Metabolic Bone Diseases

Osteoporosis - Bones Gone Brittle

• Definition: ↓Bone mass/density, normal mineralization; microarchitectural decay, ↑fracture risk.

• Types:

  • Primary: Postmenopausal (Type I - rapid trabecular loss), Senile (Type II - cortical/trabecular loss).
  • Secondary: Corticosteroids (commonest drug cause), endocrine (hyperparathyroidism, Cushing's), immobilization, myeloma, alcohol.

• Pathophysiology: ↑Osteoclast activity relative to osteoblast activity.

• Clinical: Asymptomatic until fracture. Sites: Vertebrae (compression), hip, distal radius (Colles').

• Radiographic Features:

  • Osteopenia (radiolucency), cortical thinning.
  • Prominent primary, lost secondary trabeculae.
  • Vertebrae: "Codfish" (biconcave), wedge, compression fractures.
  • Singh Index: Grade < 3 (femoral neck).

• Diagnosis:

  ⭐ DEXA (gold standard): T-score ≤ -2.5 SD (osteoporosis). T-score between -1.0 and -2.5 SD (osteopenia).

Rickets & Osteomalacia - Soft Bones Saga

*Defective mineralization of bone (osteomalacia) & growth plate cartilage (rickets).

• Etiology: Vitamin D deficiency (most common) → ↓Ca absorption. Or phosphate deficiency.
• Pathophysiology: ↓Vit D → ↓Serum Ca & ↓Serum P (eventually) → ↑PTH → ↑bone resorption.
  • Labs: ↓Serum 25(OH)D, ↓Ca (or normal), ↓P, ↑ALP (marked), ↑PTH.
• Rickets (Children - open physes):
  • Clinical: Craniotabes, rachitic rosary, Harrison's sulcus, limb bowing (genu varum/valgum).
  • X-ray: Metaphyseal widening, fraying, cupping, splaying. Growth plate widening.
• Osteomalacia (Adults - closed physes):
  • Clinical: Bone pain, proximal muscle weakness, fractures.
  • X-ray: Osteopenia, indistinct trabeculae, biconcave (codfish) vertebrae.

⭐ Looser zones (pseudofractures) are pathognomonic for osteomalacia: incomplete, transverse radiolucencies perpendicular to cortex (e.g., scapula, ribs, pelvis, proximal femur).

Hyperparathyroidism - Calcium Chaos Crew

• Excess PTH: ↑Serum Ca, ↓Serum P (or normal/↑ in secondary/tertiary), ↑ALP.
• Types:
  • Primary: Adenoma (~80%), hyperplasia, carcinoma.
  • Secondary: CKD (most common), Vit D deficiency.
  • Tertiary: Autonomous PTH after prolonged secondary.
• Mechanism: ↑PTH → ↑Osteoclast activity → ↑Bone resorption.
• 📌 Classic: "Bones, stones, abdominal groans, psychic moans."
• Radiological Features (Osteitis Fibrosa Cystica - OFC):
  • Subperiosteal resorption (pathognomonic): Radial aspect of middle phalanges, distal clavicles.
  • Brown tumors (lytic lesions; osteoclasts & hemosiderin).
  • "Salt & pepper" skull.
  • Osteopenia/Osteoporosis.
  • Chondrocalcinosis.

⭐ Subperiosteal bone resorption on the radial aspect of the 2nd and 3rd middle phalanges is a hallmark of hyperparathyroidism.

Paget's Disease - Mixed-Up Remodeling

• Chronic disorder: excessive bone resorption, disorganized ↑ new bone.
• Phases:
  • Lytic (osteoclasts ↑): "blade of grass/flame sign" (long bones); osteoporosis circumscripta (skull).
  • Mixed: lytic + blastic activity.
  • Sclerotic (osteoblasts ↑): thickened cortices/trabeculae; "cotton wool" skull.
• Clinical: Often asymptomatic. Bone pain, deformity (bowing), fractures.
• Labs: ↑ ALP; normal Ca, PO4. ↑ Urine hydroxyproline.
• Imaging:
  • Pelvis: iliopectineal line thickening (brim sign), protrusio acetabuli.
  • Spine: "picture frame/ivory" vertebra.
  • Long bones: cortical thickening, bowing, fissure (banana) fractures.
• Complications: Pathological fractures, deafness, nerve compression, high-output CHF, osteosarcoma (<1%).

⭐ Most common site for Paget's sarcoma is the femur, followed by pelvis and humerus (in decreasing order of frequency).

High‑Yield Points - ⚡ Biggest Takeaways

• Osteoporosis: DEXA T-score ≤ -2.5; commonest cause of vertebral compression fractures.
• Rickets/Osteomalacia: Vitamin D deficiency leads to Looser's zones (pseudofractures) and cupped/frayed metaphyses in children.
• Primary Hyperparathyroidism: Subperiosteal resorption (pathognomonic), brown tumors, salt-and-pepper skull, osteitis fibrosa cystica.
• Paget's Disease: Presents with lytic and sclerotic phases, cotton wool skull, and blade of grass sign.
• Scurvy: Vitamin C deficiency causes subperiosteal hemorrhages and Wimberger's ring sign.