Decreased bone density and osteoporosis are not favorable to orthodontic treatment. Estrogen can promote alveolar bone formation and inhibit bone resorption. Estrogen regulates bone remodeling through various cytokines under orthodontic force. First, estrogen can inhibit bone resorption and promote bone restoration by upregulating the expression of osteoprotegerin and downregulating the receptor activator of nuclear factor-κB ligand. Second, estrogen can promote osteoblasts and osteocytes to secrete bone morphogenetic proteins, which accelerate alveolar bone remodeling. In addition, estrogen can control alveolar bone resorptionby significantly inhibiting tumor necrosis factor-α, interleukin-1, and interleukin-6. Estrogen can also inhibit bone resorption by suppressing the expression of interferon-γ. Estrogen deficiency caninduce osteoclastogenesis by increasing the expression of macrophage colony-stimulating factor. Therefore, administration of optimum dosagesof estrogen is necessary in patients with osteoporosis for effective and safe orthodontic treatments.
