Long-term effects of ovariectomy on the mechanical properties and chemical composition of rat bone.

One hundred Sprague-Dawley rats were ovariectomized (OVX) or sham-operated (Sham) at the age of 12 weeks. Eight or six animals from each group were sacrificed at the following time points: 0, 2, 4, 8, 16, 28, and 40 weeks. Mechanical testing of the bones showed that the maximal load of the femoral neck remained at the baseline level in OVX rats but clearly increased in Sham animals. However, after 28 weeks (at the age of 40 weeks) of ovariectomy, the femoral neck strength in Sham animals started to decrease, reaching the OVX level at the age of 1 year. The gain of torsional strength of the humerus was first slowed down after ovariectomy, but 16 weeks after operation this difference between the OVX and Sham animals disappeared. In OVX rats incorporation of 45 Ca2+ into the tibia and the number of osteoclasts on the trabecular bone surfaces were increased at 4-16 weeks and at 2-28 weeks, respectively, but they returned later to the levels observed in Sham animals. This indicates that the OVX-induced increase in bone turnover is transient. The trabecular bone volume at the distal femur decreased and the area of the marrow cavity in the distal third of the femur increased after OVX and did not show any recovery. Analysis of the inorganic composition of bone by energy-dispersion spectrometry showed that the percentage of magnesium was transiently decreased after OVX in both trabecular and cortical bone. Despite the OVX-induced loss of trabecular bone, increasing body weight or some metabolic changes may induce structural changes which may be responsible for maintaining the mechanical force of the femoral neck in aging OVX rats.