Mechanical characterisation of acetabular soft tissues: experimental and computational study

Abnormal hip contact mechanisms can be associated with acetabular soft tissue damage and the progression of osteoarthritis. One morphological cause of this abnormal mechanical environment is a cam-shaped femoral head that results in impingement with the acetabular rim and labrum during hip motions. In this thesis, cam-type femoroacetabular impingement (FAI) related loading was mimicked on the acetabular cartilage-labral junction in vitro and in silico. During loading, computed tomography scans were obtained whereby radiopaque solution was used in order to separate acetabular soft tissues in the hip during contact. Measurements of overall cartilage strain were taken at the centre of the contact region and the labral apex displacement was established in three-dimensional space. The circumferential properties of the labrum were also assessed by re-loading the tissue sample following introducing a cut to the labrum. Two-dimensional finite element models of the femoral head and acetabulum were developed based on an image slice through the centre of the contact region. Geometrical features of the acetabulum and femur at the contact site were captured in the models. Computational results were compared with experimental results. A parametric study was conducted on the models for verification and for investigation of hip parameters regarding the soft tissue behaviour under load. Contact occurred at the anterior-superior region of the acetabulum in all samples, as would be expected if the conditions of cam-type FAI were replicated. The cartilage strain ranged from 20% to 60% and the labrum maximum displacement ranged from 1.5 to 5.0 mm, measured from CT scans in all samples. The circumferential effect in the labrum was demonstrated with an averaged factor of 1.4 of increase in the labrum apex displacement per applied force in labrum-cut cases. The cartilage strain and load distribution in soft tissues were found to be sensitive to the femoral head position in the computational models, with strain differences up to 41% and cartilage contact force differences up to 237%. The ratio between the cartilage and labrum Young’s modulus affected the tensile strain at the cartilage-labral junction by up to 14%. The position of cartilage-labral junction affected the total contact force on the soft tissue by up to 49%. This work measured the soft tissue behaviour under cam-type FAI loading via an experimental approach and characterised the soft tissue behaviour under various set-ups via computational approach. The importance of adapting reliable tissue alignment and three-dimensional modelling were highlighted. It can be concluded that, stiffer labrum compared to the cartilage, along with focused loading at the cartilage-labral junction, would cause high strain in the cartilage and concentrated tensile strain at the junction, suggesting the damage mechanism in hips with FAI.