Tibio-femoral Joint Contact Mechanics: An In-vitro Simulation with a 6 DOF Static Knee Simulator

Abstract

Introduction: Understanding the relationship between muscle loads crossing the knee joint and knee joint mechanics is critical for understanding knee stability and the effects of altered muscle forces on healthy and ACL injured knees. In vitro measurement can be used to elucidate this if the simulation is biofidelic, allowing the physiological levels of applied loads to dictate the tibiofemoral kinematics in all degrees of freedom (DoF). The objectives of this study were to describe and apply the University of Ottawa knee simulator as well as measure the reliability of the device. In addition, this device was used to quantify the effect of muscle loads and anterior cruciate ligament (ACL) resection on contact mechanics and kinematics of the tibiofemoral joint. Methods: Muscle forces were determined from an electromyography-driven musculoskeletal model of a healthy male during gait. Six knee specimens were loaded into the simulator and subjected to 100%, 75% and 50% in vivo muscle forces applied through the 6 simulated muscles, in addition to a quadriceps weakness and a hamstring weakness condition. Tibiofemoral mechanics were measured with all 5 loading conditions before and after ACL transection. Results: With the ACL intact, very high reliability in contact area and pressures among loading conditions were observed as the intra-class correlation coefficients (ICC) ranged from 0.932 to 0.99. After ACL transection, reliability remained very high as ICCs ranged from 0.926 to 0.99. In all simulated conditions, muscle forces maintained the knee joint in a stable position resulting in minimal kinematic differences, but altered contact mechanics in both the ACL and non-ACL condition. Removal of the ACL significantly reduced both the medial and lateral contact areas in all loading conditions compared to the ACL intact condition. Conclusion: In summary, the UOKS has demonstrated high reliability within repeated measures. Additionally, small, normally undetectable alterations in joint kinematics resulted in significant alterations to contact mechanics, which can be linked to the degenerative process.