Defining hyoplasia of the atlas: A cadaveric study

Abstract

Study Design. Cadaveric study. Objective. To defi ne congenital hypoplasia of the atlas. Summary of Background Data. Little has been written about hypoplasia of the atlas and it is usually described in the setting of other skeletal dysplasias or syndromes. Methods. A 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 magnifi cation was performed. Hypoplasia of the atlas was defi ned as the lowest 2.5% of measurements. The correlation between inner sagittal diameters at C1 and C3 was calculated. Results. The mean C1 inner sagittal diameter was 30.8 ± 2.4 mm (range, 23.5–38.1 mm). We defi ned C1 hypoplasia as an inner sagittal diameter value representing the smallest 2.5% of subjects. Because the mean was 30.8 mm, hypoplasia was defi ned as a diameter of ≤26.1 mm or less. Correcting for size and magnifi cation of radiographs, hypoplasia is defi ned 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 Steels 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). Conclusion. With 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.