J Korean Neurosurg Soc.  2015 Dec;58(6):528-533. 10.3340/jkns.2015.58.6.528.

Anatomical Variant of Atlas : Arcuate Foramen, Occpitalization of Atlas, and Defect of Posterior Arch of Atlas

• Kim MS

• KMID: 2151154
• DOI: http://doi.org/10.3340/jkns.2015.58.6.528

Abstract

OBJECTIVE
We sought to examine anatomic variations of the atlas and the clinical significance of these variations.
METHODS
We retrospectively reviewed 1029 cervical 3-dimensional (3D) CT images. Cervical 3D CT was performed between November 2011 and August 2014. Arcuate foramina were classified as partial or complete and left and/or right. Occipitalization of the atlas was classified in accordance with criteria specified by Mudaliar et al. Posterior arch defects of the atlas were classified in accordance with criteria specified by Currarino et al.
RESULTS
One hundred and eight vertebrae (108/1029, 10.5%) showed an arcuate foramen. Bilateral arcuate foramina were present in 41 of these vertebrae and the remaining 67 arcuate foramina were unilateral (right 31, left 36). Right-side arcuate foramina were partial on 18 sides and complete on 54 sides. Left-side arcuate foramina were partial on 24 sides and complete on 53 sides. One case of atlas assimilation was found. Twelve patients (12/1029, 1.17%) had a defect of the atlantal posterior arch. Nine of these patients (9/1029, 0.87%) had a type A posterior arch defect. We also identified one type B, one type D, and one type E defect.
CONCLUSION
Preoperative diagnosis of occipitalization of the atlas and arcuate foramina using 3D CT is of paramount importance in avoiding neurovascular injury during surgery. It is important to be aware of posterior arch defects of the atlas because they may be misdiagnosed as a fracture.

Keyword

Atlas; Arcuate foramen; Arch; Congenital defect; Occipitalization of atlas

MeSH Terms

Congenital Abnormalities
Diagnosis
Humans
Retrospective Studies
Spine

Figure

• 

  Fig. 1 Classification of posterior arch defects of the atlas. Type A : failure of posterior midline fusion of the two hemiarches. Sometimes a small separate ossicle is seen within the gap, Type B : unilateral cleft, ranging from a small gap to a complete absence of the hemiarch and posterior tubercle, Type C : bilateral clefts of the lateral aspects of the arches with preservation of the most dorsal part of the arch, Type D : complete absence of the posterior arch with a persistent isolated tubercle. This anomaly is conceivably a more extensive form of bilateral clefts. The lateral parts of the posterior arch are absent except for the posterior tubercle, Type E : absence of the entire posterior arch including the tubercle. Reproduced with permission from Currarino G, Rollins N, Diehl JT : Congenital defects of the posterior arch of the atlas : a report of seven cases including an affected mother and son. AJNR Am J Neuroradiol 15 : 249-254, 19945).

• 

  Fig. 2 Arcuate foramen. A : Three-dimensional computed tomography of a 21-year-old man showing complete arcuate foramina on both sides (arrows). B : Three-dimensional computed tomography of a 54-year-old woman demonstrating a partial arcuate foramen on her right side (arrow).

• 

  Fig. 3 Occipitalization of the atlas. A : Lateral cervical spine X-ray image of a 32-year-old man showing occipitalization of the atlas, C2–3 fusion (arrow : fusion between occiput and posterior arch of atlas, arrowhead : fusion between occiput and anterior arch of atlas). B : Coronal computed tomography demonstrating an abnormal odontoid process (arrowhead) and occipitalization of the atlas (arrow). C : Axial computed tomography showing an anomalous foramen magnum (arrows).

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  Fig. 4 Defect of the posterior arch of the atlas. A and B : Three-dimensional computed tomography of a 34-year-old man showing a Type A defect of the posterior arch of the atlas (A, arrow) and a defect of the anterior arch of the atlas (B, arrow, arrowhead : no defect in upper portion of anterior arch). C : Three-dimensional computed tomography of a 58-year-old woman showing a Type B defect (arrow). D and E : Three-dimensional computed tomography of a 41-year-old woman showing a Type D defect (arrow). F and G : Lateral cervical spine X-ray image (F, arrow : absence of the entire arch, including the tubercle) and three-dimensional computed tomography (G, arrow : absence of the entire arch, including the tubercle) of a 14-year-old girl showing a Type E defect.

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