Bone histomorphometric changes in the femoral neck of aging and ovariectomized rats

Abstract Background: This study characterizes the changes in cortical and cancellous bone and cross sectional moment of inertia of the femoral neck from aging and ovariectomized (ovx'd) rats to determine their role in the previously reported ovx‐induced reduction of mechanical strength in the femoral neck. Methods: Undecalcified double‐fluorescent lableled cross sections of femoral neck of 3.5‐, 5.5‐, 6.5‐, and 8.5‐month‐old female rats and rats ovx'd at 3.5 months for 2, 3, and 5 months of 45 rats were studied. The estimated endocortical and trabecular surfaces, cortical and cancellous bone histomorphometry, and cortical moment of inertia were determined. Results: The femoral neck was adding cortical bone between 3.5 and 5.5 months of age by increasing cortical thickness and decreasing marrow cavity area. No change of cortical bone mass was found between 5.5 and 8.5 months and the same amount of cancellous bone was observed between 3.5 and 8.5 months of age. Ovariectomy‐induced cancellous, but not cortical bone loss. The loss was due to a transient ovx‐induced negative bone balance that by 5 months post‐ovx produced a 42% loss in trabecular bone while the histomorphometry profiles were the same as controls. The crosssectional moment of inertia increased with age but did not differ significantly between ovx'd and controls. Conclusions: Our findings suggest that the ovx‐induced cancellous bone loss could be a contributing factor to the reduced mechanical strength in the femoral neck of ovx'd rats reported previously. © 1995 Wiley‐Liss, Inc.