Development of a subject specific 3D knee finite element model to estimate the effect of weight loss on cartilage biomechanics

Sammanfattning: Osteoarthritis (OA) is a common musculoskeletal disorder that degrades articular cartilage and is a leading cause of disability worldwide. Overweight has been considered a major risk factor of knee OA, and it is known that weight loss may reduce the risk of knee OA. The biomechanical mechanisms of how weight loss affects knee cartilage are however unknown. Evaluating the biomechanics of cartilage in-vivo is difficult, but an option is provided by numerical modeling of the knee joint using finite element (FE) modeling. FE modeling has proved to be effective in simulating knee joint kinematics and is therefore in this thesis proposed to estimate the effect of weight loss on cartilage biomechanics. The first and main objective of this thesis was to estimate the effect of weight loss on knee cartilage biomechanics using a subject-specific FE knee joint model. The second objective was to investigate if modeling bone as a deformable, isotropic, and heterogeneous material would affect the cartilage biomechanics. The third objective was to investigate if the addition of a subchondral bone cyst (SBC), a common symptom of knee OA, in the tibia of the FE model would affect the biomechanics of cartilage and bone. To achieve these objectives, a 3D model of a knee was manually segmented from magnetic resonance imaging (MRI) data of a participant in a weight loss focused clinical trial. The knee segmentation was meshed into a set of four different FE models and simulated using subject-specific motion analysis data for three sets of weight during the stance phase of gait. The results of the FE analysis showed that as the subjects weight decreased from 85 kg to 74 kg, contact pressure, Von Mises stress, and maximum principal strain at the surface of the tibial cartilage decreased by 6.2 \%, 6.9 \%, and 6.5 \% respectively at peak load during the stance phase of gait. Modeling the tibial bone as heterogeneous and deformable led to a 10.7 \% and 14.8 \% reduction of Von Mises stress and maximum principal strain in the tibial cartilage when compared to rigid bone. The addition of a SBC led to a marginal decrease in contact pressure, Von Mises stress, and maximum principal strain in the tibial cartilage, but an increase in minimum principal stress and strain in the tibial bone. In conclusion, this thesis has shown that weight loss simulated by FE analysis leads to a quantifiable reduction of the biomechanical load on knee tissues. Modeling bone as rigid also proved to be an effective simplification to reduce computational time while maintaining accuracy in the cartilage mechanics. Further refinement of the models investigated in this thesis, for example by the addition of ligaments or more complex material models, may in the future provide an effective means of predicting cartilage response to weight loss. This could result in a clinically viable computational method of suggesting the best possible preventative treatment of OA.