Changes in the permeability and histologic findings of rabbit menisci after immobilization.

The knees of mature rabbits were immobilized for 2 to 8 weeks by a long leg cast to examine adverse effects on the meniscus. Some joints were remobilized for 1 to 4 weeks after an immobilization period of either 6 or 8 weeks. Horseradish peroxidase was injected into knee joints, and the degree of permeation into the meniscus was visualized with diaminobenzidine tetrahydrochloride. Menisci were examined histologically by light and transmission electron microscopic study. Injected horseradish peroxidase permeated all areas of the meniscus in nonimmobilized joints. However, permeability of horseradish peroxidase gradually decreased with increasing time of immobilization and was restricted to the superficial layer of the meniscus in the 8-week immobilization group. Degenerative changes in the deep layers of the meniscus were noted in the 6-week immobilization group: 8 weeks of immobilization induced additional degenerative changes. Although permeability of horseradish peroxidase gradually recovered with remobilization, the degenerative changes remained in the deep meniscal layer in the 8-week immobilization group, even after 4 weeks of remobilization. These results clearly show that long-term immobilization of the knee joint causes adverse changes in the meniscus.