Differences in cortical bone in overloaded and underloaded femurs from ovariectomized rats: Comparison of bone morphometry with torsional testing

Osteopenia and mechanical incompetence are the defining features of osteoporosis, yet the effects of changes in bone structure on mechanical properties are not completely understood. The primary objective of this study was to determine if in a rodent model, changes in cortical bone structure, as measured by static morphometry, would correlate with functional properties, as measured by torsional testing. For this, cortical bone structure and stiffness were determined in overloaded and underloaded limbs in rats rendered ovarian hormone-deficient by ovariectomy (OVX). Rats were OVX and six weeks later a hind limb was immobilized by casting for an additional six weeks. In all cases the femur from the underloaded, casted limb was compared with that from the contralateral, overloaded limb. The success of this experimental protocol was confirmed by differences in femoral cancellous bone volume by microradiography and single photon absorptiometry, with the overloaded limb having a greater bone volume and bone mineral density than the contralateral, immobilized controls. Morphometric differences in the femoral diaphyseal cortical bone included greater endocortical perimeter and endocortical osteoclast surface, less cortical area, less minimum cortical width, and less minimum cortical width at fracture planes when the bones were tested for stiffness in the underloaded, compared with the contralateral, overloaded limbs. Using a torsional test, the ultimate torque to failure and stiffness were less in the underloaded femurs compared with the contralateral, overloaded femurs. These results emphasize the importance of mechanical loading on bones from a gonad-deficient animal. These data also demonstrate in a convenient rodent model that somewhat subtle but significant changes in cortical bone structure can result in consistent differences in mechanical competence of the bone.