Cementum-like bone production in solitary bone cysts. (so-called "cementoma" of long bones). Report of three cases. Electron microscopic observations supporting a synovial origin to the simple bone cyst.

Three cases of simple bone cysts (S.B.C) in association with cementum-like bone production are reported. Analysis of our cases and the 2 previously reported in the literature as so-called cementomas has led us to conclude that the "cementoma" of long bone is not a distinct entity, but merely, a form of S.B.C. associated with a peculiar, poorly cellular form of bone which mimics tooth cementum by light microscopy only. Electron microscopic studies of this substance demonstrate collagen fibers and numerous matrix vesicles which form the initial sites of calcification. Matrix vesicles are a product of osteoblastic activity and are never found in the cementum of the tooth or oral cementum producing tumors. We also report the first ultramiscroscopic observations on the wall of the simple bone cyst and its lining. Two cell types constitute the lining, having features identical to those described for type A and type B synovial cells. Because of this new observation, we believe, the most reasonable explanation for the simple bone cyst is that it represents a congenital "rest" of synovial tissue displaced into the thin, cortical metaphyseal region of bones at the synovial-capsularbone reflection. Its benign nature and slow growth would explain its discovery in early childhood and the marked preponderance of its proximal humerofemoral location dependent upon the fact that these 2 bones have the largest area of capsular to metaphyseal bone reflection.