Dynamic Finite Element Modeling of Knee Mechanics

Purpose: It is widely accepted that alterations in the mechanical environment within the knee is a leading cause of knee osteoarthritis (OA). However, a significant unfulfilled technological challenge in musculoskeletal biomechanics and OA research has been determining the dynamic mechanical environment of the cartilage (and other components) resulting from routine and non-routine physical movements. There are two methods of investigating musculoskeletal joint mechanics that have been used to date: 1) forward and inverse multibody dynamic simulations of human movement and 2) detailed quasi-static finite element modeling of individual joints. While the overwhelming majority of work has been focused on biomechanical multibody dynamics modeling, these types of simulations do not allow for the detailed continuum level analysis of the mechanical environment of the cartilage and other knee joint structures (meniscus, ligaments, and underlying bone) within the knee during physical activities. This is a critical gap that is required to understand the relationship between functional or injurious loading of the knee and cartilage degradation. The objective of this investigation was to develop a detailed neuromuscularly activated dynamic finite element model of the human lower body in order to simultaneously determine the dynamic muscle forces, joint kinematics, contact forces, and detailed (e.g., continuum) stresses and strains within the knee during controlled and muscle actuated movements. Methods: A finite element model of the lower body was developed using individual digitized anatomic surface models. Contact conditions were defined at the knee joint between the femoral cartilage, tibial cartilage, menisci, and patella cartilage to constrain articular joint motion based on anatomic geometry. All the ligaments of the knee were modeled as non-linear one-dimensional springs; the femoral cartilage, tibial cartilage, menisci were modeled as linear elastic materials using published material properties. Fifty-nine muscles of the lower limbs were modeled using active contraction Hill-type muscle springs using published insertion points and muscle parameters. The model was configured to simulate a seated leg extension by activating the quadriceps. We used a parameter optimization method to directly optimize muscle activation parameters within the dynamic FE model (LS-DYNA (v. 971, LSTC, Livermore, CA) based on a physiologic cost function. Once optimal muscle activations were determined, we investigated the changes in the mechanical environment within the knee due to removal of the meniscus. Results: The final optimal muscle activations required to perform a leg extension resulted in a smooth, controlled dynamic movement. The dynamic stresses in the femoral cartilage during the leg extension were significantly higher (with the peak stress also occurring significantly earlier in the motion) in the knee without a meniscus compared to the intact knee (Figure 1). Conclusions: The dynamic muscle forces, joint kinematics, contact forces, and detailed (e.g., continuum) stresses and strains within the knee (cartilage, meniscus, ligaments, and bone) were simultaneously determined for a neuromuscularly controlled seated leg extension with a weight of 30 lbs. added to the ankle. This methodology was used to investigate how structural and material alterations in this complex environment due to removal of the meniscus altered the mechanical environment within the knee. Removal of the meniscus significantly affected not only the peak cartilage stresses, but significantly altered the time course of those stresses during a dynamic knee extension. Future studies will focus on the how effects such as anatomical differences, obesity, and sarcopenia (each of which are risk factors for knee OA) potentially affect the detailed dynamic mechanics of each component of the knee joint.