Radiology of postnatal skeletal development. II. The manubrium and sternum.

Thirty-six manubriosternal composites from skeletally immature cadavers were examined morphologically and radiographically. Sternebral ossification followed certain patterns. The manubrium (first sternebra) usually had one primary ossification center and one or two smaller centers. These usually were caudad to the major center (longitudinally bifid). The second sternebra invariably had only one ossification center. The third and fourth sternebrae had latitudinal (right-left) bifid ossification centers, undoubtedly a result of the original formation of the sternum from two longitudinal mesenchymal anlagen. Occasionally the fourth sternebra exhibited longitudinally bifid ossification. The usual pattern was ossification of four sternebrae, although a fifth was intermittently present. The xiphisternum (not a true sternebra) was infrequently ossified.