# A new experimental platform for deep flexion knee tests

Franck Jourdan, Etienne Malachanne, Yasmine Boukhriss, Hervé Louche,
Cédric Reynaud, Louis Dagneaux

► To cite this version:

Franck Jourdan, Etienne Malachanne, Yasmine Boukhriss, Hervé Louche, Cédric Reynaud, et al. A new experimental platform for deep flexion knee tests. Multidisciplinary Biomechanics Journal, 2025, 50th congress of the., 10.46298/mbj.16247. hal-05191802v2

HAL Id: hal-05191802

https://hal.science/hal-05191802v2

Submitted on 23 Oct 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>
<figure><img src="image_2.png" /></figure>

# A new experimental platform for deep flexion knee tests

Franck Jourdan<sup>a*</sup>, Etienne Malachanne<sup>a, b</sup>, Yasmine Boukhriss<sup>a</sup>, Hervé Louche<sup>a</sup>, Cédric Reynaud<sup>a</sup>, Louis Dagneaux<sup>a, c</sup>

a LMGC, Univ. Montpellier, CNRS, Montpellier, France

b IMT, Mines d'Alès, France

c CHU, Montpellier, France

* Franck Jourdan: franck.jourdan@umontpellier.fr

Received date: 06/04/2025

Accepted date: 27/06/2025

Publication date: 27/10/2025

**Keywords:** knee flexion, experimental platforms, force control, total knee arthroplasty (TKA)

© 2025 The Authors

Licence CC-BY 4.0

Published by Société de Biomécanique

## 1. Introduction

Experimental platforms simulating flexion tests are essential for studying knee biomechanics using cadaveric specimen, reproducing forces and kinematics. Historically, the most referenced platform is commonly known as the “Oxford Knee Rig” (OKR) (Bourne et al. 1978). Knee flexion angle was obtained by monitoring actuator of the quadriceps force (QF), which remains a simplistic approach of knee biomechanics especially during deep flexion activities. More recently, knee flexion angle monitoring utilized platforms based on robotic actuators (Most et al. 2002). Force-driven processing allows full degrees of freedom of the knee joint, whether pre-arthritic, arthritic or implanted knee conditions, which is not the case with robotic platforms.

In this article, the purpose is to present a novel force-driven platform aiming to better replicate the lower-limb mechanics during deep flexion activities, while inspired from the historical OKR. The design of this prototype, namely the “Montpellier Knee Rig” (MKR), is presented below, including key points of novelty and modularity, as well as preliminary results comparing quadricipital forces tested in OKR or MKR configurations.

## 2. Methods

The flexion test bench was designed to simulate a squat movement. The OKR device is usually adapted to a traction/compression machine and allows to perform flexion of a cadaveric knee. This type of device

remains useful for studying knee flexion, but results that it provided are still quite far from the physiological loads. In the OKR configuration, the center of the hip joint is maintained fixed above the ankle during flexion, which is not physiological during deep knee flexion, since the rotation of the hip and the movement of the pelvis are both mandatory. In addition, the rotation of the femur around its anatomical axis is also not physiological, the femoral neck offset being neglected in OKR configuration. Finally, ankle was considered as an unconstrained center of rotation, while being limited in dorsiflexion by the actuation of Achilles tendon and tibial pilon morphology (30°, approximatively).

Therefore, we aimed to develop a new device, after considering the above limitations. A cylinder (linear electrical actuator) was fixed on the femur to replace the quadriceps muscle, in addition to force sensors (3-axis sensor at the ankle to measure the total load, and one direction load sensor in series with the actuator to measure muscle force (Fig. 1 (a)). An inclinometer was also attached to the femur to monitor flexion angle, only controlled by the actuator under the action of the device's own weight. This self-weight can be modulated by adding counterweights, to limit the forces in the actuator. The first tests were carried out on a TKA implanted on a 3D printed knee. Threaded rods were embedded in the femoral and tibial shaft to respect physiological alignment and sealed in aluminium tubes
with the knee model. The tibial shaft was connected to a ball joint, attached to the 3-axis load cell. The femoral shaft was connected to a bar by a T-link connected to a ball joint representing the hip. Extensor apparatus and collateral ligaments were approximated using robust straps to account for viscoelastic structures.

The distance between hip and knee articular centers was adjustable. The tilt of the pelvis was controlled by adding rubber bands between hip and tibia, simulating the action of the hamstrings. Three consecutive flexion/extension tests, up to 115°, were carried out for each configuration, at low speed to overcome dynamic effects.

## 3. Results and discussion

An example of QF and ankle reaction forces are shown in Fig. 1 (b) and (c), with QF according to time and 3D ankle reaction forces, respectively. The maximum QF values for each test are given in Table 1. Forces were higher in the OKR configuration than in the MKR configuration during 3 flexion tests.

This result was expected because the MKR configuration may enhance loads distribution with pelvic degree of freedom. During deep flexion, the initial load (80 N in z direction) applied to the ankle represented the half of the so-called body weight (BW), (Fig. 1 c). The sign of the vertical force changed during flexion, which occurs when the tibia was limited to 30° of dorsiflexion by a wedge sized for this purpose.

In addition, the BW was deliberately low to limit maximum QF values to limit damage of the synthetic knee resistance. Despite this precaution, the straps were damaged. The QF values were around 6 fold the BW, which was within the upper limit of literature values (Merbarki et al. 2023). This excessive load was attributed to friction created by both a misalignment of the vertical guide rails of the device and the counterweight system.

<table><caption>**Table 1.** Maximal Quadricipital Forces (N) for each configuration.</caption><thead><tr><td>QF (N)</td><td>MKR</td><td>OKR</td></tr></thead><tbody><tr><td>Trial 1</td><td>871</td><td>951</td></tr><tr><td>Trial 2</td><td>839</td><td>969</td></tr><tr><td>Trail 3</td><td>1011</td><td>1030</td></tr></tbody></table>

<figure><img src="image_3.png" /><figcaption>**Figure 1.** (a) Montpellier Knee Rig, (b) QF force (N) over time (s), (c) Ankle forces (N) over time (s).</figcaption></figure>

## 4. Conclusions

A force-driven knee flexion test platform was developed, aiming to reproduce more physiological loads during a squat than those shown with the OKR configuration. Our preliminary results from 3 flexion tests using TKA implanted on a 3D printed synthetic knee showed significant differences in quadricipital forces and ankle reaction forces between the two configurations (OKR versus MKR). The main point of improvement will be in reducing friction forces during deep flexion loading.

## Acknowledgements

Stéphan Devic, Yvan Duhamel, Jean-Benoît Perrot.

## Conflict of Interest Statement

None.

## Contributor Roles

EM, YB, HL, CR, LD: Conceptualization, Methodology, Validation, Writing-review; FJ: Methodology, Conceptualization, Supervision, Funding acquisition, Writing original draft, Writing-review & editing.

## Funding

This work was supported by the LabEx NUMEV "Digital and Hardware Solutions, Environmental and Organic Life Modeling" (ANR-10-LABX-20-01).
References

Bourne, R., Goodfellow, J. W., & O'Connor, J.J. (1978). A functional analysis of various knee arthroplasties. *Transactions of the Orthopaedic Research Society*, 24, 160.

Li, G., Most, E., Otterberg, E., Sabbag, K., Zayontz, S., Johnson, T., et al. (2002). Biomechanics of posterior-substituting total knee arthroplasty: an in vitro study. *Clinical Orthopaedics and Related Research*, 404, 214–225. https://doi.org/10.1097/00003086-200211000-00035

Mebarki, S., Jourdan, F., Canovas, F., Malachanne, E., & Dagneaux, L. (2023). Validation of a novel finite-element model for evaluating patellofemoral forces and stress during squatting after posterior-stabilized total knee arthroplasty. *Orthopaedics & Traumatology: Surgery & Research*, 109(5), 103519. https://doi.org/10.1016/j.otsr.2022.103519