Defining Hyoplasia of the Atlas

Abstract

Study Design Cadaveric Study Objective To define 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 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 radiographic 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. Results The mean C1 inner sagittal diameter was 30.8 ± 2.4mm (Range 23.5 – 38.1mm). We defined C1 hypoplasia as an inner sagittal diameter value representing the smallest 2.5% of subjects. Since the mean was 30.8mm, hypoplasia was defined as a diameter ≤ 26.1mm. 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). Conclusions With an inner sagittal diameter of 26mm or less, one may describe the atlas as hypoplastic. 10% 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.

What Is the Most Representative Parameter for Describing the Size of the Atlas? CT Morphometric Analysis of the Atlas with Special Reference to Atlas Hypoplasia

“…26–28) However, as they were primarily taken to determine surgical screw placement, measurements relating to hypoplasia are scarce. 11,24) To describe hypoplasia of the atlas, the parameters were the anterior-posterior diameter, the SCD, and the CSA of the atlas. 7–24) …”

“…11,24) To describe hypoplasia of the atlas, the parameters were the anterior-posterior diameter, the SCD, and the CSA of the atlas. 7–24) …”

“…Because earlier radiological studies applied the inner anterior-posterior diameter (IAP), the spinal canal diameter, and the cross-sectional area (CSA) of the atlas independently to evaluate atlas hypoplasia, 7–24) the parameter most useful for determining the size of the atlas remains to be elucidated. The aim of the present study was to evaluate these atlas parameters and to identify the one most useful for determining the size of the atlas.…”

Ossification of the posterior atlantoaxial membrane associated with atlas hypoplasia

“…They conclude that atlas with an inner sagittal diameter of 26 mm or less can be described as hypoplastic. [23] Besides, it generally is agreed that a reduction of the SAC to 13 mm or less may be associated with neurologic problems. [24] The patient in our case has an atlas sagittal diameter of 23 mm, indicating a hypoplastic atlas.…”

Congenital malformed posterior arch of atlas with fusion defect: a case of developmental canal stenosis causing cervical myelopathy

“…Cervical stenotic myelopathy is a common presentation in the subaxial cervical spine involvement (2,15). The midsagittal canal diameter at the level of atlas in the present case was 6.36 mm, whilst the normal sagittal diameter at the level of atlas is reported to be 16-25 mm (2,3,16). While significant cervical cord compression occurs if the sagittal canal diameter is <14 mm at the level of atlas; myelopathic signs and symptoms develop when the diameter falls <10 mm (17,18).…”