Remobilization does not fully restore immobilization induced articular cartilage atrophy

The recovery of articular cartilage from immobilization induced atrophy was studied, The right hind limbs of 29-week-old beagle dogs were immobilized for 11 weeks and then remobilized for 50 weeks, Cartilage from the immobili zed knee was compared with tissue from age matched control animals. After t he immobilization period, uncalcified articular cartilage glycosaminoglycan concentration was reduced by 20% to 23%, the reduction being largest (44%) in the superficial zone. The collagen fibril network showed no significant changes, but the amount of collagen crosslinks was reduced (13.5%) during immobilization, After remobilization, glycosaminoglycan concentration was r estored at most sites, except for in the upper parts of uncalcified cartila ge in the medial femoral and tibial condyles (9% to 17% less glycosaminogly cans than in controls). The incorporation of (SO4)-S-35 was not changed, an d remobilization also did not alter the birefringence of collagen fibrils, Remobilization restored the proportion of collagen crosslinks to the contro l level. The changes induced by joint unloading were reversible at most sit es investigated, but full restoration of articular cartilage glycosaminogly can concentration was not obtained in all sites, even after remobilization for 50 weeks, This suggests that lengthy immobilization of a joint can caus e long lasting articular cartilage proteoglycan alterations at the same tim e as collagen organization remains largely unchanged. Because proteoglycans exert strong influence on the biomechanical properties of cartilage, lengt hy immobilization mag jeopardize the well being of articular cartilage.