Osteoporosis Study Guide

📖 Core Concepts Osteoporosis – systemic skeletal disorder with ↓ bone mass, degraded micro‑architecture, ↑ porosity → higher fracture risk. Bone remodeling – balanced cycle: osteoclasts resorb bone, osteoblasts form new bone. In osteoporosis, resorption > formation. Key hormonal regulators Estrogen ↓ → ↑ osteoclast activity, ↓ osteoblast survival. PTH‑Ca‑Vit D axis – low Ca/D → PTH ↑ → bone resorption to normalize serum Ca. RANKL‑RANK‑OPG – osteoblasts release RANKL → activates osteoclasts; OPG binds RANKL (inhibits). Trabecular vs cortical bone – trabecular bone remodels faster → vertebrae, hip, wrist most fracture‑prone. 📌 Must Remember Diagnostic T‑score: osteoporosis if T ≤ −2.5 (DXA of lumbar spine/hip). Fragility fracture = fracture from fall ≤ standing height or less. FRAX: 10‑yr hip fracture risk ≥ 3 % or major osteoporotic fracture risk ≥ 20 % → treat. Screening – women ≥65 yr (or <65 with risk factors); men ≥70 yr (or risk‑equivalent). First‑line meds – oral/IV bisphosphonates (e.g., alendronate). Calcium – ≥1 g/day; Vitamin D – maintain serum 25‑OH‑D >20 ng/mL (often 800–1 000 IU/day). Lifestyle – weight‑bearing + resistance exercise ≥4 d/wk; limit alcohol ≤3 units/d; stop smoking. 🔄 Key Processes Bone Remodeling Cycle Activation → Resorption (osteoclasts) → Reversal → Formation (osteoblasts) → Quiescence. RANKL‑mediated Osteoclastogenesis Osteoblasts ↑RANKL → RANK on osteoclast precursors → differentiation → bone resorption. OPG ↔ RANKL (blocking step). FRAX Calculation (simplified) Input: age, sex, weight, height, previous fracture, glucocorticoid use, smoking, alcohol, rheumatoid arthritis, secondary osteoporosis, femoral neck BMD (optional). Output: 10‑yr probability of hip & major osteoporotic fracture → guide treatment. 🔍 Key Comparisons Trabecular bone vs Cortical bone – Resorption rate: trabecular > cortical. Common fracture sites: trabecular → vertebrae; cortical → hip (femoral neck). Bisphosphonates vs Denosumab – Mechanism: bisphosphonates bind hydroxyapatite → inhibit osteoclasts; denosumab = anti‑RANKL antibody. Reversibility: bisphosphonate effect persists after stop; denosumab effect wanes quickly (risk of rebound fractures). Oral vs Intravenous Bisphosphonate – Onset: IV = faster bone resorption suppression. Adherence: oral requires strict dosing rules; IV avoids GI irritation. ⚠️ Common Misunderstandings “Calcium alone prevents fractures.” – Only modest benefit; must be paired with vitamin D and weight‑bearing exercise. “Only postmenopausal women get osteoporosis.” – Men develop it later; secondary causes (glucocorticoids, endocrine disease) affect both sexes. “DXA T‑score of −1.5 is safe.” – Osteopenia (−1 to −2.5) still carries fracture risk, especially with high FRAX score. 🧠 Mental Models / Intuition “Bone balance seesaw” – Imagine bone mass as a seesaw: osteoclasts on one side, osteoblasts on the other. Estrogen, OPG, and exercise add weight to the osteoblast side; aging, estrogen loss, PTH excess add weight to the osteoclast side. When the osteoclast side outweighs, the seesaw tips to net bone loss. “RANKL = accelerator, OPG = brake.” – Visualize traffic: RANKL pushes osteoclasts forward; OPG pulls the hand‑brake. 🚩 Exceptions & Edge Cases High‑dose calcium (>1 g/d) – May increase MI, stroke, kidney stones – not routinely recommended. Low‑dose vitamin D (<400 IU/d) – No fracture‑prevention benefit in vitamin‑D‑sufficient adults. Drug holidays – After 5 yr oral or 3 yr IV bisphosphonate in low‑risk patients, pause therapy to reduce atypical femur fracture risk. Renal insufficiency – Bisphosphonates contraindicated if eGFR <30 mL/min; consider denosumab (which is not cleared renally). 📍 When to Use Which Fragility fracture present → start bisphosphonate (or denosumab if contraindicated). T ≤ −2.5 without fracture → bisphosphonate if FRAX high; consider denosumab for renal impairment. Very high fracture risk (multiple fractures, FRAX >30 %) → anabolic agent (teriparatide or romosozumab) before anti‑resorptives. Contraindication to bisphosphonates (esophageal disease, poor adherence) → denosumab or IV bisphosphonate. 👀 Patterns to Recognize Spine + hip + wrist = classic “osteoporotic triad” → think osteoporosis when any appear after low‑impact fall. Elevated urinary C‑telopeptide + low BMD = active resorption → consider anti‑resorptive therapy. Sudden back pain in elderly → suspect vertebral compression fracture; order lateral spine X‑ray. 🗂️ Exam Traps “Calcium supplementation reduces fracture risk in all adults.” – Wrong; only modest effect and not in those with adequate intake. “T‑score of –1.8 = normal bone.” – Misclassification; falls in osteopenia range, still at risk. “All patients with osteoporosis need hormone replacement therapy.” – Incorrect; HRT only for those with menopausal symptoms and not first‑line solely for bone. “Bisphosphonates are contraindicated in mild renal impairment.” – Overstatement; caution below eGFR 30 mL/min, but not an absolute ban. --- Use this guide for quick recall right before your exam – focus on definitions, thresholds, and the “when‑to‑use” decision trees.