• Edze Hummel and Jan C de Groot.
• Department of Radiology, University Medical Center Groningen, Hanzeplein 1, 9700 CC Groningen, The Netherlands. e.hummel@umcg.nl
• Spine J. 2013 May 1; 13 (5): e1-5.

Background ContextA bipartite atlas is a rare coincidental finding, and it is reported in only 0.1% of the general population. It is a congenital disorder characterized by incomplete fusion of the anterior and the posterior arches of C1, and it is important to differentiate it from a Jefferson fracture.Study Design/SettingCase report and literature review.PurposeTo report three cases of patients with bipartition of the atlas with a focus on imaging. To review the literature on these fusion defects, the embryologic basis, and the differentiation from a Jefferson fracture.MethodsWe report three cases of patients with a bipartite atlas as a coincidental finding in a trauma setting. The bipartite atlas was assessed by multidetector computed tomography (CT). The first case, for example, describes a 36-year-old patient who was struck by a moped. The CT of the skull showed a bipartite atlas as an additional finding. The embryologic development of C1 is reviewed and also the imaging features and the management. Furthermore, a CT image of a Jefferson fracture is provided for comparison.ResultsThe CT scans of the three patients show midline clefts of the anterior and the posterior arches of C1 with similar imaging features: smooth margins lined by cortical bone and no lateral offset. The patients had no neurological symptoms relating to the C1 abnormality, and no follow-up was performed. The clefts at level C1 are the result of the failure of three ossification centers to fuse properly. Anterior and posterior clefts are caused by hypoplasia of the hypochordal bow and lateral parts of the C1 sclerotome, respectively. Because of the risk of instability, assessing atlantoaxial stability is advised. However, patients usually have no symptoms and require no specific treatment.ConclusionsA bipartite atlas is a rare congenital abnormality, caused by a failure of anterior and lateral ossification centers to fuse. It needs to be differentiated from a Jefferson fracture in a trauma setting. It usually requires no specific treatment.Copyright © 2013 Elsevier Inc. All rights reserved.

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