Development of a Three-Dimensional Musculoskeletal Model for the Hardware-in-the-Loop Joint Simulation

Dislocation of artificial joints causes serious complications after total hip arthroplasty. There are various factors influencing the dislocation process related to implant component parameters as well as physiological conditions. Because in vivo measurements of the dislocation process are not possible, a novel, mechatronic hardware-in-the-loop joint simulator was recently introduced connecting a physical test setup with a computer simulation running in real-time. The purpose of this work is the development of the required multibody model providing the musculoskeletal structure of the lower limb. The objective of this model is the prediction of hip joint reaction forces tested for a standing-up motion. The model consists of segments of the lower right limb. Additionally, the upper body is represented so that the influence of the relocation of the upper body mass is taken into account. Skeletal structures were obtained by extracting geometric information of a male computed tomography dataset. The specific composition of the segments was realized by the use of a consistent, anatomical dataset. The resulting multibody model is scalable via body height and mass. As ligament structures, only the patella tendon was considered, modeled as a nonlinear spring. Muscles were incorporated by means of the inverse dynamic approach resulting in a distribution problem of muscle forces. This was resolved by the simplex algorithm using the sum of all muscle forces as objective function to be minimized. As simplification, the segments were oriented parallel to the sagittal plane whereas the joints allowed relative flexion and extension movements only. By comparison to experimental results derived from a patient with total hip replacement, the model was proved regarding the prediction of reaction forces in the hip joint. As a result, it showed a realistic display of a standing-up motion. The resultant of the hip reaction force corresponded with the experimental results.