Research Report 8: Current Status of Joint Lubrication

Abstract

Properties of synovial fluid and cartilage are reviewed and some thought is given to the nature of joint lubrication. It does appear, at this stage, that neither the classical hydrodynamic theory of lubrication nor the boundary lubrication can permit an adequate description of the mechanism of joint lubrication; a possible solution lies in application of the elastohydrodynamic theory of lubrication, although this theory is not yet advanced enough to handle the total complexity of joint lubrication. It is suggested that, in principle, joint lubrication depends on the existence of thixotropic and elastic (normal stress phenomenon) fluid between the articular surfaces, that the area of the load-carrying film depends on the elasticity and resiliency of cartilage, and that the latter shows most variable, time- and load-dependent rheological behaviour. Pathological changes in synovial fluid leading to the change of its rheology to Newtonian, will result in a damage to the articular surfaces; on the other hand, a presence of a healthy synovial fluid per se is not sufficient to maintain an efficient lubrication. Resiliency of cartilage is responsible for the existence of elastohydrodynamic conditions; a degeneration of cartilage will lead to boundary lubrication and mechano-chemical degradation of synovial fluid. Complexity of the problems requires multi-disciplinary collaboration.