Basilar impression and platybasia in Paget's disease.

Abstract

Basilar impression (known also as basilar invagination) and platybasia are not synonymous nor need they necessarily be coexistent simultaneously or otherwise. Their only relationship lies in the fact that both represent specific types of abnormality of shape of a portion or portions of the base of the skull which under certain conditions may coexist. Basilar impression is related to the entire foramen magnum region of the floor of the posterior fossa, while platybasia is related to the floors of the anterior and middle cranial fossae and involves only that portion of the floor of the posterior cranial fossa which extends from the dorsum sellae to the anterior margin of the foramen magnum. Accordingly, basilar impression may be defined as an abnormal cephalad invagination of the foramen magnum region as determined by the relation of the odontoid process to Chamberlain's line (1), which is drawn from the posterior margin of the hard palate to the posterior margin of the foramen magnum on the lateral roentgenogram (Figs. 1 and 2). (McGregor (2) draws a line from the posterior margin of the hard palate to the lowermost point of the occipital bone.) If the odontoid process is significantly above Chamberlain's line, basilar impression is present. In addition, a frontal view (3) may show asymmetry of the posterior cranial fossa and foramen magnum; upward displacement of the atlas as well as the axis; an abnormal relationship of the atlas and odontoid process to the lumen of the foramen magnum; abnormality of the atlanto-occipital articulations; an unusual shape and asymmetry of the petrous portions of the temporal bones, and a broadened biparietal measurement (biasterionic diameter). The lateral view (3) may reveal a rudimentary development of the occipito-atlanto-axial region; upward and forward dislocation of the atlas and axis; upward displacement of the clivus; invagination of the posterior rim of the foramen magnum; high position of the petrous portions of the temporal bones, and a relatively long and vertically narrow appearance of the cranial cavity. While one or more of these abnormalities may be noted in anyone case, their existence is not reliably constant in all cases, so that the only accurate index is a significant projection of the odontoid process above Chamberlain's (or McGregor's) line. Platybasia is an abnormal obtuseness of the relationship of two specific segments of the cranial floor. The first segment is represented by a line (Figs. 1 and 2) which extends from the nasion to the center of the sella turcica, and the second segment by a line extending from the latter point to the anterior margin of the foramen magnum on the lateral roentgenogram. The inferior angle subtended by these two lines is termed the basal or sphenoid angle. This angle represents the fundamental shape of the base of the skull only as far posterior as the anterior margin of the foramen magnum.