Imaging description

There are several skeletal variants of the C1 and C2 vertebrae which may be confused with injury. Knowledge of the normal development of these vertebrae is essential for distinguishing anatomic variants from pathology.

The atlas typically develops from three primary ossification centers: one anterior arch and two neural arches. Two neurocentral synchondroses separate the anterior arch from the neural arches (Figure 19.1). A single posterior synchondrosis separates the neural arches. The atlas is normally fused by eight years of age [1].

The axis typically develops from five primary ossification centers: two odontoid centers, two neural arches, and one centrum. The odontoid centers usually fuse prior to birth (Figure 19.2). The remaining primary centers are usually fused by six years of age. A secondary center of ossification, known as the os terminale, forms at the odontoid tip and usually fuses by 12 years of age [2].

Incomplete fusion of the atlas may result in a cleft, usually at the site of a synchondrosis. The cleft will usually demonstrate a smooth margin at a characteristic location, and should not be mistaken for a fracture (Figure 19.3). If there is non-development of the anterior arch, the neural arches may overgrow and attempt to fuse anteriorly, resulting in an anterior midline cleft (Figure 19.4).