The anterior arch develops from one or more ossification centers which usually appear during the first year of life. Fusion of the anterior arch to the lateral masses forming the neurocentral synchrondrosis, normally occurs by the seventh year of life.

Posterior mid-line clefts of the atlas are common whereas anterior mid-line clefts are much rarer and usually associated with a posterior defect.

It is important to differentiate these defects from Jefferson burst fractures by the presence of smooth corticated margins in contrast to the sharp non-corticated appearance of a fracture.  In addition, developmental anomalies may be associated with an atlanto-axial lateral offset of 1-2 mm if there are both anterior and posterior defects.