Cryoscanning electron microscopy of loaded articular cartilage with special reference to the surface amorphous layer

The surface layer (i.e. the surface lamina) of articular cartilage, which is devoid of a collagen fibril network or cells, was investigated in the pig and human. It overlies the collagenous main part of the articular cartilage which contains chondrocytes and is thought to be important biomechanically. In order to examine morphological changes in this layer when under load, knee articular cartilage of the pig, along with the underlying subchondral bone, was compressed with a cylindrical indenter. The specimen was frozen by immersion in liquid nitrogen to maintain the loaded condition and was then freeze-fractured at the indented region. The fracture face was examined with a cryoscanning electron microscope. The surface layer was compressed beneath the indenter regardless of loading pressure or period and was expanded around the indenter to form a triangular bulge in cross section. The height of the bulge was related to the applied pressure and not to the loading period. Recovery of the cartilage from indentation was also examined. Immediately after removal of the indenter, the bulge of the surface layer moved back into the previously indented region. The region was covered by a thick surface layer after 2 s. The response of the surface layer to and recovery from indentation was largely instantaneous and elastic. Under heavy load conditions, the main part of the cartilage under the indenter was observed to have a striped pattern which was made up of bands of densely packed collagen fibrils with fibrillar networks remaining between them. These morphological findings agree well with previously reported biomechanical hypotheses and can be explained by the flow of interstitial fluid provoked by stress application.