Unicameral/ solitary bone cyst
Incidence- 5% of primary bone lesions.
Etiology- - trauma (synovial entrapment at capsular reflection)
- Vascular anomaly (blockage of interstitial damage)
Age- 3- 19 yrs; occurs during active phase of bone growth.
- M: F = 3:1
Clinical presentation- asymptomatic unless fractured
Location- Proximal femur, proximal humerus, fibula, at base of calcaneal neck, talus, rarely in ribs, ileum, small bones of hand and feet.
Site- intramedullary centric metaphyseal,
- adjacent to epiphyseal cartilage ( during active phase)
- migrating into diaphysis with growth ( during latent phase)
- does not cross epiphyseal plate
Radiographic Features-
- 2-3 cm oval radiolucency with long axis parallel to long axis of host bone
- fine sclerotic boundary
- scalloping or erosion of inner cortical margin
- photopic area on bone scan ( if not fractured)
- “Fallen fragment sign†if fractured ( 20% cases) that is seen as centrally dislodged fragment falls into a dependent position.
Complication- pathologic fracture (65%)
D/D- Enchondroma (calcific stippling)
- Giant cell tumor
- Aneurysmal bone cyst ( eccentric)
- Chondroblastoma ( epiphyseal)
- Chondromyxoid fibroma ( more eccentric and expansile)