Swelling effects in cartilage arise from a high density of negatively-charged proteoglycans which generate a large swelling pressure. Swelling effects in cartilage are regulated by a balance between this swelling pressure and elastic forces generated within the cartilage solid matrix (1,2). Increased swelling is a characteristic of osteoarthritic cartilage and reflects a pathological alteration in the balance between swelling pressure and elastic restraining forces (3). Recently, we have developed a new technique to measure the nonuniform swelling-induced strain fields in samples of articular cartilage in situ, i.e., while attached to the subchondral bone (4). We observed variations in swelling behavior of cartilage with depth from the articular surface, reflecting the nonhomogeneous composition and structure of the material. In this study, we apply our newly-developed experimental technique to measure the swelling-induced strain fields in cartilage following removal of weight-bearing forces with cast immobilization. Periods of joint immobilization have been shown to produce compositional changes characteristic of osteoarthritis including a decreased proteoglycan content and changes in proteoglycan conformation (5, 6). We hypothesize that the changes associated with joint immobilization will alter the swelling behavior of articular cartilage, which will be detected as a change in swelling-induced strain fields in cartilage in situ.

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