DIMINISHED MATERIAL PROPERTIES AND ALTERED BONE-STRUCTURE IN RAT FEMORA DURING PREGNANCY

Pregnancy and lactation are known to cause structural and mechanical changes in bone, but the effects of pregnancy alone have not been evaluated thoroughly, This study used radiographic measurements, torsion testing, mineral analyses, and histological evaluation to determine whether there are changes in bone material and geometric properties during pregnancy in the growing rat, as implied by earlier biochemical and histological studies. The bones of pregnant 9 to 12-week-old rats and controls that were not pregnant and were matched by age (but not weight) were evaluated at times corresponding to 5, 10, 15, and 20 days of the 23-day gestation period to address the following questions: (a) How is the growth of whole bone affected by pregnancy in the growing rat (as determined by radiographic analyses)? (b) How are the mechanical properties (structural and material) of whole bone affected by pregnancy (as assessed by torsion testing)? (c) Are there changes in the characteristics of bone mineral during pregnancy (as determined by measurement of mineral content and x-ray diffraction analyses)? and (d) Are there detectable morphological or ultrastructural differences between the bones of pregnant and control rats (as assessed by analyses based on histology and back-scattered electron imaging)? The presence of statistically significant differences in this study was determined initially on the basis of a two-factor analysis of variance. In general, significant differences were noted only at late gestation (day 20), when the bones were longer and had a greater outer radius and cortical thickness; this indicates that more growth occurred during pregnancy. At day 20, biomechanical testing showed that the femora of pregnant rats had a 20% decrease in rotational angle to failure and a 30% increase in shear stiffness. No alterations in maximum torque or energy to failure were noted. Calculation of the properties of bone material revealed a 54% decrease in ultimate shear stress, a 50% decrease in shear modulus, and no alteration in shear strain when the bones from rats pregnant for 20 days were compared with controls. The bones of the pregnant rats at day 20 also differed from controls in terms of both mineral characteristics and morphology. X-ray diffraction showed larger mineral crystallites in the specimens from pregnant rats as compared with controls, whereas the mineral contents (ash weights) were similar. Scanning electron microscopy revealed qualitative differences, including increased periosteal vascularity and numerous large cortical cavities thought to be areas of resorption in the bones from rats pregnant for 20 days as contrasted with controls. These changes are consistent with the observed alteration in mechanical properties.