Mineral density and histomorphometric assessment of bone changes in the proximal tibia early after induction of type II collagen-induced arthritis in growing and mature rats.

Bone changes in both actively growing (6-week-old) and mature (6-month-old) rats with collagen-induced arthritis (CIA) were investigated in order to clarify the mechanisms of osteoporosis near inflamed joints in patients with early rheumatoid arthritis (RA) and juvenile RA. In female Sprague-Dawley rats, the proximal tibiae from the CIA and control groups early after immunization, when any influence of immobilization due to joint pain and swelling is minimal, were studied using dual X-ray absorptiometry and histomorphometry after double-labeling with tetracycline. Arthritis developed within 10-14 days after immunization in both growing and mature rats. Physical activity, growth, and body weight continued to resemble that of the control group for at least 10 days. The bone mineral density in the proximal tibia did not differ significantly between the CIA and control groups. In growing rats, a highly significant increase in bone resorption, and decreases in bone formation and trabecular bone volume became evident histomorphometrically before visible signs of arthritis had developed. In mature rats, bone formation was markedly decreased without an increase in bone resorption. The differences in the reaction between growing and mature rats reflected a difference in the number of remodeling sites (units) and an uncoupling between osteoblasts and osteoclasts. We conclude that osteoporosis near inflamed joints results from an imbalance between bone resorption and formation caused by immune reactions in the CIA rats. Moreover, a decrease in bone formation may, in part, precede the clinical onset of arthritis.