmer Biomet (Warsaw, Indiana,
U.S.). All custom cases were obtained keeping the joint
space in flexion and in extension equal to that of the refer-
ence case (posterior referencing). To that aim, the femoral
component geometry was translated and rotated in the sag-
ittal plane, with the aid of the 3-D manipulation software
Meshlab [8], such that its outline would always match
tangentially the outline of the reference case at the most
posterior and most distal ends of the implant (Fig.1). This
allowed for preservation of the post-operative PCO and did
not alter the joint line in extension. Geometrical wrapping
surfaces guided the path of muscles and ligament around
the knee joint, and were adapted for each combination of
implant size and FFC. The same size of the tibial compo-
nent as of the reference case was used in all custom cases.
In addition, an intact knee case was implemented, based
on pre-operative CT images of the same patient. Given
the scarce visibility of menisci and cartilaginous tissues

Knee Surgery, Sports Traumatology, Arthroscopy
1 3
on CT images, the articular surfaces of the tibial, patellar
and femoral cartilage were estimated using an offset of
the bony surfaces of tibia, patella and femur, respectively.
The amount of offset was made equal to the average car-
tilage thickness found in the literature for each respective
compartment [9]. Menisci were not modelled. The anterior
cruciate ligament was modelled as a spring with mechani-
cal properties adapted from the literature [3].
The model was configured to simulate a rising-from-a-
chair activity, which was recorded using standard motion
capture techniques and available as part of an open-access
dataset (PS_chairrise1) [12]. The trial consisted of a ris-
ing phase followed by a sitting phase for a total duration
of 4.375s. The range of knee flexion, as measured, was
approximately 10°–96° and the chair-rise task was per-
formed without the aid of the arms. Additional movie files
show the musculoskeletal model in motion during a repre-
sentative simulation [see Additional file2 and 3]. The fol-
lowing parameters were continuously recorded as output
of the simulations: patellar tendon moment arm (PTMA),
patellar tendon force (PTF), quadriceps muscle force (QMF),
quadriceps tendon-to-femur force (QTFF), PF contact force
(PFCF), PF antero-posterior translation, the force in the
PCL and medial patellofemoral ligament (MPFL) and the
kinematics of the TF contact point. The PF antero-posterior
translation was defined using a well-established knee joint
coordinate system [14], adapted to describe PF kinematics.
The femoral reference frame was built from the mechani-
cal and transepicondylar axes of the femur, and the patellar
reference frame was built based on anatomical landmarks
identifying the most proximal and most distal, and the most
medial and most lateral points of the patella.
A total of thirteen (three sizes and four FFC angles, plus
one intact case) simulations were executed. The results of
the custom post-operative cases were compared to those of
the reference case (neutrally aligned, size 4). The PTMA
and the PF antero-posterior translation from all post-oper-
ative cases were also compared to those obtained with the
intact knee simulation. Joint forces were expressed as frac-
tions of body weight (BW) and the ligament forces were
expressed in units of newton (N).
Results
Patellar tendon moment arm
At knee flexion, both size and FFC had negligible effects
on the PTMA. At knee extension, the PTMA increased
with FFC and with a bigger size, and decreased with a
smaller size, compared to the reference case (Table1;
Fig.2). In all post-operative cases, the PTMA was about
6% smaller than in the intact case. Detailed values of
PTMA for all simulated cases are provided separately (see
Additional file4).
Fig. 1 Twelve simulated post-operative cases with three different
sizes and four degrees of flexion of the femoral component. Illustra-
tion of the twelve custom post-operative cases simulated in this study.
From left to right four degrees of flexion of the femoral component
are shown: 0°, 3°, 6°, 9°. Three sizes of the femoral component (blue:
size 3, red: size 4, yellow: size 5) plus the pre-operative bone are
shown in overlay for each flexion of the femoral component (FFC)
angle. Note that in every case the most distal and most posterior ends
of the outlines of the femoral component are made to match tangen-
tially, to simulate a posterior referencing and to preserve the posterior
condylar offset
Table 1 Changes in knee extensor parameters due to flexion and size
of the femoral component
Changes of patellar tendon moment arm at knee flexion (PTMAflex),
at knee extension (PTMAext), peak patellar tendon force (PTF),
quadriceps muscle force (QMF), quadriceps tendon-to-femur force
(QTFF), and patellofemoral contact force (PFCF) during rising-from-
a-chair simulations due to varying size and flexion of the femoral
component (FFC). Variations are expressed as average percentage
increase (+) or decrease (-) relative to the reference case (size 4, 0°
FFC) for every 3° increase of FFC (+ 3° FFC) and for a bigger size
(Size +) and a smaller size (Size −)
+ 3° FFC Size+ Size−
PTMAflex 0% 0% 0%
PTMAext + 1% + 6% −7%
PTF −2% −5% + 7%
QMF −2% −7% + 8%
QTFF + 2% + 11% −15%
PFCF −2% −10% + 12%

Knee Surgery, Sports Traumatology, Arthroscopy
1 3
Forces ontheknee extensor mechanism
The forces in the knee extensors mechanism peaked during
the ascending phase, at a knee flexion angle of about 90
degrees. Peak values of PTF, QMF, QTFF, and PFCF for all
simulated cases are depicted in Fig.3, and their variations
relative to the reference case are summarised in Table1.
Detailed peak values for all simulated cases are provided
separately [see Additional file4].
Patellofemoral kinematics
Changes in FFC and size affected the patellar antero-
posterior translation (Fig.4), and the effect was smaller
with increased knee flexion. At knee extension (approxi-
mately 10° knee flexion), the patella shifted by 0.6, 0.8,
and 1.1mm more anteriorly for every 3° increase of FFC,
with size 3, 4, and 5, respectively, and it shifted about
5mm more anteriorly with a bigger size of the femoral
component. Compared to the intact case, the patella was
located 10.2, 5.6, and 0.3mm more posteriorly, at knee
extension, with size 3, 4, and 5, respectively.
Ligament forces
The ligament forces were rather sensitive to changes in
size and FFC. The MPFL force peaked at knee extension
and the PCL force peaked at approximately 90° of knee
flexion (Fig.5), in the reference case. On average, the peak
force in the MPFL increased by 80% for every 3° increase
of FFC, especially with knee extension and mid-flexion,
and increased by 314% with a bigger size. The MPFL
remained slack with size 3 regardless of the FFC angle.
The peak force in the PCL increased by 18%, for every 3°
increase of FFC, increased by 96% with a bigger size and
decreased by 56% with a smaller size.
Fig. 2 Patellar tendon moment arm. Patellar tendon moment arm
(PTMA) at varying knee flexion angle during a rising-from-a-chair
simulation. From left to right the results in mm for size 3, 4 and 5 are
shown. Each line series correspond to a flexion of the femoral com-
ponent (FFC) angle. The flexion angle in the abscissa indicates the
phases of the rising and sitting motion
Fig. 3 Peak forces on the knee extensor mechanism. Peak forces on
the knee extensor mechanism during a rising-from-a-chair simulation.
From left to right: patellar tendon force (PTF), quadriceps muscle
force (QMF), quadriceps tendon-to-femur force (QTFF), and patel-
lofemoral contact force (PFCF). Results are reported in body weights
(BW)

Knee Surgery, Sports Traumatology, Arthroscopy
1 3
Kinematics ofthetibiofemoral contact point
The effect of FFC on the kinematics of the TF contact point
was very small. The size of the femoral component had
a slightly larger effect on the kinematics (see Additional
file5). A comparison of the kinematics of the TF contact
point with the intact case is also provided separately (see
Additional file6).
Discussion
The two most important findings of this study are that flex-
ing the femoral component: (1) while keeping the size,
increases the knee extensor moment arm in extension,
reduces the quadriceps and patellofemoral contact forces in
flexion, and provided stable kinematics throughout the range
of knee flexion and extension; (2) in combination with a
smaller size, results in similar forces and kinematics as with
a bigger size which is neutrally aligned. These results con-
firm our hypothesis and suggest that the femoral component
can be downsized and flexed, to prevent both mediolateral
overhang and anterior notching of the femur, and that this
would result in an equally stable reconstruction of the knee
extensors mechanism as with a neutrally-aligned upsized
implant.
The computational approach used in this study presented
some key novel aspects. It enabled the study of size and
sagittal alignment of the femoral component in a single sub-
ject case, while all the other variables were unchanged, such
as the PCO, the size and alignment of the tibial and patel-
lar components, and the level of the joint line in extension.
This aspect overcomes one big limitation of clinical studies,
in which confounding variables are present inevitably. For
instance, Antony etal. found a correlation between higher
FFC and larger maximal post-operative flexion angle in CR-
TKA [2], whereas Murphy etal. observed a larger maximal
knee flexion angle at surgery, which did not translate in a
functional benefit at 1year post-operatively [22]. In both
studies, the PCO was not controlled for, which may have
acted thus as a confounding parameter.
Flexing the femoral component provided some positive
effects. On the one hand, a more flexed implant increased
the patellar tendon moment arm at knee extension and, to
a lesser extent, in mid-flexion, which may be relevant for
those activities involving large quadriceps action in the
Fig. 4 Tibiofemoral distraction and patellofemoral antero-posterior
translation. Kinematics of a patellofemoral antero-posterior transla-
tion and b tibiofemoral distraction, at varying knee flexion angle dur-
ing a rising-from-a-chair simulation. From left to right the results in
mm for size 3, 4, and 5 are shown. Each line series correspond to a
flexion of the femoral component (FFC) angle. Kinematics from the
custom cases are plotted relatively to the intact case. The rising and
sitting phases for each curve are overlapped

Knee Surgery, Sports Traumatology, Arthroscopy
1 3
first arc of the knee range of motion. This first mechanism
can be explained by the trochlear groove positioned more
anteriorly and distally with more FFC. In other words, the
patellofemoral joint becomes overstuffed. On the other hand,
more FFC increased the QTFF in (mid-)flexion. This second
mechanism redistributes some of the patellofemoral joint to
the quadriceps tendon–femur compartment. A higher QTFF
may result in larger stresses at the implant-bone (or implant-
cement) interface, which may have an effect on implant fixa-
tion. However, these aspects were not investigated in the pre-
sent study and warrant further attention. Summed together,
the abovementioned effects of FFC provided a means for
reducing the quadriceps and patellofemoral contact forces
during dynamic and weight-bearing exercise.
A larger size of the femoral component, leaving the PCO
unchanged and increasing the offset of the trochlea (poste-
rior referencing), relative to the reference case, resulted in an
even larger reduction in the quadriceps and PF forces with
knee flexion from 0 to 100°, in the present study. This seems
to be in contrast with the finding of Kawahara etal., who
found higher PF contact forces at flexion angles of 90° and
more with larger femoral components [16]. These authors,
however, adopted an opposite approach: they increased the
antero-posterior dimension of the femoral component by
increasing the PCO and leaving the position of the anterior
flange unchanged (anterior referencing). Moreover, they only
evaluated PF contact forces in deeper flexion under static
and non-weight-bearing conditions, and they used PS-TKA.
In contrast, we estimated PF contact forces in a CR-TKA
model during a dynamic and weight-bearing knee exercise,
involving quadriceps muscle activity. Their findings, in
essence, do not conflict with our results.
Ligament tensions here presented were in line with previ-
ous studies on ligament length changes in TKA [1, 15, 18].
With a bigger size, the both PCL and MPFL forces increased
substantially, and much more than observed after variations
in FFC alone. Higher tension in the MPFL resulted from
an oversized femoral component (mediolateral overhang),
and this may be detrimental to the results of TKA [4]. For
this reason, over-sizing the femoral component is generally
discouraged. Larger PCL forces with a bigger size of the
femoral component were in agreement with findings of pre-
vious studies [10], and could be explained both by a larger
TF distraction and a larger posterior tibial translation with
knee flexion. In contrast, a smaller femoral component slack-
ened the MPFL nearly entirely, due to a posterior patellar
Fig. 5 Ligament forces. Ligament force of the a medial patellofemo-
ral ligament (MPFL) and b posterior cruciate ligament (PCL), at var-
ying knee flexion angle during a rising-from-a-chair simulation. From
left to right the results in N for size 3, 4, and 5 are shown. Each line
series correspond to a flexion of the femoral component (FFC) angle.
The flexion angle in the abscissa indicates the phases of the rising
and sitting motion

Knee Surgery, Sports Traumatology, Arthroscopy
1 3
translation (understaffing) and a smaller mediolateral size
of the femoral component, and the PCL force was halved,
compared to the reference size. This scenario is also discour-
aged, as slackening of the MPFL may increase the risk of
patellar instability (although no aberrant PF kinematics were
observed in this study) [24] and slackening of the PCL may
destabilise the knee in flexion. Flexing the femoral compo-
nent could partially restore the tension in these ligaments.
The post-operative PTMA in (mid-)flexion was consist-
ently smaller than in the intact case, which may indicate a
failed reconstruction of the PTMA for other reasons. At knee
extension, similar PTMA was obtained in the intact case,
with size 5, and with size 4 with additional FFC. There-
fore, increasing the FFC may also increase the PTMA in
extension. Implant size had the largest influence on patellar
antero-posterior translation. Post-operatively, the patella was
consistently less anterior than in the intact case, throughout
the range of flexion–extension. In extension and mid-flexion,
additional FFC could partially restore the antero-posterior
translation.
From a purely anatomical point of view, and if we con-
sider only the femoral antero-posterior dimension, the size
5 of the femoral implant would likely provide the best fit
(Fig.6). However, such a choice could be less favourable
concerning mediolateral overhang, as it could consequently
cause an irritation of the soft tissues. Virtually, an equally
good antero-posterior fit as with size 5 could be achieved
using a smaller femoral component (size 4) which is flexed
by about 6°. Despite the downsizing, flexing the femoral
component while preserving the PCO would also ensure a
proper reconstruction of the flexion space, without concerns
of anterior notching of the femoral cortex.
In light of these findings, flexing and downsizing the
femoral component seem to provide similar biomechanical
results as using a bigger size with neutral alignment, but
without the problem of mediolateral overhang and anterior
notching. Moreover, flexing the femoral component does
not appear detrimental to TF and PF kinematics. Therefore,
surgeons may consider flexing the femoral component as
an option to limit anterior femoral notching in downsized
implant. Surgeons should also be aware that downsizing the
femoral component might decrease the tension in the PCL
and MPFL, and flexing the femoral component may partially
restore this tension, as shown in this study.
The present study elucidates biomechanical aspects
of sagittal alignment and size of the femoral component
in CR-TKA with posterior referencing. Caution should
be used when generalising the present findings to other
implant types (e.g. PS-TKA), designs (e.g. single-radius)
and surgical techniques (e.g. anterior referencing), and
cases of large anatomical deformity, as these were not
investigated. Furthermore, given our choice to preserve
the PCO with posterior referencing, some of the simulated
cases (e.g. size 3 with 0° and 3° FFC and size 5 with 6°
and 9° FFC) are not plausible in practice. These hypo-
thetical cases were included as well, to provide a more
Fig. 6 Illustrative case for the alignment in flexion of a downsized
femoral component. Illustrative case for the alignment in flexion of
a downsized femoral component with preservation of the posterior
condylar offset (PCO). Size 5 with 0° FFC fits the antero-posterior
dimension of the femur, however, mediolateral overhang is observed,
which is detrimental. Downsizing the femoral component (Size 4, 0°
FFC) reduces the mediolateral overhang, but creates anterior notching
of the femoral cortex, if the PCO is preserved. Flexing the smaller
component by a few degrees in the sagittal plane (Size 4, 6° FFC)
may concomitantly preserve the PCO, while limiting mediolateral
overhang and preventing anterior notching

Knee Surgery, Sports Traumatology, Arthroscopy
1 3
comprehensive overview of the parameters investigated.
The use of a computer model to simulate the effect of size
and alignment involved many assumptions and simplifi-
cations. The musculoskeletal model was based on only
one patient and implant design, which minimised possible
confounding variables. Future research should assess the
influence of anatomical variability and validate these find-
ings in a clinical setting; this study provides clues as to
which parameters could be included.
Conclusion
Flexing the femoral component increases the knee exten-
sors moment arm and reduces the quadriceps and patel-
lofemoral contact forces in posterior-referencing CR-TKA.
There seems to be little risk associated with flexing the
femoral component in a downsized implant, which could
have advantages in terms of preventing mediolateral over-
hang and anterior notching, and would result in similar
patellofemoral forces and kinematics as in a neutrally-
positioned upsized component.
Acknowledgements We would like to thank Dr. Darryl D’Lima
(The Scripps Research Institute, Department of Molecular Medicine,
California Campus, La Jolla, California, U.S.), Marc Vogels (Zimmer
Biomet, Warsaw, Indiana, U.S.), Michelle Zawadzki (Zimmer Biomet,
Warsaw, Indiana, U.S.), Chuck Perrone (Zimmer Biomet, Warsaw,
Indiana, U.S.) for their courtesy and kind assistance in providing the
computer files for the additional sizes of the femoral component used
in this study.
Authors’ contributions MAM implemented the analyses through mus-
culoskeletal modelling, analysed the data and drafted the manuscript.
MS helped to draft the manuscript and carry out the analyses. DWJ,
BFJMK, SAWvdG helped in the analysis and interpretation of the data
and critically revised the manuscript for intellectual content. PJCH
participated in the design and coordination of the study and helped to
draft the manuscript. ABW and NJJV conceived of the study and con-
tributed to the interpretation of the data. All authors read and approved
the final manuscript.
Funding This project was supported by the European Research Coun-
cil under the European Union’s Seventh Framework Programme
(FP/2007–2013), ERC Grant Agreement no. 323091 awarded to N.V.
Compliance with ethical standards
Conflict of interest The authors declare that they have no conflict of
interest.
Ethical approval Approval was not required, as neither human partici-
pants nor animals were involved in this study.
Informed consent Informed consent was not applicable for this study.
Open Access This article is distributed under the terms of the Crea-
tive Commons Attribution 4.0 International License (http://creat iveco
mmons .org/licen ses/by/4.0/), which permits unrestricted use, distribu-
tion, and reproduction in any medium, provided you give appropriate
credit to the original author(s) and the source, provide a link to the
Creative Commons license, and indicate if changes were made.
References
1. Amis AA, Firer P, Mountney J, Senavongse W, Thomas NP
(2003) Anatomy and biomechanics of the medial patellofemoral
ligament. Knee 10:215–220
2. Antony J, Tetsworth K, Hohmann E (2017) Influence of sag-
ittal plane component alignment on kinematics after total
knee arthroplasty. Knee Surg Sports Traumatol Arthrosc
25:1686–1691
3. Blankevoort L, Kuiper JH, Huiskes R, Grootenboer HJ (1991)
Articular contact in a three-dimensional model of the knee. J
Biomech 24:1019–1031
4. Bonnin MP, Schmidt A, Basiglini L, Bossard N, Dantony E (2013)
Mediolateral oversizing influences pain, function, and flexion after
TKA. Knee Surg Sports Traumatol Arthrosc 21:2314–2324
5. Brar AS, Howell SM, Hull ML, Mahfouz MR (2016) Does kin-
ematic alignment and flexion of a femoral component designed
for mechanical alignment reduce the proximal and lateral reach
of the trochlea? J Arthroplasty 31:1808–1813
6. Chen S-C, Zeng Y-M, Yan M-N, Yue B, Zhang J, Wang Y
(2017) Effect of femoral component flexion implantation on
the mediolateral bone-prosthetic fit in total knee arthroplasty.
Orthop Surg 9:91–96
7. Chen S, Zeng Y, Yan M, Yue B, Zhang J, Wang Y (2017) Mor-
phological evaluation of the sagittal plane femoral load-bearing
surface in computer-simulated virtual total knee arthroplasty
implantation at different flexion angles. Knee Surg Sports Trau-
matol Arthrosc 25:2880–2886
8. Cignoni P, Callieri M, Corsini M, Dellepiane M, Ganovelli F,
Ranzuglia G (2008) MeshLab: an open-source mesh processing
tool. In Vittorio S, Rosario DC, Ugo E (eds) Eurographics Ital
Chapter Conf Eurographics Association, pp129–136
9. Cohen ZA, McCarthy DM, Kwak SD, Legrand P, Fogarasi F,
Ciaccio EJ, Ateshian GA (1999) Knee cartilage topography,
thickness, and contact areas from MRI: in-vitro calibration and
in-vivo measurements. Osteoarthr Cartil 7:95–109
10. Donadio J, Pelissier A, Boyer P, Massin P (2015) Control of
paradoxical kinematics in posterior cruciate-retaining total knee
arthroplasty by increasing posterior femoral offset. Knee Surg
Sports Traumatol Arthrosc 23:1631–1637
11. Fokin A, Heekin R (2013) Anterior referencing versus posterior
referencing in total knee arthroplasty. J Knee Surg 27:303–308
12. Fregly BJ, Besier TF, Lloyd DG, Delp SL, Banks S, Pandy
MG, D’Lima DD (2012) Grand challenge competition to predict
invivo knee loads. J Orthop Res 30:503–513
13. Gromov K, Korchi M, Thomsen MG, Husted H, Troelsen A
(2014) What is the optimal alignment of the tibial and femoral
components in knee arthroplasty? Acta Orthop 85:480–487
14. Grood ES, Suntay WJ (1983) A joint coordinate system for the
clinical description of three-dimensional motions: application
to the knee. J Biomech Eng 105:136–144
15. Halewood C, Risebury M, Thomas NP, Amis AA (2014) Kin-
ematic behaviour and soft tissue management in guided motion
total knee replacement. Knee Surg Sports Traumatol Arthrosc
22:3074–3082
16. Kawahara S, Matsuda S, Fukagawa S, Mitsuyasu H, Naka-
hara H, Higaki H, Shimoto T, Iwamoto Y (2012) Upsizing the

Knee Surgery, Sports Traumatology, Arthroscopy
1 3
femoral component increases patellofemoral contact force in
total knee replacement. J Bone Joint Surg Br 94:56–61
17. Kim Y-H, Park J-W, Kim J-S, Park S-D (2014) The relationship
between the survival of total knee arthroplasty and postopera-
tive coronal, sagittal and rotational alignment of knee prosthe-
sis. Int Orthop 38:379–385
18. Li G, Zayontz S, Most E, Otterberg E, Sabbag K, Rubash HE
(2001) Cruciate-retaining and cruciate-substituting total knee
arthroplasty: an invitro comparison of the kinematics under
muscle loads. J Arthroplasty 16:150–156
19. Lustig S, Scholes CJ, Stegeman TJ, Oussedik S, Coolican MRJ,
Parker DA (2012) Sagittal placement of the femoral component
in total knee arthroplasty predicts knee flexion contracture at one-
year follow-up. Int Orthop 36:1835–1839
20. Marra MA, Vanheule V, Fluit R, Koopman BHFJM., Rasmussen
J, Verdonschot N, Andersen MS (2015) A subject-specific mus-
culoskeletal modeling framework to predict invivo mechanics of
total knee arthroplasty. J Biomech Eng 137:20904
21. Matziolis G, Hube R, Perka C, Matziolis D (2012) Increased flex-
ion position of the femoral component reduces the flexion gap
in total knee arthroplasty. Knee Surg Sports Traumatol Arthrosc
20:1092–1096
22. Murphy M, Journeaux S, Hides J, Russell T (2014) Does flexion
of the femoral implant in total knee arthroplasty increase knee
flexion: a randomised controlled trial. Knee 21:257–263
23. Nedopil AJ, Howell SM, Hull ML (2017) What clinical character-
istics and radiographic parameters are associated with patellofem-
oral instability after kinematically aligned total knee arthroplasty?
Int Orthop 41:283–291
24. Panagiotopoulos E, Strzelczyk P, Herrmann M, Scuderi G (2006)
Cadaveric study on static medial patellar stabilizers: the dynam-
izing role of the vastus medialis obliquus on medial patellofemoral
ligament. Knee Surg Sports Traumatol Arthrosc 14:7–12
25. Roßkopf J, Singh PK, Wolf P, Strauch M, Graichen H (2014)
Influence of intentional femoral component flexion in navigated
TKA on gap balance and sagittal anatomy. Knee Surg Sports Trau-
matol Arthrosc 22:687–693
Aliations
MarcoA.Marra1· MartaStrzelczak1· PetraJ.C.Heesterbeek2· SebastiaanA.W.vandeGroes3· DennisJanssen1·
BartF.J.M.Koopman4· NicoVerdonschot1,4· AteB.Wymenga5
Marta Strzelczak
strzelczak.marta@gmail.com
Petra J. C. Heesterbeek
P.heesterbeek@maartenskliniek.nl
Sebastiaan A. W. vande Groes
Sebastiaan.vandeGroes@radboudumc.nl
Dennis Janssen
Dennis.Janssen@radboudumc.nl
Bart F. J. M. Koopman
h.f.j.m.koopman@utwente.nl
Nico Verdonschot
Nico.Verdonschot@radboudumc.nl
Ate B. Wymenga
A.wymenga@maartenskliniek.nl
1 Orthopaedic Research Laboratory, Radboud Institute
forHealth Sciences, Radboud University Medical Center,
Postbus 9101, 6500HBNijmegen, TheNetherlands
2 Sint Maartenskliniek Research, Postbus 9011,
6500GMNijmegen, TheNetherlands
3 Orthopaedic Department, Radboud University Medical
Center, Postbus 9101, 6500HBNijmegen, TheNetherlands
4 Department ofBiomechanical Engineering, University
ofTwente, Postbus 217, 7500AEEnschede, TheNetherlands
5 Sint Maartenskliniek Orthopaedics, Postbus 9011,
6500GMNijmegen, TheNetherlands