"Boosted lubrication" in synovial joints by fluid entrapment and enrichment.

The manner in which a human load-bearing synovial joint is able to carry the applied loads has been the subject of speculation for many years. In the past, a number of theories have been advanced each using as a basis one of several modes of lubrication well known to mechanical engineets. More recently some of the more unique characteristics of human cartilage have been recognized, and theories have been modified accordingly. Thus MacConaill (1932) proposed an explanation based upon a type of lubrication known to engineers as "hydrodynamic", Charnley (1960) proposed an explanation based upon "boundary lubrication", and McCutchen (1962, 1966) explored the possibilities of "weeping lubrication". In the light of further study an explanation has been proposed by Linn (1967). Davies, Barnett, CocIrane, and Palfrey (1962) and Edwards (1967) have investigated the properties of articular surfaces. Negami (1964) along with Davies (1966) has studied the rheological properties of synovial fluid. Rydell (1965) and Paul (1967) have investigated the forces which act through joints in normal circumstances. Certainly, if an understanding is to be reached concerning the role of synovial fluid in a human load-carrying joint, the physical characteristics of the articular surfaces, in addition to those of the synovial fluid, must be taken into account. In the present study, a number of friction experiments were designed to investigate the extent to which the various modes of lubrication were present in physiological conditions. The overall geometry of the fluid-filled gap between the mating articular surfaces has been investigated, and the microscopic surface quality of cartilage has been recorded. On the basis of these observations, the features of normal joint lubrication are described and the factors which encourage full fluid film lubrication are listed. "Boosted lubrication", the name given to a mode of lubrication not previously described,