gms | German Medical Science

During the early development of this important and complicated region the axial sclerotomes of the first 4 somites are incorporated into the skull base and soon form a structure extending across the segments, the basioccipital [Ref. 8]. On the other hand, parts of the segmental material incorporated into the skull are involved in the construction of the upper cervical vertebrae also. This applies particularly to parts of the proatlas (Froriep's occipital vertebra). In this sense the body of the proatlas forms later the ossiculum terminale Bergmann, which represents the apex dentis, while the material of the dorsal arch of the proatlas is included in the atlas, especially into the dorsal part of the superior articular facet. It is remarkable, that the atlas is only an annular structure and that the axis features an osseous process, the dens axis (formerly called processus odontoideus epistrophei s. dens epistrophei). The embryological development of these special and characteristic details of the first two vertebrae is likewise of decisive importance for the understanding of the variations occurring in this region.

The development of the axis can be described as follows: the apex dentis is formed by the body of the proatlas. This body develops into the ossiculum terminale Bergmann, which is then incorporated into the tip of the dens. The dens axis and its socket correspond to the body of the atlas, and therefore possess a base plate. Together with the intervertebral disc, located between atlas and axis and the top plate of the corpus axis, this base plate forms the "ossiculum Albrechti". This bony disc provides the attachment of the dens axis to the body of the axis, and is situated somewhat below the level of the superior articular facets of the axis. According to Cattell & Filtzer [Ref. 2], the remnant of the intervertebral disc can be regularly demonstrated radiologically in man up to the 3^rd year of life. Even in adults relics of this "subdental synchondrosis" may persist and must not be misinterpreted as a fracture. Very rarely the combined persistence of the ossiculum terminale Bergmann and the subdental synchondrosis results in a dens tripartitus [Ref. 15], which impressively demonstrates the development just described. The anterior arch of the atlas develops from a condensation of mesenchyme located in front of the axial section of the sclerotome and is called the "hypochordal blastema" [Ref. 3]. The adjacent sclerotomal segments also possess such a hypochordal blastema. The hypochordal blastema of the proatlas is located in the area of the anterior border of the foramen magnum, and later vanishes completely. Disturbance of the described developmental processes may cause two categories of variations: assimilation of the atlas, if too much material is incorporated into the skull base, or in manifestation of occipital vertebra, if too few material is incorporated or if the hypochordal arch of the proatlas persists.

Even today the term "assimilation of the atlas" is used for very different and non-homogeneous formations. According to Pfitzner [Ref. 9], the expression "assimilation" ("internal absorption") of an element should be used only if the respective part merges to a neighbouring anatomical structure by loosing its shape and identity. In most "assimilations of the atlas" this type of complete merging of the atlas to the occiput is not observed, or only to a slight extent (Figure 1 a [Fig. 1]). Accordingly, many so-called assimilations of the atlas actually represent only atlantooccipital fusions or synostoses. Differentiation from atlanto-occipital ankyloses resulting from pathological processes (e.g. tuberculosis, syphilis, spondylitis ankylopoetica Bechterew-Strümpell-Marie) is also very important. The assimilation of the atlas (Figure 1 b [Fig. 1]) represents a clinically important entity. Vertebragenous vestibular crises are most frequently observed [Ref. 14], and a progressive atlantoaxial subluxation is noticeable in approximately 50% of the cases [Ref. 1]. The fixed malrotation of the assimilated atlas (Figure 1 b [Fig. 1]) may also cause a torticollis osseus. It must be stressed, that the assimilation of the atlas, first described by Realdo Colombo in 1559, has an incidence between 0.1% and 0.4% and is often combined with other irregularities of the craniocervical region.

If material of the proatlas is not completely incorporated into the base of the skull or into the upper cervical vertebras, or if its hypochordal blastema persists, the occipital vertebra will occur. This term was intoduced by Kollmann [Ref. 4], [Ref. 5] and characterizes the phenomenon very well. The occipital vertebra can appear in many different entities [Ref. 11], either representing accessory osseous bumps, accessory isolated elements or irregularities in the segmentation of the skull base.

The processus basilares are small bony excrescences located at the anterior edge of the foramen magnum. These tubercles may occur unilaterally or bilaterally, be firmly attached, or form accessory bony elements. If they are strongly developed, the processus basilares fuse together in the median sagittal region and a pseudocondylus tertius results. This structure is, however, still perforated by a fine sagittal canal (canalis intraoccipitalis s. canalis intrabasalis s. canalis intrabasalis Kollmanni). This fine bony canal differentiates the pseudocondylus clearly from the condylus tertius. If the processus basilares more closely resemble bulges, they are sometimes also described as "labia foraminis magni anteriora". The processus basilares can be demonstrated by conventional frontal tomography or in CT [Ref. 10]. According to Misch [Ref. 7] the processus basilares have an incidence of approximately 4%. Etiologically the processus basilares can be deduced from the hypochordal blastema of the proatlas. If the lateral parts of the hypochordal bar persist while the central part degenerates, the processus basilares result. They do not seem to have any clinical significance.

The third condyle constitutes a bony extension located in the median sagittal plane at the anterior rim of the foramen magnum. The third condyle may end freely or assume an articulated connection with the anterior arch of the atlas or to the apex dentis, if more fully developed [Ref. 13]. Various authors quote an incidence between 0.25% and 1%. The condylus tertius results if the hypochordal blastema of the proatlas degenerates in the lateral parts, but persists in the central part. It represents a clinically significant variant, as the formation of a joint with the atlas or the axis seriously disturbs the head articulations. The third condyle may also form an isolated accessory element articulating at the tip of the dens axis. Differentiation from an Ossiculum terminale Bergmann persistens or a so-called os odontoideum is easy, since the isolated third condyle is not accompanied by hypoplasia of the dens.

An ossified bulge at the entire anterior rim of the foramen magnum is called an arcus praebasioccipitalis. In very rare cases the arcus praebasioccipitalis may also manifest itself as an isolated bony arch shaped like a horseshoe, cranially articulating with the basilar part of the occipital bone and caudally with the anterior arch of the atlas as well as with the tip of the dens. Based on an anatomical specimen Prescher et al. [Ref. 10] point out the particular structure of this remarkable element resembling the "hat of Napoleon", and describe ways of obtaining this outline radiologically. Lombardi [Ref. 6] published a typical radiograph of this element and mentions having observed this configuration only once in 4000 radiographs (0.025%). The arcus praebasioccipitalis develops if the complete hypochordal blastema of the proatlas persists, and therefore practically represents a combination of bilateral processus basilares with a condylus tertius. Clinically, the arcus praebasioccipitalis may impair the function of the joints of the skull. This applies particularly to the isolated type, since this manifestation is often accompanied by severe arthrosis.

The processus paracondylicus represents a cone-shaped bony extension, with a broad base emerging from the exooccipital and directed caudally. The shape of the paracondylar process is quite variable. The processus may terminate freely in the soft tissues or assume an articulated or bony connection with the transverse process of the atlas. If the processus exists in rudimentary form only, the shallow bump is called tuberculum paracondylicum. The firm connection with the occipital bone may also get abolished and an isolated bony cone, the massa paracondylica, will result. If this massa paracondylica fuses with the processus transversus atlantis, this is called a processus epitransversus, which in turn may again form an articulated connection with the occiput. If the massa paracondylica forms an articulated connection with the processus transversus atlantis, but not with the occipital bone, v.Torklus & Gehle [Ref. 14] recommend that this configuration should be called an isolated processus epitransversus. For the processus paracondylicus, Misch [Ref. 7] quotes an incidence of 2%. The paracondylar process develops if the material of the transverse process of the proatlas persists. The processus paracondylicus will become clinically significant if an articulated or osseous connection to the atlas exists. This configuration seriously impairs the function of the joints and may cause a torticollis osseus. As mentioned by v. Eicken [Ref. 12], the paracondylar process may cause technical problems in surgery of the dural sinuses or performing a neck dissection.