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Split or bipartite atlas is the rare congenital anomaly where the atlas is split into two halves by fusion defects in both the anterior and posterior arches. The osseous defects are spanned by fibrous tissue.
They are usually asymptomatic and found incidentally on spinal imaging. Patients may have neck pain and rarely develop cervical myelopathy.
The anterior arch defect is mostly in the midline and usually smaller than the posterior arch defect, which is almost always in the midline.
Care should be taken to avoid misdiagnosing a Jefferson fracture, noting that fusion defects will have broad or tapered osseous margins that are rounded and well-corticated.
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