Osteonal remodeling and mechanical properties of the femoral cortex in rabbits treated with 24R,25(OH)2D3

Summary The increase of bone mass by therapeutics does not always mean the enhancement of bone quality. The purpose of this study is to clarify the changes of osteonal remodeling and the mechanical properties of femoral cortex in rabbits treated with 24 R, 25 (OH)₂D₃. Fifteen NZW rabbits (3 kg B.W.) were divided into three groups of 5 animals each. Groups 1, 2, and 3 were given vehicle, 10 g/kg, and 100 g/kg 24R,25(OH)₂D₃, respectively, daily for 8 weeks. At the end of the experiment, the left femur was removed and bone mineral content (BMC) was measured with single photon absorptiometry. Serum 24,25(OH)₂D concentrations reached levels of approximately 15 and 200 times that of the controls in groups 2 and 3, respectively. Neither 25(OH)D nor 1,25(OH)₂D level showed any significant change in either group. Group 3 showed significant increase in mineral content and density in the epimetaphyseal regions, but the increase at the diaphyseal region did not reach a statistically significant level. Mechanical test for torsion was conducted for mid-cortical regions. After the test, bone pieces were bonded together with adhesive to reconstruct the original form, and undecalcified cross-sectional sections were made at the diaphyses. Fluorescent microscopy disclosed a marked reduction of remodeling in secondary osteonal bone area. The numbers for double-labeled osteons for groups 1, 2, and 3 were 2.47±0.819, 1.14±1.02^* and 0.137±0.307^* N/mm², respectively, and the numbers for osteons with resorption lacunae were 1.37±0.721, 0.412±0.370^* and 0.268±0.339^** N/mm², respectively. However, neither structural stiffness nor strength correlated with the indices of osteonal remodeling; instead, they were significantly correlated with bone mineral contents. Normalized mechanical parameters for torsion were almost the same for all three groups. This study clearly demonstrated that reduced osteonal remodeling by 24R,25(OD)₂D₃ does not affect the mechanical properties of the cortex, and the increase in bone density by the agent is considered to be accompanied by an increase in its mechanical strength (^*P< 0.05, ^**P< 0.01).