Incomplete restoration of immobilization induced softening of young beagleknee articular cartilage after 50-week remobilization

The aim of this study was to characterize the biomechanical and structural changes in canine knee cartilage after an initial 11-week immobilization an d subsequent remobilization period of 50 weeks. Cartilage from the immobili zed and remobilized knee was compared with the tissue from age-matched cont rol animals. Compressive stiffness, in the form of instant sheer modulus (I SM) and equilibrium shear modulus (ESM) of articular cartilage, was investi gated using an in situ indentation creep technique. The local variations in cartilage of glycosaminoglycan (GAG) concentration were measured with a mi crospectrophotometer after safranin O staining of histological sections. Us ing a computer-based quantitative polarized light microscopy method, collag en-related optical retardation, T, of cartilage zones were performed to inv estigate the collagen network of cartilage. Macroscopically, cartilage surf aces of the knee joint remained intact both after immobilization and remobi lization periods. Immobilization caused significant softening of the latera l femoral and tibial cartilages, as expressed by ESM (up to 30 %, p < 0.05) . Remobilization restored the biomechanical properties of cartilage in the lateral condyle of tibia, but in the lateral condyle of femur ESM remained 15% below the control level (p = 0.05). The instant shear modulus was not c hanged either after immobilization or remobilization. The GAG content of th e cartilage was slightly decreased after immobilization, especially in the superficial zone of cartilage, but the change was not statistically signifi cant. After remobilization the intensity of safranin O content rose to cont rol level. Neither immobilization nor remobilization had any effect on the Gamma value of collagen fibril network either in the superficial or the dee p zone at any of the test points. The changes of ESM were positively correl ated with the alterations in GAG content of the superficial and deep zones after immobilization and remobilization. This confirms the key role of prot oglycans in the regulation of the equilibrium stiffness of articular cartil age. As a conclusion, immobilization of the joint of a young individual may cause long-term, if not permanent, alterations of cartilage biomechanical properties. This may predispose joint to degenerative changes later in life .