Calvarial defect in human anencephaly.

The calvarial defect in human anencephaly has been studied on 21 anencephalic skulls in all of which the defect extended into the vertebral column. Based on the boundaries of the defect, a nomenclature of the anomaly has been proposed that is simpler, more scientific, and more informative than previous ones because it expresses the precise extent of destruction of the cranial vault. In the present study, the defect was "fronto-occipito-vertebral" in in 71.4%, "occipito-vertebral" in 23.8%, and "parieto-occipito-vertebral" in 4.8% of cases. The morphologic details of the cranial bones bounding the calvarial defect have been described.