Metabolic Bone Diseases

Bone Physiology & Metabolism - Skeleton's Blueprint

• Cells: Osteoblasts (form bone, secrete osteoid), Osteoclasts (resorb bone), Osteocytes (mechanosensors). 📌 Blasts Build, Clasts Chew.
• Matrix: Organic (Collagen Type I), Inorganic (Hydroxyapatite $Ca_{10}(PO_4)_6(OH)_2$).
• Hormones:
  • PTH: ↑ Serum Ca, ↑ resorption (via RANKL).
  • Vit D (Calcitriol): ↑ Ca/PO4 absorption, mineralization.
  • Calcitonin: ↓ Serum Ca, inhibits osteoclasts (minor).
• RANKL/OPG System: RANKL activates osteoclasts; OPG (decoy receptor) inhibits.

⭐ OPG (Osteoprotegerin) is a decoy receptor for RANKL, preventing osteoclast activation and reducing bone resorption.

Osteoporosis - Fragile Framework

• Definition: Systemic skeletal disease with ↓ bone mass & microarchitectural deterioration of bone tissue, leading to ↑ bone fragility & fracture risk. Mineralization is normal.
• Types & Causes:
  • Primary: Postmenopausal (Type I - rapid loss, trabecular bone), Senile (Type II - slower loss, cortical & trabecular).
  • Secondary: Glucocorticoids (most common drug-induced), alcohol, smoking, immobilization, endocrine disorders (e.g., hyperparathyroidism, hyperthyroidism), malnutrition (Ca/Vit D deficiency).
• Pathogenesis: Imbalance between bone resorption (↑ osteoclast activity) & formation (↓ osteoblast activity). Estrogen deficiency → ↑RANKL, ↓OPG.
• Clinical Features: Often asymptomatic until fracture. Common sites: vertebrae (compression fractures → height loss, kyphosis), femoral neck, distal radius (Colles' fracture).
• Diagnosis:
  • Dual-energy X-ray absorptiometry (DEXA): T-score ≤ -2.5 SD.
  • (Osteopenia: T-score between -1.0 and -2.5 SD).
  • Serum calcium, phosphate, and alkaline phosphatase (ALP) are typically normal.
• Morphology: Thinned cortex, reduced number & thickness of trabeculae, especially horizontal trabeculae, leading to loss of interconnectivity.

⭐ Vertebral compression fractures are the most common clinical manifestation of postmenopausal osteoporosis due to predominant loss of trabecular bone.

Rickets & Osteomalacia - Bendy Bone Blues

• Defective bone mineralization: Rickets (children; affects growth plates & osteoid), Osteomalacia (adults; affects osteoid).
• Etiology:
  • Vitamin D deficiency (↓intake, ↓sunlight, malabsorption).
  • Impaired Vitamin D metabolism (liver/kidney disease, certain drugs).
  • Phosphate depletion (e.g., renal tubular acidosis, Fanconi syndrome).
• Pathophysiology: ↓Active Vit D → ↓Intestinal $Ca^{2+}$ & $PO_4^{3-}$ absorption → ↓Serum $Ca^{2+}$ → ↑PTH → ↑Bone resorption & ↓Renal $PO_4^{3-}$ reabsorption → Mineralization defect.
• Labs: ↓Serum 25(OH)D, ↓/$Ca^{2+}$ (or normal), ↓Serum $PO_4^{3-}$, ↑Alkaline Phosphatase (ALP), ↑PTH.
• Rickets: Craniotabes, rachitic rosary, pigeon chest (pectus carinatum), Harrison's sulcus, bowing of legs (genu varum/valgum), widened epiphyses (cupping, fraying).
• Osteomalacia: Diffuse bone pain, muscle weakness, pathologic fractures, pseudofractures (Looser's zones).

⭐ Looser's zones (pseudofractures) are characteristic radiologic findings in osteomalacia, appearing as transverse radiolucent bands, often bilateral and symmetrical, perpendicular to the bone cortex (e.g., femoral neck, pubic rami).

Hyperparathyroidism & Paget's Disease - Chaotic Construction

• Hyperparathyroidism (HPT): Excess PTH → ↑bone resorption.
  • Primary: Adenoma. Labs: ↑Ca²⁺, ↓PO₄³⁻, ↑PTH, ↑ALP.
  • Secondary: CRF, Vit D def. Labs: ↓/N Ca²⁺, ↑PO₄³⁻ (CRF), ↑PTH, ↑ALP.
  • Bone: Osteitis fibrosa cystica (OFC), brown tumors, subperiosteal resorption, "salt & pepper" skull.
  • 📌 "Stones, bones, groans, moans."
• Paget's Disease (Osteitis Deformans): Chaotic bone remodeling; ?Paramyxovirus, SQSTM1.
  • Phases: Lytic → Mixed → Sclerotic (woven bone).
  • Clinical: Bone pain, deformity (↑hat size), fractures, deafness.
  • Labs: Markedly ↑ALP; Ca²⁺ & PO₄³⁻ normal.
  • X-ray: Osteoporosis circumscripta, "cotton wool" skull, "blade of grass" sign.
  • Histo: Mosaic/jigsaw pattern (cement lines).
  • Complication: Osteosarcoma (<1%).

  ⭐ Isolated, markedly elevated serum ALP is a hallmark of Paget's disease.

Renal Osteodystrophy - Kidney's Skeletal Storm

• Bone disease in CKD from dysregulated $Ca^{2+}$, $PO_4$, Vitamin D, and PTH.
• Key driver: Secondary hyperparathyroidism (↑PTH) due to ↓active Vit D, ↑$PO_4$, ↓$Ca^{2+}$.
• Spectrum:
  • Osteitis fibrosa cystica (OFC): High-turnover state with marrow fibrosis.
  • Adynamic bone disease: Low-turnover state, common with dialysis.
  • Osteomalacia: Low-turnover, impaired mineralization due to Vit D deficiency.

⭐ Classic X-ray signs: "Rugger jersey spine", subperiosteal resorption (e.g., phalanges). Brown tumors (cysts) in severe OFC.

High‑Yield Points - ⚡ Biggest Takeaways

• Osteoporosis: Reduced bone mass, normal mineralization; vertebral crush fractures classic.
• Rickets/Osteomalacia: Defective mineralization from Vitamin D deficiency; Looser's zones, bowing legs.
• Primary Hyperparathyroidism: ↑PTH causes osteitis fibrosa cystica (brown tumors), "stones, bones, groans".
• Paget's Disease (Osteitis Deformans): Disordered bone remodeling; mosaic pattern histology, ↑ALP, risk of osteosarcoma.
• Renal Osteodystrophy: Bone changes in CRF, includes secondary HPT, osteomalacia.
• Fibrous Dysplasia: Ground-glass X-ray; McCune-Albright syndrome (polyostotic, café-au-lait spots).