DEVLIN, Maureen*; LIEBERMAN, Daniel; LEDOUX, Nicole; Harvard University; Harvard University; Harvard University: Estradiol, strain, and periosteal bone growth

Recently we showed that estradiol (E₂) increases osteogenic responses to loading, so that the same mechanical loading produces more periosteal bone growth in individuals with higher E₂ levels. This hypothesis was supported in sheep (Ovis aries, N=32) exposed to either low or high E₂ levels and either sedentary or high activity levels. Periosteal growth in exercised animals was 25-75% greater in high than in low E₂ animals, but only 15-20% greater in sedentary high vs. low E₂ animals. Although E₂ clearly stimulates bone growth, it is unknown whether this growth improves resistance to deformation. This study tests the hypothesis that E₂-induced periosteal growth coincides with areas of peak tensile strain at midstance. We calculated midshaft cross-sectional properties (polar moment of area, J_N; section moduli of tension and compression, Z_Nt, Z_Nc) in the tibia and metatarsal using the experimentally determined neutral axis (Lieberman et al., 2004) and compared them to areas of bone growth during the experiment. Results indicate little variation in J_N, although it is greatest in high-E2 animals. In the tibia, section moduli are similar in the low-E₂ and high-E₂ sedentary groups. However, Z_Nt and Z_Nc are 17-20% greater in high-E₂ vs. low-E₂ exercised animals, corresponding to periosteal deposition on the anterior and posterior surfaces of the bone. In the metatarsal, Z_Nt and Z_Nc are 13-24% greater in the high-E₂ exercised vs. the sedentary groups, corresponding to deposition on the posterior surface. These results support the hypothesis that E₂-induced periosteal growth is concentrated in areas of highest tensile strain, improving bone�s resistance to bending.