Impact loading of articular cartilage
Impact loads have been implicated in the initiation of secondary osteoarthritis but in the absence of defined injury this is difficult to rest rigorously. The response to controlled impacts of samples of cartilage and bone in isolation and together, may yield valuable insights into how tissue properties may influence degenerative changes associated with osteoarthritis. A rigid instrumented drop tower was constructed and interfaced to a LabVIEW software oscilloscope modified to capture and store data to disk. Controlled impact loads were applied to cores of articular cartilage, both isolated and in situ on the underlying bone or bonded to substrates of different material properties. Bovine tissue from the carpometacarpal joint and human cartilage from elderly femoral heads was used. The response of the samples was investigated in terms of a dynamic stiffness, energy absorbed and coefficient of restitution. In addition the quasistatic modulus was measured from compression tests in order to compare the values for the stiffness of cartilage and bone at different rates of stress and strain. Composition analysis was then performed on human cartilage samples to investigate if there was any correlation between the biochemical constituents and mechanical factors. The dynamic stiffness of the cartilage samples was governed by peak stress and did not show a high sensitivity to strain rate. Cartilage had good force attenuating properties in situ on bone and the substrates. The greater volume of the stiffer underlying substrate dominated the response of the composite samples. For the human cartilage samples the dynamic stiffness was most correlated to percentage collagen whereas the quasistatic modulus was most correlated with water content. Overall the biochemical composition was a poor predictor of stiffness which indicates the importance of interactions between the matrix constituents in the tissue response to an applied load.