Quantitative morphological and biochemical investigations on the effects of physical exercise and immobilization on the articular cartilage of young rabbits.

The effects of physical exercise and immobilization on the knee joint articular cartilage of young rabbits (2.5 to 5 months of age) were investigated by quantitative histology, semiquantitative stereomicroscopy and scanning electron microscopy as well as biochemical methods, during a period of 8 weeks. Physical exercise increased the cell size in all zones of the uncalcified cartilage without notable concomitant change in the cell density. It also increased the volume density of the cells in the intermediate and deep zones. Increased striation of the cartilage surface was observed after 1 week's running period, suggesting superficial appearance of collagenous fibres and depletion of the proteoglycans on account of the exercise. However, after 4 to 8 weeks of running the increase of the striated surface was no longer significant. At the same time the proteoglycan content increased with preponderance of keratan sulfate-rich proteoglycans. Immobilization of the knee joint decreased slightly the cell size but increased the cell density in the superficial zone. Immobilization caused derangement of the smoothness of the articular surface already at the end of the 1st week with superficial splitting of the surface. Reduction of the cartilage matrix proteoglycans and collagen was observed after 1 week's immobilization. In the whole femoral surface, but not in the weight-bearing area of tibial lateral condyle, the proteoglycan content was replenished with chondroitin sulfate-rich molecules by the end of the 8th week after immobilization. Both physical exercise and immobilization caused significant intracondylar remodelling of the articular cartilage.