Osteoarthritis (OA) is a degenerative disease of articular cartilage that may lead to pain, limited mobility and joint deformation. It has been reported that abnormal stresses and irregular stress distribution may lead to the initiation and progression of OA. Body weight and the frontal plane tibiofemoral angle are two biomechanical factors which could lead to abnormal stresses and irregular stress distribution at the knee. The tibiofemoral angle is defined as the angle made by the intersection of the mechanical axis of the tibia with the mechanical axis of the femur in the frontal plane. In this study, reflective markers were placed on the subjects’ lower extremity bony landmarks and tracked using motion analysis. Motion analysis data and force platform data were collected together during single-leg stance, double-leg stance and walking gait from three healthy subjects with no history of osteoarthritis (OA), one with normal tibiofemoral angle (7.67°), one with varus (bow-legged) angle (0.20°) and one with valgus (knocked-knee) angle (10.34°). The resultant moment and forces in the knee were derived from the data of the motion analysis and force platform experiments using inverse dynamics. The results showed that Subject 1 (0.20° valgus) had a varus moment of 0.38 N-m/kg, during single-leg stance, a varus moment of 0.036 N-m/kg during static double-leg stance and a maximum varus moment of 0.49 N-m/kg during the stance phase of the gait cycle. Subject 2 (7.67° valgus tibiofemoral angle) had a varus moment of 0.31 N-m/kg, during single-leg stance, a valgus moment of 0.046 N-m/kg during static double-leg stance and a maximum varus moment of 0.37 N-m/kg during the stance phase of the gait cycle. Subject 3 (10.34° valgus tibiofemoral angle) had a varus moment of 0.30 N-m/kg, during single-leg stance, a valgus moment of 0.040 N-m/kg during static double-leg stance and a maximum varus moment of 0.34 N-m/kg during the stance phase of the gait cycle. In general, the results show that the varus moment at the knee joint increased with varus knee alignment in static single-leg stance and gait. The results of the motion analysis were used to obtain the knee joint contact stress by finite element analysis (FEA). Three-dimensional (3-D) knee models were constructed with sagittal view MRI of the knee. The knee model included the bony geometry of the knee, the femoral and tibial articular cartilage, the lateral and medial menisci and the cruciate and the collateral ligaments. In initial FEA simulations, bones were modeled as rigid, articular cartilage was modeled as isotropic elastic, menisci were modeled as transversely isotopic elastic, and the ligaments were modeled as 1-D nonlinear springs. The material properties of the different knee components were taken from previously published literature of validated FEA models. The results showed that applying the axial load and varus moment determined from the motion analysis to the FEA model Subject 1 had a Von Mises stress of 1.71 MPa at the tibial cartilage while Subjects 2 and 3 both had Von Mises stresses of approximately 1.191 MPa. The results show that individuals with varus alignment at the knee will be exposed to greater stress at the medial compartment of the articular cartilage of the tibia due to the increased varus moment that occurs during single leg support.
Design and analysis of a lower extremity exoskeleton with the humanoid knee joint