Abstract
Introduction
Despite decades of clinical research in artificial joints and underlying failure mechanisms, systematical and reproducible identification of reasons for complications in total knee replacements (TKR) remains difficult. Due to the complex dynamic interaction of implant system and biological situs, malfunction eventually leading to failure is multifactorial and remains not fully understood. The aim of present study was to evaluate different TKR designs and positions with regard to joint kinematics and stability under dynamic conditions by using a robot-based hardware-in-the-loop (HiL) setup.
Material & methods
An industrial 6-axis robot with 6-axis force-torque sensor mounted into its end-effector moved and loaded real, commercially available TKR (bicondylar, cruciate-retaining) that were in virtual interaction with a subject-specific computational multibody model representing the anatomical situs of the knee joint while performing passive seated deep knee flexion. The subject-specific musculoskeletal multibody model (MMB) included rigid bones of the lower right extremity. Bone and cartilage geometries were reconstructed from MRT/ CT data sets preserving anatomical landmarks and allowing for the calculation of inertial properties. M. quadriceps femoris was modeled as single passive tensile force elements. Knee ligaments were modelled as elastic spring elements with a nonlinear force-displacement characteristic. Providing the flexion angle, the robot moved and loaded the mounted femoral implant component with respect to the tibial component while being in continuous interaction with the MMB. Several influencing parameters like implant position (internal/external rotation, varus/valgus alignment) and design (fixed vs. mobile bearing, tibia-insert height) as well as ligament insufficiency and joint loading on joint kinematics and stability was systematically analysed.
Results
Improper implant positioning caused joint instability, which was demonstrated in higher magnitudes of the relative kinematics. Negative effects by incorrect implant positioning could be partially compensated by a mobile bearing design. However, this was accompanied with an increase in tibiofemoral contact forces. High correlation of tibia-insert height on ligament and contact force was found. After releasing ligament structures, lower tibiofemoral contact forces and joint opening during deep knee flexion were observed.
Conclusion
By means of HiL simulation different clinical and technical parameters of TKR were evaluated in a systematical and reproducible fashion under physiological-like boundary conditions with regard to joint kinematics and stability. The proposed HiL test setup combining robot-based testing with MMBs can contribute to deeper understanding of knee joint function and improvement of total knee implant systems.
Acknowledgement
The authors would like to thank the Deutsche Forschungsgemeinschaft (grant numbers: WO WO 452/8-1, BA 3347/3-1 and KL 2327/4-1) for supporting the project.
