Cineradiography of a cracking joint

Abstract

Human synovial joints are superb bearings which operate on a mixture of boundary and fluid-film lubrication mechanisms. The fluid in the joint, though present in very small amounts, is subjected to a wide range of pressures, from highly positive during loading to negative during rest and parts of the walking cycle. Just as the pressure in the synovial fluid is raised when the joint faces move towards each other, the pressure is lowered as they move apart. Cavitation is a well documented engineering phenomenon (Knapp et al, 1970) which describes the growth and collapse of gas or vapour bubbles in a liquid. Gaseous cavitation occurs where bubbles grow by gaseous diffusion, for example in carbonated drinks, and is usually harmless. Vapourous cavitation occurs where the local pressure in a liquid drops to its vapour pressure. Bubbles of vapour are then formed, often explosively. This form of cavitation can be extremely noisy and harmful because the vapour bubbles are unstable and collapse violently when a higher ambient pressure is encountered. Fluid machinery such as propellers, pumps and valves are highly susceptible to erosion from this mechanism. The cracking sound produced by synovial joints has been the subject of scientific curiosity for many years. Roston and Wheeler-Haines (1947) were the first to make a study of the distracting loads needed to produce the sound from the metacarpophalangeal (MCP) joint. Mennel (1947) mentioned that the MCP joint frequently cracked during rigorous examination or therapeutic manipulation.