Changes in articular cartilage following intraarticular injection of tritiated glyceryl trioleate

The effects of intra-articular injections of non-radioactive and tritium-labelled glyceryl trioleate into the mandibular and knee joints of adult rabbits have been investigated using autoradiographic and histochemical techniques and electron microscopy. As observed at the fourth day after operation, fat droplets accumulate in cells of the fibrous, intermediate and cartilaginous layers of mandibular condylar, and in the superficial and upper middle (rather than the deeper) zones of femoral condylar cartilage. Autoradiography of frozen sections shows that numerous silver grains are located over these fat-laden cells following injection of trioleate which has been labelled in the fatty acid moiety of the molecule. In the knee joint the number of grains is directly related to the amount of lipid in the cell. Following injection of glyceryl-labelled trioleate no such result is obtained; it seems doubtful whether or not there is any uptake of this label. However, synovial membrane from the knee joint appears to take up both kinds of trioleate. Results of histochemical methods of NADH diaphorase, lactic dehydrogenase, acid phosphatase and beta-glucosaminidase are consistent with ultrastructural evidence of degeneration in some chondrocytes and of loss of ground substance from the matrix. A raised level of alpha-glycerophosphate dehydrogenase activity is probably associated with synthesis of endogenous glycerol for re-esterification of absorbed fatty acids, and enhanced activity of UDPglucose dehydrogenase with the chondrocytic reaction to matrix depletion. Apart from the increase in fat content, ultrastructural features in injected knee joints include flattening of cell processes against the chondrocyte surface and more abundant intracytoplasmic filaments. Injected mandibular joints show little evidence of these changes although the number of cells in the cartilage appears to be greatly reduced. No extracellular fat droplets occur in femoral cartilage, but material similar in electron density to intracellular fat is observed at the external aspect of some mandibular chondrocytes. The findings indicate that the fatty acid portion of triglyceride injected intra-articularly is taken up by the chondrocytes and retained until at least the fourth day after injection. It is suggested that prior lipolysis takes place either in the synovial cavity (or membrane) or at the chondrocyte surface, but it is uncertain how or in what form fat traverses the matrix. Lipoarthrosis appears to produce changes in the chondrocytes which are thought to be pathological; a number of cell deaths occur. The possibility that gross degeneration of the articular cartilage may ensue is subject to further investigation.