THE term “basilar impression,” often called “basilar invagination,” is generally used to designate different types of deformities involving the posterior cranial fossa and the upper cervical vertebrae. One of the most common causes of this deformity is an abnormal structure of the skeleton; for example, the decalcification of bones in rickets or Paget's osteitis deformans. Another type of “basilar impression” is seen in cases of long standing, increased intracranial pressure causing atrophy of the cranial capsule (hydrocephaly, oxycephaly). The third type corresponds to a congenital malformation of the atlanto-occipital region (“assimilation of the atlas”).2 The anatomic findings in the different types of “basilar impression” are variable. As a rule the weighted parts of the occiput, forming the frame of the foramen magnum, are pushed up toward the cranial cavity (therefore the term “basilar impression”). In addition, the petrous portion of the temporal bone is dislocated upward and turned axially, so that...