Pediatric Unicameral Bone Cyst

  • Etiology:
  • Imaging:
    — Location: Metaphysis, intramedullary
    — Lytic or blastic: Lytic with thin sclerotic margin, usually unilocular, dependent bony fragment is evidence of pathologic fracture (fallen fragment sign)
    — Matrix:
    — Zone of transition: Narrow
    — Periosteal New bone or cortical destruction: None
    — Additionally:
  • DDX:
  • Complications: Common cause of pathological fracture
  • Treatment:
  • Clinical: Most common in proximal humerus

Radiology Cases of Unicameral Bone Cyst

Radiograph of unicameral bone cyst of the ulna
AP (left) and lateral (right) radiographs of the forearm show a diaphyseal lesion that is expansile and lytic in appearance with a narrow zone of transition and no periosteal reaction.

Radiology Cases of Unicameral Bone Cyst With Pathologic Fracture

Radiograph and CT and MRI of unicameral bone cyst of the femur
AP radiograph of the hip (upper left) shows a metaphyseal lesion that is expansile and lytic in appearance with a narrow zone of transition and no periosteal reaction. There is a transversely oriented linear density within it representing a fallen fragment sign. Coronal CT without contrast of the hip (upper right) better shows the cystic nature of the lesion and the fallen fragment. Coronal (left lower) T2 MRI without contrast of the hip shows the cystic nature of the lesion and axial T2 MRI (right lower) shows fluid / fluid levels within the lesion due to hemorrhage.
Radiograph of pathologic fracture through a unicameral bone cyst
AP (above) and frog-leg (below) radiographs of the hips shows an intramedullary lesion in the left hip metaphysis with multiple locules and without periosteal new bone. A lucent defect is seen along its medial cortical surface on both views.