Effects of ovariectomy on the changes in microarchitecture and material level properties in response to hind leg disuse in female rats

Background: Ovariectomy (OVX) and immobilization are known to decrease bone mineral density and alter its microarchitecture. Their effects on the material level properties of bone, a determinant of bone strength, are still largely unknown. We investigated the effect of OVX and/or disuse achieved by sciatic neurectomy (NX) in 6-month-old Sprague Dawley female rats. Methods: At baseline, animals underwent OVX or sham operation. At week 16, NX was performed on the left hindlimb while the right hindlimb was sham-operated. All animals were sacrificed at week 40. Proximal tibiae and vertebral bodies (L4) were evaluated by micro-computed tomographic morphometry (mu CT). Material level properties (elastic modulus, hardness, and dissipated energy) were evaluated by a nanoindentation test. Results: At the proximal tibia, OVX and NX decreased relative bone volume, the former mainly through a reduction in trabecular number, and the latter through a decrease in trabecular thickness. NX decreased modulus (-10%; p<0.001) and dissipated energy (-13.3%, p<0.001) in cortical bone, and modulus (-16.8%, p = 0.004), hardness (-29.3%, p =- 0.004), and dissipated energy (-17.7%, p = 0.01) in trabecular bone, while OVX decreased cortical bone dissipated energy (-14.6%, p<0.001) and trabecular bone hardness (-19.4%, p = 0.05). In the vertebral body, OVX altered mainly the trabecular microarchitecture and nanoindentation variables. Conclusion: These results show that NX with and without OVX markedly alter material level properties in addition to an alteration of bone microarchitecture, although not in the same manner. (C) 2012 Elsevier Inc. All rights reserved.