X-ray diffraction has been used to measure the preferred orientation of the collagen fibrils, and their angular distribution within the tissue, as a function of depth from the articular surface in patellar cartilage. Measurements have been made at four different sites chosen to represent differing surface curvatures and régimes of wear. The orientation of fibrils in the surface layer allows it to oppose the swelling pressure exerted by the gel of hydrated glycosaminoglycans within the cartilage. An intermediate layer (where a bimodal distribution of fibrils is sometimes resolved) allows the orientation of the fibrils to change, with increasing depth, until they are roughly perpendicular to the articular surface. In this deep layer the fibrils can tie into the underlying calcified tissue so as to firmly anchor the cartilage. In the plane of the surface the fibrils tend to be aligned in the direction of stress caused by motion.
A swelling pressure cell for X-ray diffraction test
