Aneurysmal bone cyst of the jaws. (II). Pathogenesis.

The pathogenesis of the aneurysmal bone cyst is the subject of much controversy. This study was undertaken to test the hypothesis that the aneurysmal bone cyst is a secondary phenomenon which occurs in a primary lesion of bone. Histological material from 303 bone lesions was studied for evidence of development of aneurysmal bone cyst. Microcysts and blood-filled spaces, similar to those seen in aneurysmal bone cysts, were observed in 15 of 54 central giant cell granulomas studied (28%). The frequency was lower (10%) in 49 cases of fibrous dysplasia and very low in ossifying and cementifying fibromas. 1 of 7 cases of Paget's disease of bone showed large blood spaces. It is postulated that the initiating process of the aneurysmal bone cyst is the microcyst, which forms as a result of intercellular oedema in a primary lesion with loose, unsupported stroma. Rupture of vessels into the microcysts introduces blood under haemodynamic pressure. With little resistance provided by the stroma, the blood spaces resorb the surrounding bone and lift the periosteum, which produces a thin shell of new bone. Similar histological features were observed in 6 of 42 fibrosarcomas examined (14%) and in 8 of a series of 75 osteosarcomas (11%). While many of these lesions did not present with clinical similarity to the aneurysmal bone cyst, it is postulated that this type of case may account for the so-called malignant aneurysmal bone cyst occasionally reported.