Lubrication and wear of joints

Abstract

The importance of lubrication in minimising friction between the joint surfaces is just as great in the natural as in the mechanical joint. In recent years engineers specialising in tribology (defined as 'the science and technology of interacting surfaces in relative motion') have collaborated with rheumatologists, orthopaedic surgeons and biochemists in studying the lubrication of joints and in the design of articular prostheses. Lubrication is of 3 types: 1) Fluid film lubrication, when the bearing surfaces are separated by a layer of fluid which is thicker than the sum of the surface roughnesses. Friction force then depends on the viscosity of the lubricant. 2) Boundary lubrication: because the thickness of a lubricant layer decreases at low speeds or high loads, the opposing surfaces will eventually come into contact at raised spots or asperities and friction is increased. Friction force no longer depends on viscosity of the lubricant but on the properties of thin films formed by chemical or physical reaction on the surface of the solids by an additive introduced into the lubricant. 3) Mixed lubrication: the surfaces are not completely separated by fluid and the friction characteristics differ from those of either of the other two. In synovial joints the bearing material is articular cartilage, supported by bone and lubricated by synovial fluid. Articular cartilage is a relatively soft, porous material with a surface roughness 10 times greater than that of ordinary engineering surfaces. Synovial fluid contains an additive, hyaluronic acid, which endows it with a characteristic property that its coefficient of viscosity falls rapidly as the shear rate (relative sliding velocity/lubricant film thickness) increases: when the joint is almost stationary the viscosity of the synovial fluid approximates that of glycerine, whle at the highest rates it is little higher than that of water. The possibility of treating osteoarthrosis with intraarticular introduction of a synthetic lubricant is under investigation, but the results cannot yet be assessed. Prosthetic joints present 2 tribological problems: choice of a material with the lowest possible rate of wear, and minimisation of friction force between the bearing surfaces. Many materials, both metals and plastics, have been used but the search for the ideal material is still going on.