Increased mechanical strength of the vitamin D-replete rat femur by the treatment with a large dose of 24R,25(OH)2D3.

The mechanical properties of the rat femur treated with a large dose of 24R,25(OH)2D3 were examined and their relationship with the mass and mineral contents of the bone were investigated. Male Wistar rats were fed diet containing 0, 0.025, 1.25, 4.0, or 12.5 ppm 24R,25(OH)2D3 for two years starting six weeks after birth. The rats were killed and their right femurs were removed. The adhering soft tissue were stripped off. Radiographs were made of the femur and its bone mineral content was measured by single photon absorptiometry. Then a three-point bending test was done with pressure exerted in the plane of natural extension. After mechanical testing, non-decalcified cross-sections of the femur were prepared at the mid diaphysis as close to the test fracture site as possible. On x-ray images, the cortical thickness was clearly increased in groups treated with larger doses, and the mid-cortical segmental mineral content of the femur increased dose-dependently to about 150%. The mechanical parameters in the treated animals also increased significantly, strength to 120% (p less than 0.01), energy-absorption capacity to 124% (p less than 0.05), and structural stiffness to 183% (p less than 0.01). Segmental bone mineral contents showed the positive correlations with strength and structural stiffness in both the control and the 24R,25(OH)2D3 treated groups as well. We concluded that the increase in the bone density in animals administered high doses of 24R,25(OH)2D3 was accompanied by an increase in mechanical strength of the bone.