• Michael P Kelly, Yasushi Oshima, Jin S Yeom, Rashmi Agarwal, Navkirat S Bajwa, and K Daniel Riew.
• *Department of Orthopedic Surgery, Washington University School of Medicine in St. Louis, Saint Louis, MO †Department of Orthopaedic Surgery, The University of Tokyo, Tokyo, Japan ‡Department of Orthopaedic Surgery, Seoul National University, Seoul, Republic of Korea §Department of Orthopaedic Surgery, University of Pittsburgh, Pittsburgh, PA; and ¶University of South Alabama, College of Medicine, Mobile, AL.
• Spine. 2014 Oct 1; 39 (21): E1243E1247E1243-7.

Study DesignCadaveric study.ObjectiveTo define congenital hypoplasia of the atlas.Summary Of Background DataLittle has been written about hypoplasia of the atlas and it is usually described in the setting of other skeletal dysplasias or syndromes.MethodsA total of 543 cervical spine specimens were randomly selected from the Hamann-Todd collection. Sagittal and coronal diameters of the atlas, axis, and C3 (when available), and the dens diameter were measured using digital calipers. Correction for modern size and radiographical magnification was performed. Hypoplasia of the atlas was defined as the lowest 2.5% of measurements. The correlation between inner sagittal diameters at C1 and C3 was calculated.ResultsThe mean C1 inner sagittal diameter was 30.8 ± 2.4 mm (range, 23.5-38.1 mm). We defined C1 hypoplasia as an inner sagittal diameter value representing the smallest 2.5% of subjects. Because the mean was 30.8 mm, hypoplasia was defined as a diameter of ≤26.1 mm or less. Correcting for size and magnification of radiographs, hypoplasia is defined as an inner sagittal diameter of the atlas of 28.9 mm. Approximately 10% of cases had a dens that occupied more than 40% of the spinal canal at C1, thus not following Steel's Rule of Thirds. There was only a moderate correlation between the spinal canal diameter at C1 and at C3 (r = 0.483, N = 345; P < 0.001).ConclusionWith an inner sagittal diameter of 26 mm or less, one may describe the atlas as hypoplastic. Ten percent of the specimens had an odontoid process that occupied more than 40% of the spinal canal at C1. There was little correlation between the inner sagittal diameter at C1 and the diameter at C3.Level Of EvidenceN/A.

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