Osteomalacia
Aetiology
Summary
Think: vitamin D, phosphate, renal, drugs, rare tumour / genetic.
- ↓ Vitamin D:
- Nutritional deficiency, low sunlight exposure
- Malabsorption (coeliac, IBD, bariatric surgery)
- Cholestatic liver disease (↓ bile salts → impaired Vit D absorption)
- Renal:
- CKD with phosphate retention, ↓ 1α-hydroxylase → ↓ active Vit D (overlaps with renal osteodystrophy)
- Phosphate-wasting:
- Tumour-induced osteomalacia
- Hereditary hypophosphataemic rickets/osteomalacia
- Fanconi syndrome (proximal RTA)
- Drugs:
- Anticonvulsants (enzyme-inducing → ↑ Vit D metabolism)
- Long-term anticonvulsants / antiresorptives can predispose to insufficiency fractures
Pathophysiology
- Normal osteoid formation but impaired mineralisation (↓ Ca/PO₄, or both).
- Bone becomes soft → deformity + microfractures:
- Insufficiency fractures → Looser zones
- Bone pain, muscle weakness, waddling gait
- Biochemically often:
- ↑ ALP, low/normal Ca, low/normal PO₄ (pattern depends on cause)
- Abnormal Vit D / PTH.
Imaging
Summary
Main imaging pattern = generalised osteopenia + Looser zones ± deformity.
Early disease can be normal on radiographs.
Plain radiographs
General features
- Diffuse osteopenia:
- Thinned cortices
- Coarser, sparse trabeculae
- Bones may appear “washed out”
- Deformities in long-standing cases:
- Bowing of long bones
- Biconcave “codfish” vertebrae
- Pelvic deformity (e.g. protrusio acetabuli)
Looser zones (pseudofractures) – key sign
- Short, transverse or oblique lucent lines, usually:
- Perpendicular to the cortex
- Do not extend fully across bone
- Often with sclerotic margins
- Represent incomplete insufficiency fractures through unmineralised osteoid.
- Classic locations:
- Medial femoral neck
- Pubic rami
- Lateral scapula
- Ribs (axillary margins)
- Medial aspect of proximal femur / femoral shaft
- Ulna, fibula
- Often bilateral and symmetric.
Spine
- Generalised vertebral osteopenia
- Biconcave “codfish” vertebrae (endplate concavity)
- Insufficiency fractures (wedge/compression) in severe disease.
Pelvis and lower limbs
- Osteopenia + Looser zones in pubic rami, femoral neck, sacrum
- Long-standing disease:
- Coxa profunda / protrusio acetabuli
- Bowing deformity of long bones (esp. femur, tibia).
Skull
- Usually less dramatically involved than in hyperparathyroidism:
- Osteopenia, loss of sharp trabeculae
- No classic salt-and-pepper pattern.
CT
- More sensitive for:
- Insufficiency fractures and Looser zones
- Cortical thinning and subtle deformity
- Helps exclude other focal lesions (e.g. metastasis) when Looser zones are atypical.
MRI
- Looser zones / insufficiency fractures:
- Linear low-signal line on T1, with surrounding high T2/STIR oedema
- Enhancement of fracture line and adjacent bone marrow after contrast
- Non-specific marrow signal changes from osteopenia and microfractures.
- Very sensitive for early insufficiency fractures when X-ray still normal.
Nuclear medicine (bone scan)
Tip
Classic: hot Looser zones in a background of increased general bone turnover.
- Increased uptake along Looser zones (linear, symmetric, often multiple)
- Generalised increased skeletally uptake possible (high turnover osteomalacia)
- Very sensitive to detect number and distribution of pseudofractures.
Key differentials
Osteomalacia vs osteoporosis
| Feature | Osteomalacia | Osteoporosis |
|---|---|---|
| Core problem | Defective mineralisation of osteoid | Reduced quantity of normal bone |
| Lab tests | Often abnormal (Vit D, Ca/PO₄, ↑ ALP, ± PTH) | Usually normal biochemistry |
| Radiographs | Osteopenia + Looser zones, deformity | Osteopenia; no Looser zones |
| Vertebral changes | Codfish vertebrae, fractures | Wedge/compression fractures common |
| Bone pain | Diffuse bone pain, proximal myopathy | Often asymptomatic until fracture |
Exam hook: presence of Looser zones + biochemical abnormalities → favour osteomalacia over simple osteoporosis.
Osteomalacia vs renal osteodystrophy
| Feature | Osteomalacia | Renal osteodystrophy |
|---|---|---|
| Clinical context | Various causes, not necessarily CKD | CKD/dialysis almost always present |
| Key imaging sign | Looser zones, osteopenia | ROD pattern: subperiosteal resorption, rugger-jersey spine, salt-and-pepper skull, soft-tissue calcifications |
| Vascular calcification | Not prominent | Very common (vascular & periarticular) |
| Brown tumours | No | Can be present (secondary/tertiary HPT) |
Remember: renal osteodystrophy often includes a low-turnover osteomalacia component, so overlap exists.
Exam nuggets
Summary
Buzzwords:
- Generalised osteopenia
- Looser zones / pseudofractures
- Insufficiency fractures (pelvis, femoral neck, ribs)
- Bone pain, proximal myopathy, Vit D deficiency / phosphate wasting.
- Typical patient:
- Adult with bone pain, muscle weakness, difficulty walking
- Low Vit D or phosphate-wasting disorder.
- Looser zones:
- Incomplete, transverse lucencies with sclerotic margins
- Bilateral and symmetric, classic in medial femoral neck, pubic rami, ribs.
- Bone scan:
- Markedly increased uptake in pseudofractures.
- Early disease:
- Radiographs can be normal → bone scan or MRI may show changes earlier.
- Tumour-induced osteomalacia:
- Consider when severe osteomalacia + isolated hypophosphataemia + high FGF23
- Small mesenchymal tumours (often extremities, craniofacial); role for whole-body MRI / PET in localisation.
