= UNICAMERAL / SIMPLE BONE CYST

Frequency: up to 5% of primary bone lesions

Etiology: ? trauma (synovial entrapment at capsular reflection), ? vascular anomaly (blockage of interstitial drainage)

Histo: cyst filled with clear yellowish fluid often under pressure, wall lined with fibrous tissue + hemosiderin, giant cells may be present

Age: 3–19 years (80%); occurs during active phase of bone growth; M÷F = 3÷1

• asymptomatic, unless fractured

Location: proximal femur + proximal humerus (60–75%); fibula; at base of calcaneal neck (4%, >12 years of age); talus; rare in ribs, ilium, small bones of hand + feet; NOT in spine / calvarium; solitary lesion

Site: intramedullary centric metaphyseal, adjacent to epiphyseal cartilage (during active phase) / migrating into diaphysis with growth (during latent phase), does not cross epiphyseal plate

• 2–3 cm oval radiolucency with long axis parallel to long axis of host bone
• fine sclerotic boundary
• scalloping + erosion of internal aspect of underlying cortex
• photopenic area on bone scan (if not fractured)
• “fallen fragment” sign if fractured (20%) = centrally dislodged fragment falls into a dependent position

Prognosis: mostly spontaneous regression

Cx: pathologic fracture (65%)

DDx:

1. Enchondroma (calcific stipplings)
2. Fibrous dysplasia (more irregular lucency)
3. Eosinophilic granuloma
4. Chondroblastoma (epiphyseal)
5. Chondromyxoid fibroma (more eccentric + expansile)
6. Giant cell tumor
7. Aneurysmal bone cyst (eccentric)
8. Hemorrhagic cyst
9. Brown tumor