Effet de la force de compression sur la réponse passive de l'articulation du genou: Une étude numérique non-linéaire

We investigated the effect of compressive load on the stiffness and passive resistant moments of the knee joint. Passive response of the knee joint has been studied at four flexion positions; from 0° to 45° with an increment of 15°, under a compressive load of up to 1800 N. At the first step, the undeformed joint reference or resting configuration was initially established by considering the joint response under the prestrains in ligaments. The femur was then flexed about its medial-lateral axis to the desired joint flexion angle (0°-45°) and was fixed in all directions thereafter. However, the tibia was left without constraints throughout. To avoid the artifact moments, the preload compression forces were applied onto the tibia at a location iteratively identified not to generate flexion-extension and varus-valgus angulations. This novel MBP location varied with compression preload and the joint flexion angle. In fact, it moves medially with the compressive load and posteriorly when the femur flexed from 0° to 15°.

For each studied configurations (knee flexed at the desired angle and compressive load applied at the corresponding MBP), the passive moments of the tibio-femoral (TF) articulation are calculated on the primary node of the femur that is located between the epicondylar centers and taken as the joint center in the musculoskeletal models of human gait. For all flexion angles, the TF flexion moment increases with the compressive load and reaches the maximum of 12.68 Nm at 30° with 1800 N. In contrast, the TF valgus moment is greatest here at the full extension reaching 22 Nm at 0° with 1800 N and varies from about 5 Nm to 10 Nm in the other flexion angles.

The instantaneous TF rigidity has been studied in both sagittal and frontal planes, by applying linear perturbation at the final deformed configurations (knee flexed and compressive load applied at the corresponding MBP). The femur remains fixed with an infinitesimal rotation (0.001°) applied in both sagittal and frontal planes, on the tibia at its instantaneous MBP, while constraining the remaining degrees-of-freedom. For each studied configuration (load/angle), the corresponding tangent stiffness values were determined as the ratio of the required moments over the applied infinitesimal rotations. These values, in sagittal and frontal planes significantly increase with the compressive load for all flexion angles. However, the instantaneous rigidity in the frontal plane is larger than that in sagittal plane. For a compressive load of 1800 N, the instantaneous stiffness in the sagittal plane is between 1.36 and 3.09 Nm/deg, while that in the frontal plane is between 16.06 and 19.65 Nm/deg.

Finally at the full extension position under various compression preloads applied at their respective MBP, the varus/valgus response of the TF joint is studied under up to 20 Nm varus-valgus moments. The femur remains fixed and the tibia is left free to translate, with the internal-external rotation of the tibia fixed. A non-linear response is observed. In the unloaded cases, the lateral ligaments are the primary restrictions against the applied moments. However, the resistive contribution of these ligaments decreases significantly in presence of the compressive load. Additional case with the internal-external rotation of the tibia left free is also analyzed.

The results of the present work have implications in various aspects of the knee biomechanics. The predicted results are found in general agreement with reported measurements supporting thus the accuracy of the model. The novel MBP allowed for the application of the compressive load in the most ideal position (mechanical equilibrium position or joint rotation center) with no artifact moments. It also allowed for the evaluation of instantaneous tangent stiffness in frontal and sagittal planes. Our results stresses the need to consider the contribution of the compression-dependent passive moments in resisting external moments when attempting to estimate muscle forces and joint loads in musculoskeletal models of the lower extremity. (Abstract shortened by UMI.)