Alterations in Knee Joint Muscle Activation and Tissue Stresses-Strains During Gait Associated With Knee Osteoarthritis
Ecole Polytechnique, Montreal, QC, Canada
Introduction: Osteoarthritis (OA) is the leading cause of pain and disability in the elderly worldwide with the knee being the most affected weight bearing joint. Though its pathomechanics is not yet well understood, mechanical factors are recognized to play an important role in the initiation and progression of knee OA. Earlier studies have identified marked differences in gait kinematics-kinetics between asymptomatic and OA subjects [2]. An accurate knowledge of activation levels in joint musculature as well as resulting contact stresses and stresses/strains within articular cartilage layers of both subject groups is crucial. Such data help identify the changes in the knee joint mechanical environment with the development of OA and are hence of great value in effective prevention and treatment management of knee OA. In the current work, we use a validated finite element (FE) model [1] to compute lower extremity muscle forces and knee joint stresses/strains in subjects with and without OA during the stance phase of gait. These FE analyses are driven by reported kinematics and kinetics data collected during gait of asymptomatic subjects as well as subjects with severe OA [2]. In the model simulating kinematics-kinetics data reported during gait of OA subjects, the material properties of the articular cartilage are also either left unchanged as in the model of the asymptomatic group (intact) or altered (i.e., reduced matrix and fibril moduli) to represent the disease.
Methods: An iterative kinematics-driven FE model that accounts for the passive structures of the knee joint and active musculature of the lower extremity is employed [1] (Fig. 1). This model incorporates the hip as 3D and the ankle as 1D spherical joints [4] whereas the knee is represented as a complex FE model with nonlinear depth-dependent fibril-reinforced cartilage and menisci, ligaments with distinct nonlinear properties and initial strains, patellofemoral and tibiofemoral joints. Based on reported in vivo measurements [2, 3], hip/knee/ankle joint rotations/moments and ground reaction forces (GRF) at foot during the gait stance phase collected in asymptomatic subjects and subjects with severe knee OA are used to separately drive both groups. Analyses are performed at 6 time instances corresponding to beginning 0% (heel strike), 5%, 25%, 50%, 75% and 100% (toe off) of the stance phase. At each stance period, muscle forces at the hip, knee and ankle joints are predicted using static optimization (sum of cubed muscle stresses) with moment equations as constraints (3 at the knee, 3 at the hip and 1 at the ankle). The Knee joint response is subsequently analyzed with updated muscle forces as external loads and iterations at deformed configurations continue till convergence is reached. Apart from changes in input kinematics/kinetics, the OA model accounts also for likely alterations in material properties associated with the disease. Additional analyses are thus performed at 5% and 50% stance periods with the cartilage fibril and matrix moduli reduced both by 25% from their intact values while the cartilage Poisson’s ratio is either left unchanged or also reduced.
Results: In OA subjects compared to normal ones (N in Fig. 2), forces significantly decreased in all muscle groups at most instances of stance (Fig. 2). Mean force over the stance phase in the anterior cruciate ligament remained nearly the same. Total contact forces/stresses deceased by an average of 25% with a larger portion of load transmitted via menisci. Alterations in cartilage material properties simulating OA had negligible effects on muscle forces but reduced contact pressures (Fig. 3, 50% of stance phase) while increasing cartilage strains and load transmission via menisci.
Discussion: This study quantified muscle forces and detailed knee joint response during the stance phase of gait in both asymptomatic and OA subjects based on distinct kinematics-kinetics data in each group [2]. No co-activity was considered in either normal or OA models. In accordance with the alterations in the joint moments during gait, muscle forces at the knee joint decreased overall in OA subjects leading to lower contact forces and mean/peak contact pressures. The portion of the tibiofemoral contact load transmitted via menisci (as compared to that at the cartilage-cartilage interface) increased in OA group. In addition to altered gait input data, cartilage fibril and matrix moduli were reduced to represent the expected deterioration in material properties associated with OA. These changes had smaller effects on muscle, ligament and contact forces but increased contact areas, maximum tensile strains in superficial and deep cartilage layers, and the load transfer via menisci. The peak contact pressures however decreased.
Significance: OA-associated alterations in rotations and moments at lower extremity joints recorded during gait influence activation levels in lower extremity musculature as well as contact forces/stresses and stresses/strains in knee articular cartilage. Reductions in mean and peak contact stresses, increases in the contact areas and tissue strains and transfer of larger tibiofemoral contact loads via menisci are due both to the altered kinetics-kinematics of gait and to the deteriorations in cartilage material properties.
Acknowledgements: Work has been supported by the NSERC-Canada and MUTAN-Tunisia.
References: [1] Adouni et al, JB 45:2149, 2012. [2] Astephen et al, JOR 26:332, 2008. [3] Hunt et al, CB 16:592, 2001. [4] Delp et al, IEEE BE 54 :1940, 2007.
