Simple Bone Cyst
Overview
- Benign, fluid-filled medullary bone lesion
- Usually monostotic and centrally located
- Often discovered incidentally or after pathologic fracture
Epidemiology
- Age: 5–20 years (rare in adults)
- Male > Female (2–3:1)
- Common sites: proximal humerus and proximal femur (metaphysis)
Pathophysiology
- Etiology unclear — theories include venous obstruction or fluid accumulation
- Lined by fibrous membrane, filled with serous fluid
- Often migrates from metaphysis toward diaphysis with skeletal growth
Clinical Features
- Often asymptomatic
- May present with mild pain or pathologic fracture
Imaging Features
X-ray
- Centrally located, well-defined, lytic lesion in metaphysis
- No matrix calcification
- Thin sclerotic margin
- Cortex may be thinned but usually intact unless fractured
- May cause pathologic fracture, classically with:
→ Fallen fragment sign = cortical bone fragment lying dependently within cyst
CT
- Confirms fluid density and thin cortex
- Helps exclude internal matrix or septations (which may suggest other entities)
MRI
- T1: Low signal
- T2: High signal (fluid)
- No solid enhancement; mild rim enhancement possible
Differential Diagnosis
| Diagnosis | Distinguishing Features |
|---|---|
| Aneurysmal bone cyst | Expansile, eccentric, fluid-fluid levels, septations |
| Fibrous dysplasia | "Ground-glass" matrix, expansion, no fluid, common in ribs/femur |
| Non-ossifying fibroma | Eccentric, multiloculated, metaphyseal, sclerotic rim |
| Eosinophilic Granuloma | Often painful, aggressive-appearing lysis in younger children |
Management
- Observation if asymptomatic and non-fractured (may resolve with skeletal maturity)
- Steroid injection (methylprednisolone) or bone marrow injection
- Curettage and bone grafting if recurrent or symptomatic
- Surgical fixation if associated with fracture
Pearls
- Fallen fragment sign is pathognomonic if present
- Tends to resolve after skeletal maturity
- Proximal humerus is most common site
- Exam Ddx of lucent lesion in the calcaneus
