Kontinuumsmechanische Modellierung von Gelenkknorpel unter physio-dynamischer Kontaktbeanspruchung
Final Report Abstract
The first goal of this research project was to develop a sophisticated model based on the TPM for the purpose of a precise modelling of articular cartilage. The main features of articular cartilage were the low permeability, the high anisotropy due to the presence of collagen fibres and the osmotic properties. The inclusion of these specific features in the modelling required the formulation of a highly complex, computational FE model. In order to make such a model usable, calibration techniques were elaborated and applied to a set of indentation tests performed by the project partners PD Dr.-Ing. Hurschler and Dr. med Abedian. Even though a unique identification of the parameters could not be achieved, a satisfying model calibration was performed under the given restrictions. The second goal of the project addressed applications of the calibrated model to realistic hip-joint mechanisms. Therefore, a 3-d, patient-specific hip-joint structure was recreated from MRI scans executed and delivered by the project partners Prof. Schick and Eibofner. Based on this, hip-joint mechanisms were approached in three different ways. A contact-based approach treated the articulating cartilage layers and the synovial fluid trapped in the hip-joint capsule by means of a contact algorithm and specific conditions at the synovial fluid-cartilage interface. This procedure was mostly considered by the project partners Prof. Nackenhorst and Fietz. A second approach resided in the continuum-mechanical modelling of each hip-joint constituent. In this case, numerical convergence could not be achieved due to the rapid distortion of the fluid elements under shear loading. For the last alternative, a reduced hip-joint geometry, comprising the femoral cartilage and the underlying femur, was used in combination with contact loads provided by the associated research partners PD Dr.-Ing. Hurschler and Schwarze. As walking represents a daily-life loading situation, physiological and pathological walking processes were investigated. The investigations were further refined under the consideration of healthy and OA cartilage states, since OA is the most common form of cartilage degeneration affecting millions of people worldwide. The influence of OA and pathological loading on the stress distribution at the cartilage surface and the pore-fluid pressure at the cartilage-bone interface was examined and compared with results given in the literature. Due to cartilage degeneration, the loads were distributed on a smaller contact surface and, therefore, higher contact stresses in OA cartilage than those in healthy cartilage were detected for normal walking loads. Furthermore, pathological walking loads reduced the occurrence of high stresses in OA cartilage. This is most likely due to the modified loading pattern of the participant to reduce local pains in the hip joint. Following this, the influence of the mechanical environment given by the loads resulting from the walking processes on the cartilage cells was investigated. The stresses at different cell positions in the cartilage layer were represented and confronted with the presented model and a model extended with the inclusion of cells. It appeared that a distinction between these two models is not meaningful due to the low volume fraction of the cells within the ECM. After the research project had been concluded successfully in accordance with the work schedule in the proposal and the progress report, the future step might include novel applications of the MBS related to FE models to achieve realistic boundary conditions for the computational model. Besides, the full set of material parameters is still not fully obtained due to the lack of experimental data, which would be required for a final evaluation of the obtained parameters. Relevant cartilage tests should be preconceived and designed only after a controlling process given by the presented model calibration scheme.
Publications
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Continuum-Mechanical Modelling of Hip Cartilage under Physio-Dynamical Loading. Proceedings in Applied Mathematics and Mechanics, 10 (2010), 693–694
Mabuma J., Markert B. & Ehlers W.
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Towards a Method for Parameter Estimation of Articular Cartilage and a Staggered Procedure for Synovial Fluid-Cartilage. Proceedings in Applied Mathematics and Mechanics, 12 (2012), 129–130
Mabuma J., Markert B. & Ehlers W.
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Towards a Standardised Method for Visualisation of Stress Distribution at the Cartilage-Bone Interface. Proceedings in Applied Mathematics and Mechanics, 13 (2013), 69–70
Mabuma J., Markert B. & Ehlers W.
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Multi-Field Modelling and Simulation of the Human Hip Joint. Dissertation thesis, Institute of Applied Mechanics (CE), University of Stuttgart, 2014
Mabuma J.
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Effects of Osteoarthritis and Pathological Walking on Contact Stresses in Femoral Cartilage. Biomechanics and Modeling in Mechanobiology, 14 (2015), 1167–1180
Mabuma J., Schwarze M., Markert B., Hurschler C. & Ehlers W.
