The “instantaneous” deformation of cartilage: Effects of collagen fiber orientation and osmotic stress

The present study was undertaken with two objectives in view. The first was to distinguish between the “instantaneous” deformation and creep of articular cartilage when subjected to a step loading in unconfined compression. This was done by observing changes in the specimen’s diameter rather than its thickness. The second objective was to investigate experimentally the anisotropic behaviour of cartilage in a compressive loading mode, corresponding to the physiological situation. An apparatus was thus developed and constructed which enabled us to follow the “instantaneous” changes of the surface area of the sample as the latter was being loaded in unconfined compression. Specimens of human articular cartilage from normal femoral heads and condyles were tested. Full thickness specimens were tested with and without the underlying bone, as well as partial thickness specimens, characterizing the different zones of cartilage. Solutions of different ionic strength were used to vary the osmotic stress and specimens covering a considerable range of proteoglycan concentrations were selected. The effects of hydration and proteoglycan removal on the “instantaneous” deformation were also studied. The “instantaneous” deformation was found to be of a strongly anisotropic nature in all zones. The deformation was always smaller along the Indian-ink prick pattern than at 90° to it, and this effect was most pronounced in the superficial zone of cartilage. The results reveal an analogy with the tensile properties of cartilage and indicate that the collagen network is mainly responsible for controlling the “instantaneous” deformation. The proteoglycans play an indirect role by modulating the stiffness of the collagen network through their osmotic pressure.