Ultrastructure of aneurysmal bone cyst.

The ultrastructure of 5 cases of primary aneurysmal bone cysts is described. The most frequent cells found are fibroblasts containing varying amounts of glycogen. Primitive mesenchymal cells in different stages of differentiation are seen. The foci of osteoid and bone originate through metaplastic changes of the spindle cell stroma. We feel that the fibroblastic proliferation and bone formation are manifestations of a reactive process. The giant cells are similar to those of other giant cell lesions of bone. A case of cystic chondroblastoma with secondary aneurysmal bone cyst features was studied. It shows, in addition to chondroblastoma cells, elongated cells similar to the reactive fibroblasts of primary aneurysmal bone cyst. We feel that aneurysmal bone cyst is a benign non-neoplastic condition that can develop in bone as a primary bone lesion; it may also be associated with a pre-neoplastic condition that can develop in bone as a primary bone lesion; it may also be associated with a pre-existing bone condition, and occasionally develops following trauma with subperiosteal hematoma.