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Biomechanical effects of femoral component flexion in TKA: a musculoskeletal modeling analysis



INTRODUCTION: One of the main goals in Total Knee Arthroplasty (TKA) is to restore natural knee kinematics and physiological loads in tibiofemoral (TF) and patellofemoral (PF) joint. The extent to which this is achieved may differ substantially, depending on many factors. Adding extra flexion of the femoral component (FFC) in the sagittal plane during implantation is one of these factors. More flexion may result in a more anatomical placement of the component, although its effect on the post-operative PF joint mechanics is still unknown. We hypothesize that more FFC will result in a more efficient extensor mechanism. Therefore, the aim of this study is to study the effects of varying FFC angle on the PF joint mechanics and quadriceps muscle forces. METHODS: A previously validated patient-specific musculoskeletal model of Cruciate-Retaining (CR) TKA was used [1]. Briefly, the model included full lower extremity of the implanted side (Fig. 1a); the TF and PF joint owned 12 degrees of freedom, of which all but knee flexion/extension were solved quasi-statically, and included ligaments and rigid volume penetration to solve contact at the joint interfaces (Fig. 1b). To investigate the effect of varying FFC, we varied the FFC by 3°, 6°, 9° compared to the original (0°) post-operative alignment, leading to four cases in total. Rotation occurred around a mediolateral axis placed at the most posterior aspect of the femoral condyles (Fig. 2), in order to preserve the post-operative flexion gap. At each flexion angle, the femoral component was also raised in the proximal femur direction to preserve the post-operative extension gap. A rise-from-a-chair activity from the existing model dataset was simulated in each case, which consisted of a rise phase followed by a sit phase. Marker trajectories were input to an inverse-kinematics analysis to compute joint angles. Subsequently, ground reaction forces and joint angles were input to an inverse-dynamics analysis coupled with Force-Dependent Kinematics to simultaneously solve muscle forces, ligament forces, joint contact forces and knee kinematics. We analyzed and compared PF contact forces, quadriceps muscle forces, and forces in the medial and lateral ligaments of the PF joint. RESULTS: Maximum quadriceps forces decreased by 48 N, on average, for every 3° of increase of FFC at 90° of knee flexion angle (Fig. 3a). The PF contact forces followed a similar trend (Fig. 3b): they decreased by 64 N, on average, for every 3° of increase of FFC at 90° of knee flexion. On the contrary, medial and lateral PF ligaments forces increased with increasing FFC (23 N and 25 N, on average, for every 3° of increase of FFC at knee extension, respectively), although the effect was more marked when the knee was extended (Fig. 3c-d). DISCUSSION: We showed that the FFC had an effect on the PF joint mechanics as predicted with a patient-specific model of TKA. The decrease of PF contact forces observed at higher FFC in the flexed knee position can be explained by the contemporaneous decrease of the quadriceps forces, which therefore relieves the load on the PF joint. It is speculated that a higher FFC produces an advantage for the quadriceps extensor mechanism, due to an increased moment arm. However, depending on the pre-operative situation, too high FFC may ‘over-tension’ the PF retinacula, which should be avoided. On the contrary, with a lower FFC higher quadriceps muscle forces are needed to provide the same knee extension torque, which in turn increases the PF joint contact forces. SIGNIFICANCE: Based on this study, we concluded that adding extra FFC during TKA is advisable because it is beneficial for the quadriceps extensor mechanism and it relieves the PF joint loads. This study provides orthopedic surgeons with important indications about the effect of increased FFC, as it may potentially decrease post-operative PF joint pain and improve the knee extensor muscles function. REFERENCES: [1] M. A. Marra, V. Vanheule, R. Fluit, B. H. F. J. M. Koopman, J. Rasmussen, N. J. J. Verdonschot, and M. S. Andersen, “A Subject-Specific Musculoskeletal Modeling Framework to Predict in Vivo Mechanics of Total Knee Arthroplasty.,” J. Biomech. Eng., Nov. 2014. ACKNOWLEDGEMENTS: This study was conducted within the ERC ‘BioMechTools’ project, funded by the European Research Council.
The main goals in Total Knee Arthroplasty (TKA) are restoring
knee and patellofemoral joint (PFJ) kinematics and loads to
normal values, and reduce knee pain.
Flexion of the femoral component (FFC) is a surgical option to
address flexion instability and avoid notching of the anterior
femoral cortex. More FFC also allows femoral component
down-sizing, when necessary.
However, the effect of FFC on post-operative range of motion
is controversial [1], and the mechanism by which it acts on
PFJ mechanics are not yet well documented.
We hypothesize that increasing the FFC would benefit the
knee extensor mechanism in daily activities such as rising
from a chair, and would help reducing the PFJ contact forces.
We studied the effect of FFC in combination with up- and down-
sizing the femoral component on peak quadriceps forces and
moment arms, peak PFJ contact forces, and peak medial
patellofemoral ligament (MPFL) forces when rising from a chair.
Materials and Methods
A validated musculoskeletal model of posterior cruciate-
retaining (CR) TKA [2] (Fig. 1a) was used to analyze a rise-
from-a-chair activity from a publicly available dataset [3].
Three sizes (‘3’, ‘4’, ‘5’) of a left Zimmer Natural-Knee CR-TKA
femoral component were virtually implanted into the model,
with ‘4’ being the original post-operative size (Fig. 1b).
The femoral component was flexed in the sagittal plane in
four steps of 3°, starting from the post-operative case (0°). We
positioned the implant so to preserve the post-operative
flexion and extension gap (Fig. 1c).
a) Quadriceps forces b) PFJ contact forces
c) Quadriceps moment arms d) MPFL forces
Discussion and Conclusion
FFC increases quadriceps moment arm more pronouncedly
when small sizes are used, whereas it is almost non
influential with size 5.
MPFL tension slighlty increases with more FFC when small
sizes are used, which can be expected since the trochlea
becomes more prominent. Size 5 produces a much larger
effect and the tension may become too high.
The post-operative situation of the present study showed
notching of the anterior femur cortex. FFC could potentially
avoid this notching, without resorting to a larger size.
FFC increases the efficiency of the knee extensor mechanism,
relieving both quadriceps and PFJ loads. It may be a valid option
for preventing anterior notching when down-sizing is desirable.
We are thankful to Zimmer-Biomet for providing us the computer models of CR-TKA implants.
Biomechanical effects of femoral component
flexion in TKA: a musculoskeletal modeling analysis.
M.A. Marra¹, M. Strzelczak¹, S. van de Groes², P. Heesterbeek³, A. Wymenga⁴,
H.F.J.M. Koopman⁵, D. Janssen¹, N. Verdonschot¹,
¹Orthopaedic Research Lab, Radboud University Medical Center, Nijmegen, The Netherlands, ²Orthopaedic Department, Radboud
University Medical Center, Nijmegen, The Netherlands, ³Research Department, Sint Maartenskliniek, Nijmegen, The Netherlands,
⁴Department of Orthopaedics, Sint Maartenskliniek, Nijmegen, The Netherlands, ⁵Department of Biomechanical Engineering, University
of Twente, Enschede, The Netherlands
Marco Marra, MSc
Orthopaedic Research Laboratory, Radboud umc
P.O. Box 9101, 6500 HB Nijmegen, The Netherlands
Figure 1. (a) Full-body musculoskeletal model used to simulate a rising-from-a-
chair activity with a detail view of the knee joint. Virtual implantation with (b)
three implant sizes and (c) FFC variations (0° and 9° shown). Note how the
extension and flexion spaces are preserved in all cases.
Figure 2. Peak values of (a) quadriceps forces, (b) patellofemoral joint contact
forces, (c) quadriceps moment arms, (d) medial patellofemoral ligament forces.
Note: the ‘size’ axis of the upper row was reversed for a better visibility.
The research leading to these results has received funding from the
European Research Council under the European Union's Seventh
Framework Programme (FP/2007-2013) / ERC Grant Agreement n. 323091
[1] Murphy et al., “Does flexion of the femoral implant in total knee arthroplasty increase knee flexion: a randomised
controlled trial”, Knee. 2014 Jan;21(1):257-63; [2] Marra et al., “A Subject-Specific Musculoskeletal Modeling Framework
to Predict in Vivo Mechanics of Total Knee Arthroplasty”, J Biomech Eng. 2015 Feb 1;137(2):020904; [3] Fregly et al.,
“Grand challenge competition to predict in vivo knee loads., J Orthop Res. 2012 Apr;30(4):503-13
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