Canalis basilaris medianus and related defects of the basiocciput.

Anatomic variants and developmental defects of the basiocciput are uncommon. The best known include transverse segmentation , longitudinal segmentation, "key-hole defect," and canalis basilaris medianus (CBM). Transverse segmentation [1-3] may be complete or incomplete. In the complete form the basiocciput is divided by a transverse fissure into an anterior part, sometimes called os basioticum, and a posterior part. In the incomplete form there is a transverse cleft in the lateral aspect of the basiocciput on one or both sides. In a case of unilateral cleft of the basiocciput reported by Kruyff [2] there was an associated partial absence of the basiocciputal-exoccipital synchondrosis on the same side. In longitudinal segmentation [3], a longitudinal fissure divides the basiocciput in two lateral segments. The "key-hole" or deltashaped ossification defect [4] consists of a deep cleft in the posterior margin of the basiocciput resembling a key-hole; the CBM [3, 5-16] is a channel through the basiocciput that may be complete or incomplete. This paper reports two presumed cases of the incomplete form of CBM defect with a review of the literature.