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There is a great deal of experimental evidence that the frictional forces between sliding cartilage surfaces lubricated with synovial fluid are very low under normal conditions. Considering coefficient of friction as the meaningful measure of friction (i.e. tangential force resisting sliding divided by the normal load forcing the sliding surfaces together), typical values for cartilage on glass have been found to be 0-002 to 0-01 (McCutchen, 1962; Walker, 1969). A similar range was found for the ankle joints of dogs (Linn, 1968). However, under certain conditions, the coefficient of friction can be much higher than the range quoted. For instance, a test on a dog ankle joint was run for up to 6 hours under conditions of starved lubrication (Linn, 1968), and the coefficient of friction was found to be 0 1. For cartilage on glass, even higher figures have been obtained (Walker, Dowson, Longfield, and Wright, 1968).So far, the broad conclusions have been that low friction is associated with a full film of lubricant completely separating the surfaces, whether of synovial fluid or a derivative, whilst high friction is associated with thin films and localized boundary friction due to asperity contact.The object of this paper is to show some characteristics of the fluid film formed between cartilage surfaces. Friction and squeeze-film tests were conducted for several samples of cartilage and synovial fluid running against glass and the resulting fluid films examined in the scanning electron microscope. Theoretical analysis has then been undertaken to assess the significance of the experimental observations in terms of joint lubrication.Material and methods Specimens of articular cartilage were obtained from the femoral condyles of knee joints from cadavers. Extraction of the specimens was usually carried out at the same time as the necropsy within 12 hours of death.Some complete condyles were removed; other specimens were cut from the condyles with a scalpel and then stored in Ringer's solution adjusted to pH 7 -8 with Triss. While the joint was open, as much synovial fluid as possible was obtained, and in the cases where the volume was sufficient, it was used with its own cartilage for friction tests. The experiments were carried out as soon as possible after removal but some specimens had to be stored at 2°C. until used. FRICTION EXPERIMENTSA reciprocating friction machine was used to measure the coefficient of friction between cartilage and glass, the moving part being a flat glass plate. The reciprocating motion had an amplitude of 1 cm. and completed the full cycle in two seconds. This gave a maximum sliding speed of 3 cm./sec.The specimen was held at one end of a pivoted arm so that it could be loaded against the glass plate while the other end of the arm contacted a force transducer which recorded the frictional force at the cartilage-glass interface and displayed this on a chart recorder. The accuracy of the recording was 0*1i± 0 002 kg.The cartilage specimen was cut in such a way that the bone remained attac...

Abstract. Observations were made of changes in red cell morphology in blood stored in acid citrate dextrose over a period of six weeks using the scanning electron microscope. It was found possible to demonstrate the various stages of red cell transformation from biconcave discs, through a crenated stage to the final spheroidal state. Adenine was shown to slow down these changes. It is postulated that the red cell membrane changes are reversible up to the stage of the crenated spheroid cell, but subsequent changes result in a rapid destruction of the cell if transfused. The scanning electron microscopy studies can, therefore, be a useful method of observing the effect on red cell shape of new anticoagulants, red cell storage procedures and biochemical changes.

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