Differential effects of ovariectomy on the mechanical properties of cortical and cancellous bone in rat femora and vertebrae

The purpose of this study was to evaluate the effects of ovariectomy on the mechanical properties of bone in the aged Sprague-Dawley rat model of osteoporosis. Eight female rats were sacrificed at the start of the study, at the age of four months. Twenty-four remaining rats were then bilaterally ovariectomized (OVX), and another twenty-four served as controls. Eight rats from each group were sacrificed at five, ten or fifteen weeks. The mean density of L1 vertebral bodies from OVX rats was lower than in their control counterparts, as was the compressive modulus, the ultimate compressive stress and the toughness. The second lumbar vertebra from each rat was cored to remove the bulk of the cancellous bone and tested in compression to failure. Ovariectomy did not affect the mechanical properties of the cored vertebrae, suggesting that the mechanically relevant changes take place in the cancellous bone. The gravimetric density and bone mineral density (measured by dual-energy x-ray absorptiometry) of whole femora were lower in OVX rats than control rats. However, the gravimetric density of the cortical bone was unchanged, and when femora were tested to fracture in three-point bending, the mechanical properties of the midshaft were found to be unaffected. This study suggests some limitations to the mature ovariectomized rat model of osteoporosis: mechanically significant loss of cancellous bone occurs in vertebrae much more rapidly than suggested by other studies. Further, there is little evidence of mechanically important bone loss in cortical bone within four months of ovariectomy.