Microstructural response and fluid flow mechanisms in cartilage loading: new insights using a novel indentation method

A novel indentation method was used to investigate the response of articular cartilage in the non-directly loaded region. The indenter contained a relief channel that allowed a tissue bulge to develop within it under load. Healthy bovine tissue samples were statically loaded at a nominal compressive stress of 3.6 MPa. The tissue's equilibrium deformed state was chemically fixed. Differential interference contrast microscopy was used to obtain high-resolution images of the deformed microstructure in the region of the tissue bulge. At the submicro-level, the fibrillar resistance to load is highly complex such that regions of relative compression and tension coincide along a single radial direction. This fibrillar level response is manifested as large-scale matrix shear effects within the bulge region. Further, the Surface layer, besides being strain-limiting in the tangential direction, has an intrinsic resistance to axial load. Finally, the pattern of load-induced fluid flow is seen to traverse zonal depths and hence Suggests an added complexity to the overall permeability in the deformed tissue matrix.