Radiology of postnatal skeletal development. XII. The second cervical vertebra.

The development of the second cervical vertebra is complex. The dens (odontoid process) develops two primary ossification centers that usually coalesce within three months following birth. These centers are separated from the primary ossification center of the vertebral centrum by a cartilaginous region--the dentocentral synchondrosis. This synchondrosis is a slow growing, bipolar physis similar to the triradiate cartilage of the acetabulum. It contributes to the overall heights of both the dens as well as the vertebral body. Anatomically the dentocentral synchondrosis is below the level of the C1-C2 articulations. This cartilaginous structure is continuous throughout the vertebral body with similar cartilage in both the facet regions as well as the neurocentral synchondroses. These various cartilaginous continuities progressively close--first, the connections to the facet regions, next the neurocentral synchondroses, and finally the dentocentral synchondrosis. Remnants of the incompletely closed dentocentral synchondrosis must be distinguished from a fracture, which usually propagates along this structure as a physeal injury in infants and children. The cartilaginous epiphysis at the tip of the dens may be transverse or may form a cleft ("V") shape. At eight to ten years, a secondary ossification center--the ossiculum terminale--develops in this proximal dens epiphysis. Fusion of the ossiculum terminale with the rest of the dens occurs between ten and thirteen years.