Simulation of the Knee Joint Motion by Stewart Platform

The knee is one of the most commonly studied human body joints in the field of biomechanics. Biomechanical knee joint studies aim to understand joint mechanics by utilizing kinematics and dynamics. Understanding the mechanics of intact joints provides insight into the mechanics of injured, deteriorated and reconstructed joints and help to improve current technologies in the field of orthopedics. The aim of this project is to model a Steward Platform (SP) with six degrees of freedom (6DOF) based on the kinematics of anatomic knee joint. The constructed inverse kinematics equations for the SP can then predict the anatomic knee joint kinematics and major knee joint ligaments length changing. The model of the SP was used to perform the knee joint kinematics motion within a certain range of movement between 0° to 30° flexion. This application leads to investigate the similarity between the changes in the platform actuator leg lengths and the knee joint ligament lengths. The initial lengths of the platform actuator legs were adjusted to 170 mm at 0° joint flexion. Then the platform angle changes were applied to extend it up to 30° flexion angle through taking into account the center of mass (COM) of the SP. The COM of the platform was assumed as the COM of the tibia bone of the knee joint and based on the kinematic movement of the platform the lengths of actuator legs were analyzed. From the constructed inverse kinematics equations, the SP mimicked the anatomic knee joint kinematics and the platform actuator legs predicted the anatomic knee joint ligament length changes. It was found that the lengths of the platform actuator legs varied with knee joint flexion angles during the platform motion. Also it was seen that between 0° to 30° flexion, the platform performed valgus rotation dominantly and the actuator leg lengths decreased which represented the Anterior Cruciate Ligament (ACL), Medial Collateral Ligament (MCL), and Lateral Collateral Ligament (LCL). The average changes of the platform actuator legs were found as 0.119% for actuator leg 1, 0.035% for actuator leg 2, 0.1285% for actuator leg3, 0.1285% for actuator leg4, 0.035% for actuator leg5 and 0.119% for actuator leg6. The current findings were compared with the literature data and the kinematics of the SP and the changes in the platform actuator legs were validated. The total time of the analysis and the simulation took two hours. Using modelling based study such as the SP model can provide an insight into the biomechanics and the orthopaedics field to reveal the knee joint kinematics and ligament length changes without using cadavers or invasive experiments. Keywords: Stewart platform, Kinematics, Knee, Biomechanics.