THE ROLE OF SYNOVIAL-FLUID FILTRATION BY CARTILAGE IN LUBRICATION OF SYNOVIAL JOINTS .2. SQUEEZE-FILM LUBRICATION - HOMOGENEOUS FILTRATION

A mathematical model of synovial film filtration and synovial gel form ation at normal approach of cartilage surfaces in the human hip joint is presented. The biphasic mixture model presented in Part I of this p aper [Hlavacek, J. Biomechanics 26, (1993)] for synovial fluid and tha t of Mow and his collaborators [J. Biomech. Engng 102, 73-84 (1980)] f or cartilage are used. A general analysis of filtration of an axially symmetric synovial squeeze-film between two cartilage layers at normal approach is given. The geometrically simple case much idealising the human hip joint is also considered: two cartilage discs with a synovia l film in between are compressed by the steady loading of the half hum an weight. The homogeneous film filtration process where the synovial gap remains parallel and the macromolecular concentration in the gap i s spatial homogeneous (to the thin-film approximation) and time-depend ent is numerically analysed. If the hyaluronic acid concentration of t he synovial gel at equilibrium is 50 mg/ml at least, the resulting sta ble gel layer thickness for the homogeneous filtration in the human hi p joint and for normal synovial fluids is about 0.1 mum, being almost independent of the loading. Inflammatory synovial fluid shows values s everal times lower.