Some Rare Osseous Variations of the Cranio-Cervical Junction

Introduction: The cranio-cervical junction is a developmentally restless region, where several osseous variations can occur. Such traits are of clinical importance because they may cause symptoms or differential diagnosis-related problems. Furthermore these abnormalities lead to a markedly higher vulnerability of the cranio-cervical junction due to an increased mobility or stiffness. Principally these variations can be classified into atlas-assimilations and manifestations of the occipital vertebra. In the present contribution some elected entities will be presented.

Material and Method: Three hundred eighty specimens of the cranio-cervical junction were investigated with a thorough maceration technique.

Results: From the large number of abnormal findings some rare and interesting cases are selected for the contribution. The isolated arcus prebasioccipitalis represents a horseshoe-like osseous element which is formed, if the whole hypochordal blastema of the proatlas persists. The supernumerary element rests on the apex of the dens axis like a “Napoleonshut,” and it fits exactly between the anterior margin of the foramen occipitale magnum and the anterior arch of the atlas where it forms articulation facets. The diagnosis of an arcus prebasioccipitalis can be assumed in the arcus prebasioccipitalis x-ray, because it projects into the otherwise free space between the anterior border of the foramen occiptale magnum and the anterior arch of the atlas. The diagnosis can be ensured by conventional tomography or CT (Prescher et al, 1996). The arcus prebasioccipitalis appears with a frequency of 1%, whereas the isolated element can be found in 0.025%. The fixed and isolated third occipital condyle results, if only the median part of the hypochordal blastema persists. This variation can seriously disturb the mechanics of the cranio-cervical junction. In contrast to these findings the paracondylar process results, if the blastema of the transverse process of the proatlas persists between the lateral part of the occipital bone and the transverse process of the atlas. This entity may cause problems concerning some surgical procedures as well as serious clinical symptoms. The processus condylicus posterior is a minor osseous variation without any known clinical importance. This entity results from the dorsal arch of the proatlas and can be seen with a frequency of 0.66%. The processus condylicus posterior was first described by Bolk (1922) and the term was introduced by Schmidt and Fischer (1960). These entities and related findings will be discussed concerning their developmental aspects and from the viewpoint of their clinical importance and differential diagnosis.