Cortical and trabecular osteopenia after immobilization. A quantitative histological study of the rat knee.

The aim of this investigation was to examine the effect of short-term immobilization on subchondral cortical and trabecular bone tissue in the rat tibia and to determine whether there was any difference when the knee was immobilized in extension or flexion. Thirty-six male rats were used in this study, and in 18 the knee was fixed in extension and in 18 in flexion. The time of immobilization was for 1, 2 and 3 weeks, with 12 animals in each group and 6 knees in extension and 6 in flexion. The following parameters were measured: (1) the mean thickness of subchondral and both periosteal cortices; (2) cortical porosity; (3) trabecular bone volume; (4) relative osteoid volume, and (5) relative osteoid surface. The mean cortical thickness decreased during the period of immobilization and the cortical porosity significantly increased. The trabecular bone volume was unaltered in the first week, but after 3 weeks it had decreased. The relative osteoid volume decreased significantly during the three weeks, and the relative osteoid surface moderately increased. No relationship was found between the quantitative osteopenic alteration of subchondral bone and the position of immobilization.