Developmental anatomy of the atlas and axis in childhood by computed tomography

The CT modality plays a central role in the diagnosis of cervical spine fractures. In childhood, radiolucent synchondroses between ossification centers can resemble fractures, and they can be the sites of fractures as well. Recognition of cervical spine fractures in children requires familiarity with normal developmental anatomy and common variants as they appear on CT scans.

A convenience sample of 932 CT scans of the cervical spine accessible on the picture archiving and communications system (known as PACS) at a single children's hospital was examined. Scans were excluded from further analysis if they did not include the atlantoaxial region or were otherwise technically unsatisfactory; if the patient carried the diagnosis of a skeletal dysplasia; or if there were developmental lesions noted at other levels of the spine. No more than 1 scan per patient was analyzed. Synchondroses were graded as radiolucent, not totally radiolucent but still visible, or no longer visible. Their locations and symmetries were noted. The presence or absence of the tubercles of the transverse ligament was noted as well.

After exclusions, 841 studies of the atlas and 835 studies of the axis were analyzed. The 3 common ossification centers of the atlas arose in the paired neural arches and the anterior arch, but in as many as 20% of cases the anterior arch developed from paired symmetrical ossification centers. The 5 common ossification centers of the axis arose in the paired neural arches, in the basal center, in the dentate center (from which most of the dentate process develops), and in the very apex of the dentate process. The appearance of each synchondrosis was noted at sequential ages. The tubercles for the transverse ligament generally did not appear until the ossification of the synchondroses of the atlas was far advanced. Anomalies of the atlas included anterior and posterior spina bifida, absence of sectors of the posterior arch, and anomalous ossification centers and synchondroses. Anomalies of the axis were much less common. What appeared possibly to be chronic, incompletely healed fractures of the atlas were discovered on review for this analysis in 6 cases. No fractures of the axis were discovered.

There is substantial variation in the time course and pattern of development of the atlas, and anomalies are common. Some fractures of the atlas may escape recognition without manifest sequelae. Variation in the time course of the development of the axis is notable as well, but anomalies seem much less common.