The reversibility of disuse osteoporosis. Studies of bone density, bone formation, and cell proliferation in bone tissue.

One hind leg of 80 adult rats of the Sprague-Dawley strain was made osteoporotic by immobilization for 9 weeks. Osteoporosis was noted in both the femur and the tibia when the hydrated gross bone density and the bone surface areas were measured. No signs of reversibility were observed during 10 weeks after the period of immobilization. Tetracycline and DCAF labelling failed to show significant signs of increased bone formation during the 10 weeks after remobilization. At the moment of remobilization and for some weeks thereafter, there were signs of depressed mitotic activity in the bone cells when expressed as the 3H-thymidine/DNA ratio. The conclusion was that neither the cell-proliferation rate nor the cellular activity increases sufficiently for restitution of the disuse osteoporosis.