Collagen Structure Orientation at the Bone–Cartilage Interface

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Abstract The articular cartilage overlying subchondral bone consists of variously oriented collagen fibres, the detailed organisation of which is essential for cartilage integrity that has suffered damage in degenerative joint disease such as osteoarthritis (OA). A technique attracting particular interest is that of coherent small-angle X-ray scattering mapping; the present investigations are supported by PILATUS, a highly pixelated two-dimensional detector system developed at the Paul Scherrer Institute (PSI). The system has yielded information on the anatomical features that correspond to the large-scale organisation of collagen and the mineralised phase contained within the collagen fibres in the deep cartilage zone. Results obtained from a decalcified cartilage–bone sample have been plotted in terms of the orientation of cartilage and bone components for particular intervals of k-space, showing the organisation of collagen-II within superficial layers and collagen-X in the chondrocyte-rich deeper layers of cartilage. It is apparent in undamaged cartilage that there is a gradual reorientation of the collagen-II fibres of the cartilage, from parallel to the surface of the joint, to normal to the cartilage–bone interface. A similar pattern of orientation is seen below the cement line (the surface at which cartilage anchors to the bone), for the collagen type-I prevalent in bone. Changing the interval of k-space allows subchondral features such as the microvasculature to be seen.