Findings: These are three images of the right wrist in a 13 year old female. There is an expansile lytic lesion involving the metaphysis of the distal ulna. The margins are well-defined and there are multiple internal septations. There is no discrete evidence for periostitis.

Diagnosis: Aneurysmal bone cyst.

Discussion: The aneurysmal bone cyst is an unusual entity which is most likely the result of trauma or a tumor induced process. In approximately one third of cases, the pre-existing lesion can be identified. The most common of these pre-existing lesions are giant cell tumors which account for approximately one fourth of aneurysmal bone cysts. Other possible underlying etiologies include osteoblastomas, angiomas, and chondroblastomas. The vast majority of patients present with pain and/or swelling. Patients are most typically under the age of 20 with the vast majority of aneurysmal bone cysts presenting in patients under the age of 30. Greater than 50% of aneurysmal bone cysts occur in long bones with approximately 12 to 30% occurring in the spine. Aneurysmal bone cysts of the flat bones are uncommon, of which about 50% occur in the pelvis.

Radiographic findings include an eccentric, lytic lesion, with an expanded, remodeled bony contour. Radiographs on occasion may show internal densities characteristic of chondroid matrix. In addition, internal septations are common. CT scanning is helpful in defining the lesion, especially in areas in which the bony anatomy is complex. MR imaging will generally demonstrate a well-defined, lobulated lesion with internal septations. In addition, multiple fluid fluid levels will be identified on MRI. The appropriate treatment of an aneurysmal bone cyst include identifying its specific pre-existing lesion. Treatment is generally directed at surgical removal of the entire lesion versus curettage with bone grafting.

Reference:
Kransdorf MJ. Aneurysmal Bone Cysts: Concepts, Controversy,
Clinical Presentation, and Imaging. AJR 1995;164:573-580.

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