Finite element modelling of load transfer through articular cartilage
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AbstractArticular cartilage is a hydrated connective tissue lining apposing bones in synovial joints. Articular cartilage is essential for load transmission of joint forces and joint lubrication. Mechanical factors are implicated in joint diseases, such as osteoarthritis, therefore knowledge of mechanical function and properties is necessary in prevention and treatment of joint diseases. Indentation testing of intact cartilage is one method of evaluating the mechanical function and properties of articular cartilage. The first hypothesis is that indentation stiffness is affected by structural heterogeneity and mechanical anisotropy. Numerical finite element models revealed that indentation stiffness was insensitive to variations of anisotropy in the plane of the surface layer of articular cartilage. Cartilage was modelled as a poroelastic material, consisting of separate fluid and solid phases, to simulate viscoelastic mechanical behaviours due to expression of water from the matrix upon loading. A poroelastic finite element model successfully replicated mechanical behaviours of cartilage in confined compression and indentation. Also demonstrated was sensitivity of indentation stiffness to indentor roughness and permeability, cartilage layer geometry and the displacement rate of the indentor. A plane strain poroelastic finite element model of a typical whole joint tested the hypothesis that whole joints are affected by hydraulic boundary conditions which exist along the contact surface. This model simulated a displacement controlled relaxation test Results showed that hydraulically sealed boundaries at the contacting interface of apposing cartilages are a mechanically admissible condition for transmission of joint forces developed during stress relaxation. Observation of transient tensile stresses in cartilage revealed that collagen appears to function as tissue reinforcement. Vertical stress distribution in the cartilage layer was similar in shape and behaviour to that found in numerical simulations of idealized indentation tests. This suggests that indentation testing is a valid means to simulate cartilage compression behaviours in vivo. The full joint model was modified to model joint articulation. Tensile stresses, developed near the contacting interface, demonstrated that superficial collagen has a function in joint articulation. The methods used in this dissertation show promise in further investigations of mechanical behaviours of joints and mechanisms of joint disease and repair.
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