A new model describing the lubrication of synovial joints

The purpose of this study is to explain why opposing cartilage surfaces in synovial joints do not wear, particularly under sustained loading. A new hypothesis, termed the immobile hyaluronic acid model, is developed to explain the loading problem and the role of hyaluronic acid in joints. In this model, the hyaluronic acid chains form an entangled immobile network spanning the gap between cartilage surfaces. During a squeezing motion, water and other solutes flow through this network, which significantly retards the outward flow and therefore the squeezing rate. An analysis is carried out, deriving the squeezing behaviour of a fibrous porous material between two parallel discs and a disc and parabolic surface, and the effect of relevant physiological parameters on the time for opposing surfaces to contact is studied. Under physiological conditions, cartilage contacts in 10 to 11 hours depending on whether additional solutes become trapped within the hyaluronic acid network.