Fine structural changes in the articular cartilage of the rat's knee following short-term immobilisation in various positions: a scanning electron microscopical study.

The knee joints of 36 rats were immobilised in a padded plaster cast (18 in the flexed and 18 in the extended position) for 1, 2 and 3 weeks. The femoral articular cartilage was then examined under the scanning electron microscope. After 1 week's immobilisation in the flexed position, the surface layer of the cartilage showed disruption and the superficial chondrocytes were necrotic. The sites of the necrotic chondrocytes were seen as pits on the cartilage surface. In the animals whose knees were immobilised in the extended position, the changes were not so great in severity or extent. After 2 weeks, more advanced changes could be detected in both groups, but they were more severe in the flexed group. After 3 weeks the changes in articular cartilage were diffuse in the flexed, but focal in the extended group. Diffuse necrosis of chondrocytes, with pit formation on the surface and disruption and disintegration of the collagen fibres, was seen to some extent in all the animals. In the flexed group the changes extended to the whole weightbearing surface of the articular cartilage of the femur, except the intercondylar sulcus. In the extended group, a superficial fibre network was found without the formation of closely packed collagen fibre bundles, and the changes in the chondrocytes were seen only in localised zones. The authors suggest that during immobilisation, especially in the flexed position, articular hypoxia occurs due to a decreased amount of synovial fluid, the increased compression of the cartilage surfaces and the increased intra-articular pressure. This causes degeneration and necrosis of the superficial chondrocytes and the superficial cartilage layer.