[Effects of early estrogen replacement therapy on bone stability of ovariectomized rats. A biomechanical and radiologic study of the tibial plateau].

Postmenopausal osteoporosis leads to a significant increase in bone fragility. In this study we used the rat tibia plateau fracture model to investigate the efficiency of estrogen replacement therapy (ERT) to mitigate the post-ovariectomy decrease in fracture load. A total of 73 virgin Sprague Dawley rats had been ovariectomized and 26 animals underwent sham operation. The ovariectomized animals were either untreated (n = 35) or treated with estrogen injections (10 micrograms/kg per day 3 days a week until sacrifice), starting treatment at either 0, 5, 8, or 13 days post surgery. Before starting ERT and at 50 days post surgery, the trabecular structure of the right proximal tibial metaphysis of each animal was imaged non-invasively using high resolution X-ray topography. The animals were then sacrificed and the right knee from each animal was harvested and mounted into a servo-hydraulic materials testing system so that the distal femoral condyle could be forced into the proximal tibial plateau until fracture occurred. The failure load (F) of the ovariectomized group without estrogen administration was significantly less than that for the sham group. The mean stiffness (K) of the ovariectomized group was 22 percent less than that of the sham group, though this difference did not reach statistical significance. Across all groups, the failure load and stiffness were significantly correlated with the trabecular bone volume. Our data suggest that prompt ERT can increase the fracture load and stiffness of trabecular bone by allowing bone formation to continue in previously activated bone remodeling units while suppressing the production of new remodeling units. This may be the mechanism by which estrogen and other antiresorptive agents increase bone mass, and thereby reduce the risk of osteoporotic fractures in postmenopausal women.