When incidentally found and stable, conservative management may be appropriate. Instability may lead to central canal stenosis at the craniocervical junction with associated cord compression. The clinical manifestations of an os odontoideum are variable, ranging from relatively asymptomatic to unstable with significant neurological deficits. It plays an integral role in cervical spine mobility, as well as stability of the craniocervical junction and upper cervical spine. Compared to other cervical levels, the body of C-2 normally has increased thickness and a square shape. The body articulates with the adjacent intervertebral discs, while the neural arches form the posterior elements. As with the remaining cervical spine vertebrae, the base of the axis consists of one anterior ossification center for the body and paired neural arch ossification centers posterolaterally. In terms of development, the axis (C-2) is by far the most intricate of all the vertebral bodies, mainly due to the inclusion of the dens in addition to development of the underlying C-2 vertebral body. Knowledge of the characteristic locations and imaging appearances will help ascertain normal development from both anatomic variants or anomalies and traumatic injury, especially in the acute setting. In evaluating imaging studies in skeletally immature patients, ossification centers within the spine occur in predictable locations, have a typical appearance, and present at fairly predictable time intervals in terms of both ossification and fusion. Somatic growth of the vertebrae continues well after fusion of the ossification centers and into early adulthood. In the developing spine, vertebral bodies begin as focal ossification centers separated by synchondroses, which serve as cartilaginous bridges. This review will focus on the normal and variant development of the axis (C-2) and the patterns of traumatic injury involving the dens. A basic understanding of normal development, developmental variants, and traumatic injury patterns is vital to correctly interpret cervical spine imaging studies. This is most evident at the craniocervical junction, where there is a higher incidence of injuries in young children and the elderly, as well as an increased incidence of congenital bony anomalies, particularly at the C-2 level. Imaging evaluation of the cervical spine can be difficult at times, especially when dealing with a skeletally immature child or in the setting of variations in normal anatomic development. This review article attempts to provide the basic framework required for accurate interpretation of cervical spine imaging with a focus on the dens, specifically covering the normal development and ossification of the dens, common congenital variants and their various imaging appearances, fracture classifications, imaging appearances, and treatment options. Key imaging features are useful to differentiate between traumatic fractures and normal or variant anatomy acutely however, the radiologist must first have a basic understanding of the spectrum of normal developmental anatomy and its anatomic variations in order to make an accurate assessment. At times, it is necessary to distinguish normal development, developmental variants, and developmental anomalies from traumatic injuries in the setting of acute traumatic injury. The most common congenital variations of the dens include the os odontoideum and a persistent ossiculum terminale. In addition, congenital anomalies are common in this region, especially those associated with the axis/dens, due to its complexity in terms of development compared to other vertebral levels. The biomechanics of the developing pediatric spine and age-related degenerative changes predispose these patient populations to injuries centered at the craniocervical junction. Accurate interpretation of cervical spine imagining can be challenging, especially in children and the elderly.

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