An Anatomically-accurate Finite Element Knee Model Accounting for Fluid Pressure in Articular Cartilage

Articular cartilage consists of a proteoglycan matrix, collagen fiber network and fluid. The interplay of these three constituents determines the mechanical behaviour of articular cartilage and greatly influences the mechanical functions of diarthrodial joints, such as human knees. Finite element analysis has been extensively used in the research of healthy, arthritic and injured knees. However, articular cartilage was considered to be elastic in previously published human knee models. The viscoelastic response of the knee, which is highly time-dependent because of the fluid flow in the cartilage, has not been explored using any finite element analysis and realistic knee geometry.;In addition to predicting the fluid pressure and velocity, the displacement and contact pressure determined by the proposed model were evidently different than those from an elastic model. The site-dependent fiber orientation and fiber reinforcement in the cartilage and menisci also clearly changed the displacement and fluid pressure in these tissues. The peak fluid velocity in the femoral cartilage was increased by 3.4 times by fibril reinforcement. Although the location of the peak contact pressure was not greatly influenced by fibril reinforcement, the peak fluid pressure in the femoral condyles was reduced by three quarters when no fibril reinforcement was assumed. The zonal differences in articular cartilage produced a great fluid pressure gradient in the superficial zone (surface layer) and an essentially constant pressure in the middle and deep zones. The peak fluid pressure in cartilage increased because of this nonuniform distribution in the tissue thickness direction.;The present studies indicate the necessity of implementing both the fluid flow and anisotropic fibril reinforcement in articular cartilage in knee joint modelling. When the fluid pressure is not modeled, the predicted influence of fibril reinforcement and site-specific collagen orientation is less significant; when the fibril reinforcement is not modeled, the predicted fluid pressure is very low. The results also indicate the necessity of implementing the zonal differences in articular cartilage if more detailed information is desired. The proposed models can be used to address bioengineering questions after further development and validation. Some present results may have implications in cartilage degeneration and injury.;Keywords: anatomically-accurate knee model; articular cartilage; meniscus; fibril-reinforcement; collagen fiber orientation; contact mechanics; finite element analysis; fluid flow; fluid pressure; knee joint mechanics; material anisotropy and inhomogeneity; nonlinear mechanics;In the present thesis studies, three dimensional anatomically-accurate finite element knee models were developed using magnetic resonance imaging of a healthy adult male. The fluid pressure and site-specific collagen fiber orientation in the cartilages and menisci were implemented into the proposed model of the knee. The inhomogeneity of femoral cartilage in the tissue thickness direction, which is associated with the zonal differences, was further considered in the refined model. The surface contact approaches in ABAQUS were used to simulate the mechanical contacts between different tissues in the knee. Relaxation loading protocols were used to gain insights into the transient mechanical behaviour of the knee joint. Comparisons were made to determine the roles of different tissue properties, e.g. the results were obtained for the cartilage with and without fluid respectively, and thus the influence of fluid pressure on the contact mechanics of the knee could be demonstrated.