Diagnostic Studies. A radiolucent lesion with a ballooned expansion of the bone cortices (“finger in balloon”) is the radiographic hallmark of an aneurysmal bone cyst. Although some lesions appear to have an aggressive, expansile quality, they remain contained by a thin rim of reactive periosteal bone. In children, these benign lesions seldom penetrate the articular surface of a joint or the growth plate; therefore, evidence of growth plate penetration by an aneurysmal bone cyst indicates the need for careful staging studies to rule out malignancy. The radiographic differential diagnosis includes simple bone cyst (see Plate 6-12), giant cell tumor of bone (see Plate 6-13), telangiectatic sarcoma, and angiosarcoma (see Plate 6-27).

Bone scans show intense radioisotope uptake in the margin of the lesion. MRI and CT are used to depict the precise anatomic extent and density of the lesion, and especially the thin, limiting margin of reactive bone (which is not well visualized on plain radiographs). A fluid-fluid level seen on the MR image generally confirms the diagnosis and represents the separation between the red cells and serum in the bloody cavity. Other tumors can, however, have fluid-fluid levels and aneurysmal bone cyst–like qualities.

Histologically, the proliferative lining tissue of an aneurysmal bone cyst is often difficult to distinguish from that of a giant cell tumor of bone. It contains a mixture of benign stromal tissue, giant cells, and large amounts of hemosiderin. The tissue usually contains large vascular lacunae lined with giant cells and filled with clotted blood that resembles cranberry sauce. When these histologic features also occur in other lesions (e.g., chondroblastoma, osteoblastoma, osteosarcoma, eosinophilic granuloma, nonossifying fibroma) they are called secondary aneurysmal bone cysts. Because of the preponderance of giant cells, many aneurysmal bone cysts are initially thought to be giant cell tumors of bone, and there is a continuum between giant cell tumors of bone and aneurysmal bone cysts.

Treatment/Prognosis. The majority of active aneurysmal bone cysts are treated with curettage and bone grafting; the recurrence rate, however, is 20% to 30%.

After incision of an active cyst, an alarming amount of bleeding may occur until the lining is completely removed. After complete excision of the cyst this bleeding generally decreases. Occasionally, however, there may still be brisk bleeding emanating directly from the bony wall, which can be controlled with coagulation. Curettage may be augmented with cementation with methyl methacrylate, which appears to reduce the risk of recurrence or bone graft, which allows for healing and for a recurrence to be visualized earlier. Lesions in the pelvis and spine have a higher risk of recurrence, where complete surgical exposure and removal are more difficult. Nevertheless, the prognosis for primary aneurysmal bone cyst is excellent and recurrences can usually be managed with more aggressive curettage or excision.