Alteration in the mechanical competence and structural properties in the femoral neck and vertebrae of ovariectomized rats

The structural and mechanical properties of bone in the femoral neck and va rious other sites were investigated in intact (INT), sham-operated (Sham), and ovariectomized (OVX) rats. Six weeks after operation, the maximal load and energy absorption of the femoral neck were significantly lower in the O VX than in the INT or Sham groups, being 73.2 +/- 1.9 (SE) N, 86.3 +/- 4.1 N, and 87.1 +/- 3.2 N, respectively (p < 0.01) for load. The total cross-se ctional area of the femoral neck did not change after OVX, but the marrow c avity area was enlarged, leading to a reduced bone area (including both cor tical and trabecular bone) (p < 0.01). Histomorphometric analysis showed th at new bone formation could not be detected at the periosteum of the femora l neck below the femoral head, but at the endocortical surfaces the double tetracycline labeling revealed an increased mineral apposition rate (MAR) a nd bone formation rate (BFR) in OVX animals (p < 0.001). In contrast, MAR a nd BFR were significantly increased in both periosteal and endocortical sur faces of the tibia, humerus, and femoral shaft, thus preventing a decrease in cortical bone area. The maximal bending loads of the tibia and humerus w ere not different in the various groups of animals. The correlation coeffic ient between maximal load and bone area revealed positive relationships in the femoral neck (r = 0.54, p < 0.01), tibia (r = 0.36, p < 0.01), and hume rus (r = 0.51, p < 0.01). Ovariectomy resulted in a decreased trabecular bo ne volume of lumbar vertebra VI (L6) decreased compressive loads of lumbar vertebrae I, III, and IV. These lumbar bone loads were positively related t o their L6 bone area (L4/L6: r = 0.66, p < 0.001). Element analyses (energy dispersion spectrometer) from trabecular and cortical areas of bone show e d some changes related to aging but not to OVX. These results indicate that ovariectomy influences the biomechanical properties of rat bone by changin g structural properties rather than material ones.