Abstract:
This thesis presents an investigation of the material response of articular cartilage on subchondral bone to multiple impacts via indentation, focussing on the effect of tissue degeneration and the validity of using a linear Kelvin-Voigt viscoelastic model to describe the material response. Analysis was performed at the macrostructural level with reference to microstructure. Indentation was chosen as the mechanical methodology to simulate non-destructive joint trauma, which can lead to degenerative diseases such as osteoarthritis. Bovine patellar cartilage was used as a model for human cartilage and was classified as either healthy or mildly degenerate depending on the surface features and material response to hydration. Multiple impacts from a slightly rounded indenter were then delivered to the cartilage surface, after which the damping and stiffness characteristics of the material were calculated and analysed.
The validity of the approach used in the analysis was tested by applying indentation experiments to standard materials, aluminium and polyethylene. The approach was determined to measure material response rather than material properties of the standard materials. The investigation into articular cartilage was therefore focused on investigating the material response instead of directly determining material properties.
The calculated peak stress and stiffness have been found to suggest a significant decrease in the cartilage material resistance to impact when mild degeneration has occurred; however, the calculated damping characteristics were not significantly different between healthy and mildly degenerate cartilage. These results are consistent with the findings of previous studies, which have suggested that tissue degeneration reduces the peak stress of cartilage on subchondral bone.
The methodology to calculate the stiffness and damping was based upon linear viscoelastic modelling, which was found to be a poor representation of cartilage material response to impact. Therefore, the author believes this thesis shows that nonlinear modelling must be applied to future studies, and that cartilage material properties both in isolation and with attached subchondral bone should be measured directly rather than secondarily.