Matrix Vol. 911989, pp. 486-488 Stress in Collagen Fibrils of Articular Cartilage Calculated from Their Measured Orientations R. M. ASPDEN 1 and D. W. L. HUKINS 2 1 Department of Medical and Physiological Chemistry, University of Lund, P. O. Box 94, S-221 00 Lund, Sweden and 2 Department of Medical Biophysics, University of Manchester, Stopford Building, Oxford Road, Manches- ter M13 9PT, England. Abstract Articular cartilage may be considered as a form of pressure vessel in which the internal swelling pressure is balanced by tensile stress in the collagen fibrils. This stress is calculated by analysing the tissue as a series of microscopically small pressure vessels. The previously measured orienta- tions of the collagen fibrils describe the structure necessary for this calculation. The stresses and strains developed in the fibrils are shown to be well within physiological limits. Key words: articular cartilage, cartilage mechanics, collagen organisation. Introduction Articular cartilage is comprised of collagen fibrils, which are strong in tension, surrounded by a highly hydrated gel of proteoglycans. Unloaded cartilage has an internal swel- ling pressure of about 0.3 MPa (Maroudas, 1976) and this swelling pressure is resisted by the collagen fibrils which are thereby put into tension. The orientation of the collagen fibrils are therefore important if they are to be able to reinforce the gel. The organisation of collagen fibrils has been measured using x-ray diffraction and polarised light microscopy in patella, femoral head and tibial plateau articular cartilage (Asp den and Hukins, 1981a, b; Yarker et aI., 1983; 1984). These measurements show that the organisation of the collagen fibrils may be described in three distinct zones that merge smoothly into each other on passing from the articular surface to the bone. There is no neccessity for an individual fibril to be continuous from one zone to the next, they will effectively reinforce the ground substance providing their length is greater than a critical length (Hukins et aI., 1984; Hukins and Aspden, 1985). Collagen fibrils at the articular surface are oriented pre- dominantly in the plane of the surface. Fibrils reorient through a transition zone, in which the preferred orienta- tion is either bimodal or random, until in the deep zone, © 1990 by Gustav Fischer Verlag, Stuttgart adjacent to the bone, the preferred orientation of the fibrils is approximately perpendicular to the articular surface. This structure is very similar to that described by Benning- hoff (1925) in which the arcades he used to illustrate the fibril directions are the locus of the fibrils. This structure lead to the proposal that articular cartilage functions in a similar way to a pressure vessel in which the internal pressure is resisted by tension in the walls of the vessel (Hukins et aI., 1984). The collagen fibrils in the surface zone are appropriately oriented to provide a tension resisting structure to such a vessel. Calculations made using this model suggested that the stress in the collagen fibrils at the articular surface was of the order of 10 MPa and that, given the degree of orientation and the volume fraction of collagen, the surface zone was strong enough to support this stress. Analysis The calculations were originally performed using pig femoral head as an example, as it has an approximately spherical surface, and the equation used was that for a spherical pressure vessel for which 0= rP/2t (1)