Osteoporosis

Osteoporosis is a systemic skeletal disorder characterized by low bone mineral density (BMD) and microarchitectural deterioration of bone tissue, leading to increased fragility and fracture risk. Bone is a dynamic tissue maintained by the continuous, coupled process of bone remodeling: osteoclasts resorb old or damaged bone, creating resorption lacunae (Howship lacunae), and osteoblasts subsequently fill these cavities with new osteoid that mineralizes into mature lamellar bone. This remodeling cycle takes approximately 4-6 months to complete and occurs at over one million sites simultaneously throughout the skeleton. The balance between resorption and formation is regulated by the RANK/RANKL/OPG axis. Osteoblasts produce RANK ligand (RANKL), which binds to RANK receptors on osteoclast precursors, stimulating their differentiation into mature, active osteoclasts. Osteoblasts also produce osteoprotegerin (OPG), a decoy receptor that binds RANKL and prevents osteoclast activation. The RANKL-to-OPG ratio determines net bone resorption or formation. Estrogen is a critical regulator of this balance: it stimulates OPG production by osteoblasts and suppresses osteoclast-activating cytokines (IL-1, IL-6, TNF-alpha). After menopause, the precipitous decline in estrogen dramatically increases the RANKL:OPG ratio, unleashing osteoclast activity and causing accelerated bone loss of 2-5% per year during the first 5-7 postmenopausal years. This high-turnover state disproportionately affects trabecular bone (vertebral bodies, distal radius, proximal femur), explaining why vertebral compression fractures are the earliest and most common osteoporotic fractures. Glucocorticoid-induced osteoporosis -- the most common secondary form -- operates through a different mechanism: corticosteroids directly suppress osteoblast differentiation and function while prolonging osteoclast survival, creating a low-formation, high-resorption state that causes rapid bone loss particularly in the first 3-6 months of therapy. Additionally, calcium and vitamin D deficiency impairs bone mineralization. Vitamin D (25-hydroxyvitamin D) is converted to its active form (1,25-dihydroxyvitamin D) in the kidneys, where it promotes intestinal calcium absorption. When vitamin D is deficient (<20 ng/mL), only 10-15% of dietary calcium is absorbed (compared to 30-40% with adequate vitamin D), triggering secondary hyperparathyroidism that further accelerates bone resorption to maintain serum calcium homeostasis.