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Occipitalization of the Atlas in ChildrenMorphologic Classification, Associations, and Clinical Relevance
Purushottam A. Gholve, MD, MBMS, MRCS1; Harish S. Hosalkar, MD, MBMS, FCPS, DNB1; Eric T. Ricchetti, MD1; Avrum N. Pollock, MD, FRCPC1; John P. Dormans, MD1; Denis S. Drummond, MD1
1 Divisions of Orthopaedic Surgery (P.A.G., H.S.H., E.T.R., J.P.D., and D.S.D.) and Pediatric Neuroradiology (A.N.P.), Children's Hospital of Philadelphia, 2nd floor, Wood Building, 34th Street and Civic Center Boulevard, Philadelphia, PA 19104. E-mail address for D.S. Drummond: drummond@email.chop.edu
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Disclosure: The authors did not receive any outside funding or grants in support of their research for or preparation of this work. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.
Investigation performed at Children's Hospital of Philadelphia, Philadelphia, Pennsylvania

The Journal of Bone and Joint Surgery, Incorporated
J Bone Joint Surg Am, 2007 Mar 01;89(3):571-578. doi: 10.2106/JBJS.F.00527
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Background: Occipitalization is defined as a congenital fusion of the atlas to the base of the occiput. We are not aware of any previous studies addressing the morphologic patterns of occipitalization or the implications of occipitalization in children. We present data on what we believe is the largest reported series of children with occipitalization studied with computed tomography and/or magnetic resonance imaging, and we provide a description of their clinical characteristics.

Methods: We retrospectively reviewed all cases of occipitalization in children included in our spine database. Patient charts and imaging studies were reviewed. A new morphologic classification of occipitalization was developed from the two-dimensional sagittal and coronal reformatted computed tomographic reconstructions and/or magnetic resonance images. The classification includes four patterns according to the anatomic site of occipitalization (Zones 1, 2, and 3 and a combination of those zones), and it was applied to this group of patients. Imaging studies were also reviewed for evidence of cervical instability and for other anomalies of the craniovertebral junction.

Results: Thirty patients with occipitalization were identified. There were twenty-four boys and six girls with a mean age of 6.5 years. The morphologic categorization was Zone 1 (a fused anterior arch) in six patients, Zone 2 (fused lateral masses) in five, Zone 3 (a fused posterior arch) in four, and a combination of fused zones in fifteen. Seventeen patients (57%) had atlantoaxial instability, and eight of them had an associated C2-C3 fusion. Eleven patients (37%) had spinal canal encroachment, and five of them had clinical findings of myelopathy. The highest prevalence of spinal canal encroachment (63%) was noted in patients with occipitalization in Zone 2.

Conclusions: Occipitalization is associated with abnormalities that lead to narrowing of the space available for the spinal cord or brainstem. The risk of atlantoaxial instability developing is particularly high when there is an associated congenital C2-C3 fusion. Two-dimensional sagittal and coronal reformatted computed tomographic reconstructions and/or magnetic resonance images can help to establish the diagnosis and permit categorization of occipitalization in three zones, each of which may have a different prognostic implication.

Level of Evidence: Therapeutic Level IV. See Instructions to Authors for a complete description of levels of evidence.

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    These activities have been planned and implemented in accordance with the Essential Areas and policies of the Accreditation Council for Continuing Medical Education (ACCME) through the joint sponsorship of the American Academy of Orthopaedic Surgeons and The Journal of Bone and Joint Surgery, Inc. The American Academy of Orthopaedic Surgeons is accredited by the ACCME to provide continuing medical education for physicians.
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