This adolescent tripped and fell on his outstretched hand, suffering a fracture through a lytic bone lesion. The differential is between an aneurysmal bone cyst, osteoblastoma, or unicameral bone cyst.
ABCs are benign-looking lytic tumor-like bone lesion of uncertain aetiology, 80% occuring in the less than 20 years of age, which consist of blood-filled spaces of variable size that are separated by connective tissue containing trabeculae of bone or osteoid tissue and osteoclast giant cells. They are not lined by endothelium. Although often primary, ABCs may also occur secondary to an underlying lesions (e.g chondroblastoma, fibrous dysplasia, giant cell tumor (GCT), osteosarcoma).
They are typically located in the metaphysis of long bones, adjacent to an unfused growth plate, although they have been described in many locations:
* Lower limb (40%)
* Upper limb (21%)
* Spine (16%)
* Pelvis and sacrum (12%)
* Clavicle and ribs (5%)
* Skull and jaw(4%)
Plain films demonstrate sharply defined, osteolytic lesions typically of the metaphysis, adjacent to a growth plate. They can be expansile, and often demonstrate fluid - fluid levels on CT and MRI, although this later finding is of little use as both benign and maligant lesions can demonstrate this finding (giant cell tumors (GCT) and telangiectatic osteosarcomas). In the spine they are usually of the posterior elements.
They can be the site of pathological fractures.
Traditionally these lesions have been treated operatively (curettage and bone grafting) with a recurrence rate of between 11.8% and 30.8%. Percutanous treatment with fibrosing agents has also been performed.
1. V Topouchian “Aneurysmal Bone Cysts in Children: Complications of Fibrosing Agent Injection” Radiology 2004;232:522
2. Primary Tumors of the Spine:Radiologic-Pathologic Correlation, Archives of the AFIP 
4. TM Hudson “Fluid levels in aneurysmal bone cysts: a CT feature” American Journal of Roentgenology, Vol 142, Issue 5, 1001-1004
Credit: Dr Frank Gaillard