Transfer of patients' tibiofemoral kinematics and loads to a six-degree-of-freedom (6-DOF) joint simulator under consideration of virtual ligaments

Abstract

Preclinical testing of total knee replacements (TKR) is crucial for evaluating new implant designs. Dynamic experimental testing focus mostly on level walking and squats, failing to represent a full range of daily activities. Moreover, the contribution of the ligament apparatus is often simplified. Therefore, this study transferred five daily activity load cases-level walking, downhill walking, stair descent, squat, and sit-to-stand-onto a six-degree-of-freedom (6-DOF) joint simulator with a cruciate-retaining bicondylar TKR and a virtual ligament apparatus. Forces and kinematics were based on telemetric data from an ultra-congruent TKR. The resulting kinematics, kinetics, and tibiofemoral contact surfaces were evaluated. Additionally, variations of the virtual ligament apparatus on the joint simulator, e.g. resection of the posterior cruciate ligament (PCL), have been used to assess its influence on the resulting joint dynamics. Results showed that tibiofemoral contact area was more influenced by dynamics than kinematics. Virtual PCL resection shifted the tibia posteriorly (up to 3 mm) and increased abduction (up to 0.5°). Different results were seen across all load cases. The exceptions are the squat and sit-to-stand load cases with similar patterns. Thus, cruciate-retaining TKR can be tested using telemetric data from ultra-congruent TKR, aiding in comprehensive evaluations.

Keywords: Activities of daily living; Biomechanical testing; Ligament apparatus; Six-degree-of-freedom joint simulator; Total knee replacement.

Fig. 1

Tibiofemoral kinematics and kinetics during different load cases. Kinematics (a-f) and kinetics (g-l) for level walking (yellow), downhill walking (blue), stairs down (red), squat (purple) and sit-to-stand (orange) with reference apparatus. (a) Lateral-medial L-M contact force, (b) anterior-posterior A-P contact force, (c) inferior-superior I-S contact force, (d) flexion-extension F-E contact moment, (e) abduction-adduction Ab-Ad contact moment, (f) external-internal rotation E-I contact moment, (g) lateral-medial L-M translation, (h) anterior-posterior A-P translation, (i) inferior-superior I-S translation, (j) flexion-extension F-E angle, (k) abduction-adduction Ab-Ad angle, (l) external-internal rotation E-I angle.

Fig. 2

Tibiofemoral contact areas during all five load cases. (a) digitally marked contact area for all load cases with reference apparatus (level walking – yellow, downhill walking – blue, stairs down – red, squat – purple and sit-to-stand – orange) on a clean surface insert, (b) contact area for downhill walking on an insert with scanning spray used to mark the untouched regions of the insert. The images were taken and processed with the software of the digital microscope VHX-6000 (V 2.8.0.110, Keyence Deutschland GmbH, Neu-Isenburg, Germany) and the contact areas were manually highlighted using Paint.NET (V5.0.1, Washington State University, Pullman, WA, USA and Microsoft Corporation, Redmond, WA, USA, https://www.getpaint.net/).

Fig. 3

Influence of the ligament apparatus. Inferior-superior forces, anterior-posterior forces, anterior-posterior translation, abduction-adduction angle with respect to the flexion (straight line) and extension (dashed line) angle under consideration of the four different ligament apparatus variations: reference (blue), resected PCL (red), stiffer medial structures (yellow) and stiffer lateral structures (green).

Fig. 4

Adjusting the coordinate system. (a) Tibial insert of the INNEX TKR (light red) with INNEX coordinate system (CS) (orange) and exemplarily visualised resultant force (dark red) as supplied by the CAMS dataset; (b) Tibial insert of the INNEX knee tilted by 6.5° tibial slope with respect to the INNEX CS (orange), tilted INNEX CS (green) and tilted resultant force (dark blue); (c) Tibial insert of the P.F.C. Sigma (light blue) with tilted INNEX CS (green) and tilted resultant force (dark blue). The images were generated with the multibody software Simpack (V 2022x, Dassault Systèmes, Vélizy-Villacoublay, France, available from: https://www.3ds.com/) and edited with Paint.NET (V5.0.1, Washington State University, Pullman, WA, USA and Microsoft Corporation, Redmond, WA, USA, https://www.getpaint.net/).

Fig. 5

Preparation of force data for integration into the joint simulator. Anterior (A)-posterior (P) force shown for the INNEX coordinate system, the P.F.C. Sigma coordinate system and the Grood and Suntay convention. The periodised data in the Grood and Suntay convention were transferred to the VIVO™ joint simulator.

Fig. 6

Illustration of the virtual ligament apparatus used in the experimental test setup with an exemplary force-strain curve of the ligament model (a). Virtual ligaments considered during experimental tests for the total knee replacement viewed from medial (b), posterior (c) and lateral (d) can be grouped in capsular structures (yellow), medial structures (blue), lateral structures (green) and the posterior cruciate ligament (red). Note that the bones are not depicted. The diagram was generated using MATLAB software (V R2022b, MathWorks Inc., MA, USA, https://de.mathworks.com/products/new_products/release2022b.html), and the images (b-d) of the test setup were rendered using the open-source software Blender (V 4.0, Blender Foundation, Amsterdam, Netherlands, https://download.blender.org/release/).