Adverse effects on rabbit anterior cruciate ligament after knee immobilization: changes in permeability of horseradish peroxidase

Changes in the permeability of horseradih peroxidase (HRP) and the histology of the rabbit anterior cruciate ligament (ACL) caused by knee immobilization were studied. The knees of adult rabbits were immobilized for 2-8 weeks by a long leg cast with the knee flexed at 120 degrees, and some joints were allowed to be remobilized for 4 weeks after 6 or 8 weeks' immobilization. After 1 mi of HRP was injected into the knee joint and allowed to permeate into the ACL, the degree of permeation at the mid-portion of the ACL between femur and tibia was visualized with diaminobenzidine tetrahydrochloride. The ACL was examined histologically by light and transmission electron microscopy. Although the HRP permeated into almost the entire area of the transverse section of the non-immobilized normal ACL, as the period of immobilization increased, the extent of permeation gradually decreased. The decrease was significant from week 4 to week 6 and from week 6 to week 8. Degenerative change with necrosis of cells at the central area of the ACL was noted in the 6-week immobilization group, and the 8-week period of immobilization increased the extent of this degeneration. Although the permeation recovered in both the 6-week and 8-week immobilization groups 4 weeks after remobilization, there were fewer cells with a normal morphology in the central area in the 8-week immobilization group than in the control. These results suggest that long-term immobilization adversely affects the ACL.