Metabolic Bone Disease

🦴 Metabolic Bone Disease

Osteoporosis, osteomalacia, hyperparathyroidism, and renal osteodystrophy — biochemistry, radiology, and management for MRCS.

Metabolic Bone Disease — Overview

Metabolic bone diseases represent disorders of bone quality or quantity resulting from systemic disturbances in calcium, phosphate, vitamin D, or parathyroid hormone (PTH) metabolism. They share overlapping biochemistry but have distinct pathologies, X-ray findings, and treatments that are high-yield for MRCS.

The Four Conditions at a Glance

Osteoporosis

Reduced bone quantity — normal mineralisation

Core defectDecreased bone mass with structurally normal mineralisation — osteoclast activity exceeds osteoblast activity

BloodsCa, PO₄, ALP all normal

X-rayGeneralised radiolucency, vertebral wedge/crush fractures, Codfish vertebrae

DiagnosisDEXA scan: T-score < −2.5

TreatmentBisphosphonates (alendronate), calcium + vitamin D supplementation

Osteomalacia / Rickets

Defective mineralisation of osteoid

Core defectUnmineralised osteoid accumulates due to vitamin D deficiency (or phosphate deficiency)

BloodsLow Ca, low PO₄, raised ALP, low 25-OH vitamin D

X-rayLooser zones (pseudofractures), bowing of long bones (rickets)

DiagnosisClinical + biochemistry + X-ray; 25-OH vitamin D level

TreatmentVitamin D replacement (colecalciferol); treat underlying cause

Hyperparathyroidism

Osteitis fibrosa cystica — PTH-driven resorption

Core defectExcess PTH → osteoclast activation → bone resorption → fibrous replacement (osteitis fibrosa cystica)

BloodsHigh Ca, low PO₄, raised ALP, raised PTH

X-raySubperiosteal erosions (radial side middle phalanges), “brown tumours”, pepper-pot skull

DiagnosisPTH + calcium; USS / sestamibi scan to localise adenoma

TreatmentPrimary: parathyroidectomy. Cinacalcet (calcimimetic) if not surgical

Renal Osteodystrophy

CKD — mixed picture of all the above

Core defectFailing kidney cannot activate vitamin D or excrete phosphate → secondary hyperparathyroidism + osteomalacia

BloodsLow Ca, high PO₄, raised PTH, low 1,25-(OH)₂D, raised ALP

X-rayMixed: subperiosteal erosions + Looser zones + osteosclerosis (“rugger jersey spine”)

DiagnosisCKD context + biochemistry panel

TreatmentPhosphate binders, activated vitamin D (alfacalcidol), treat CKD

The Surgical Connection

Metabolic bone disease is directly relevant to surgery in multiple ways: osteoporotic fragility fractures are the most common surgical emergency in the elderly (hip fracture); hyperparathyroidism may be encountered as an incidental hypercalcaemia on pre-operative bloods; and post-thyroidectomy or post-parathyroidectomy hypocalcaemia represents an acute surgical complication. Renal osteodystrophy matters in patients undergoing renal transplant or vascular access surgery.

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