Anatomical changes in occipitalization: is there an increased risk during the standard posterior approach?

Abstract

Purpose: The purpose of this study was to examine the anatomic changes in a case of occipitalization of the atlas.

Methods: Occipitalization of the atlas was found accidentally in a 64-year-old male cadaver. Anatomic dissection was carried out to examine the posterior aspect of the upper cervical region and craniocervical junction. The occipitalized atlas was then harvested and macerated to study the bony anomaly.

Results: In this case of occipitalization, fusion was observed at both lateral masses and at the left posterior hemiarch of the atlas. We found the following soft tissue changes: the rectus capitis posterior minor muscle was lacking on the left side and was atrophic on the right, the obliquus capitis superior muscle was present on both sides showing moderate atrophy and fatty changes. The posterior atlanto-axial membrane was thinner and asymmetric, had a free edge on the right side. Lateral to this edge the dura was lying free. We believe that these changes of the posterior atlanto-axial membrane together with the increased distance between the fused posterior arch of the atlas and the lamina of the axis could cause the observed "dura bulge" at this level. The vertebral artery was entering the skull through a canal on the left side.

Conclusions: In our case, occipitalization considerably changed the anatomy of the upper cervical spine and craniocervical junction. Special care must be taken when using the posterior approach to avoid neurovascular injury in cases with occipitalization.

Fig. 1

Posterior aspect of the upper cervical spine is demonstrated from dorsal. The tips of the two hemiarches of the atlas are connected by a bridge of connective tissue (*). Between the hemiarches of the atlas and the lamina of the axis spans the posterior atlanto-axial membrane forming a free edge on the right side (arrow). The C2 nerve root, its ganglion, ventral and dorsal ramuses and the vertebral artery (arrowheads) show no deviation from normal. The obliquus capitis superior muscle was preserved on both sides

Fig. 2

Photographs demonstrating the occipitalized atlas vertebra from different aspects. Macerated bone specimen. a posterior, b anterior, c caudal view, demonstrating the occipitalized atlas and the foramen magnum, d “internal view”, demonstrating the foramen magnum from cranial

Fig. 3

Pictures a and b illustrate the right and left side of the bony specimen. On the right side (picture a), the vertebral artery entered the skull the usual way. Picture b demonstrates the canal formed between the occiput and atlas through which the vertebral artery enters the skull