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Four-week-old female ddY mice were ovariectomized or sham-operated, and biochemical and biomechanical measurements were performed to examine timedependent changes in quality and quantity of bone after the operations. The body weight and the femoral length increased time dependently and no difference was observed between the ovariectomized and the sham-operated mice. On the other hand, increase in the uterine weight was completely suppressed in the ovariectomized mice, although the uterine weight of the sham-operated mice increased in a timedependent manner. The femoral weight of both the shamoperated and the ovariectomized mice tended to increase time dependently within 4 weeks after the operations. However, the increase in the ovariectomized mice was mild and stopped at 5 weeks after the operation. Calcium and hydroxyproline content, failure load, and stiffness of femur were highly correlated with femoral weight in both ovariectomized and shamoperated mice. The yield energy of femur correlated with the femoral weight in the sham-operated mice but not in the ovariectomized mice. The increase in the yield energy of the ovariectomized mice was completely suppressed after the operation in spite of time-dependent gain in femoral weight. Therefore, the yield energy of femur is the best index for morbidity of osteoporosis of all these parameters described in this study.

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