Hypoplasia of the Atlas Does Not Correlate With Subaxial Canal Stenosis: A Retrospective Cohort Study

Background and objective It remains unclear whether atlas hypoplasia is causative of symptomatic cervical canal stenosis or represents an independent and unrelated finding. There have been few dedicated studies describing atlas hypoplasia, and the existing literature has focused almost entirely on Asian populations. Therefore, the aims of the current study were threefold. First, we aimed to characterize the differences in cervical foraminal sagittal measurements by race and sex. Second, we evaluated the impact of age on cervical sagittal diameter and assessed the relationship between atlas hypoplasia and subaxial canal stenosis. Finally, we investigated the relationship between dens size and canal stenosis. Methods This was a single-institution retrospective cohort study approved by the Institutional Review Board at the University of Rochester School of Medicine and Dentistry. An overarching patient cohort was created by selecting patients with a Current Procedural Terminology (CPT) code of 72125-7 (CT of the cervical spine) and an International Classification of Diseases, 9th Edition (ICD-9) code 723.0 (cervical myelopathy) between January 1, 2014, and December 31, 2014. A total of 851 patients fulfilled the inclusion criteria. Variables collected included demographic and radiographic data. The inner sagittal diameters (ISD) at every cervical spine level, including the atlas, were measured. The prevalence of atlas hypoplasia was compared by age group. The correlation between each ISD was calculated. Among cases of atlas hypoplasia, the proportion of patients with subaxial canal stenosis was investigated. Results A total of 851 patients were included in the analysis. The mean ISD of the atlas was 31.4 ± 2.2 mm, and we defined atlas hypoplasia as an atlas ISD ≤ 27.0 mm. The mean space available for the cord (SAC) of the atlas was 19.0 ± 2.0 mm (range: 12.8-27.1). There was no significant difference in either the ISD of the atlas or the prevalence of atlas hypoplasia between the age groups. Pearson's correlation analysis showed that the ISD of the atlas had a strong correlation with the SAC of the atlas (r = 0.86, p < 0.01); a moderate correlation with both the sagittal diameter of the dens (coefficient = 0.48, p < 0.01) and the ISD of C2 (r = 0.59, p < 0.01) to C3 (r = 0.44, p < 0.01); and a weak correlation with the ISD of C4 (r = 0.37, p < 0.01), C5 (r = 0.38, p < 0.01), C6 (r = 0.36, p < 0.01), and C7 (r = 0.37, p < 0.01). A sub-cohort analysis of the 17 cases of atlas hypoplasia revealed that the mean ISD of the atlas was 26.3 ± 0.7 mm, and the mean SAC of the atlas was 14.5 ± 0.8 mm within this cohort. Regarding the rate of canal stenosis, the atlantal SAC was less than 14 mm in 17.6% of the cases, and the ISD of C2 to C7 was < 12 mm in 0%, 35.3%, 41.2%, 64.7%, 52.9%, and 47.1% of the cases, respectively. Conclusions The prevalence of atlas hypoplasia was not correlated with older age, which differs from subaxial developmental canal stenosis. Furthermore, although there was no strong correlation between the ISD of the atlas and each subaxial ISD, cases with atlas hypoplasia often involved subaxial canal stenosis.