The occipital bone in type B basilar invagination

Introduction Basilar invagination (BI) is a multifactorial abnormality of the craniovertebral junction. The embryological complexity of the occipital and cervical spine, combined with diverse etiological factors, indicates a great challenge to understand the various mechanisms involved in BI development. Objective To present the morphology of the occipital bone in a skull with BI type B. Methods This is a brief description of the anatomy of the occipital bone in the type B BI through macerated skulls. Two right hemicraniums (1 with BI and 1 control) of adults (age unknown) and a skull base of a fetus with 20 weeks were used. Parts of the occipital bone were marked to illustrate the more hypoplastic components in BI. Results The occipital bone shows four main bony components: basioccipital (lower clivus), right and left exoccipital (lateral and condylar parts), and the supraoccipital part of the occipital squama. The BI type B presented severe hypoplasia of the basioccipital and exoccipitals, which make the appearance of the foramen magnum more inclined. The petrous parts (petromastoid) of the temporal bone followed the process of occipital invagination. Conclusion The occipital bone showed a complex hypoplasia in the anterior and lateral region of the foramen magnum, with an evident tendency to clivus shortening and hypoplastic occipital condyle.