Background: Narrowness of the spinal canal is associated with the development of cervical myelopathy. While studies have addressed the sagittal diameter of the cervical spinal canal, few evaluated the correlation between the size of the spinal canal and the vertebral level. We addressed this issue. Methods: Our retrospective study included 102 patients with cranial or spinal disorders. We examined the correlation between the cervical spinal canal diameter (SCD) at C1 to C7 and the inner anteroposterior diameter (IAPD) of the atlas on CT images. Results: At C1, the SCD was largest, at C4 it was smallest. While there was a strong correlation between the IAPD and the SCD at C1 (r = 0.8), the correlation between the size of the atlas and the SCD at C4 to C7 was weak (r = 0.2–0.3). We divided our patients into a normal group (n = 34, SCD ≥ 12 mm at any levels) and a stenosis group (n = 68, SCD < 12 mm at all levels). The mean SCD at C2 to C7 was significantly larger in the normal group. There was no significant difference between the two groups with respect to the IAPD and the SCD at C1. Conclusions: The size of the subaxial spine does not necessarily affect the size of the atlas. The pathophysiology of spinal canal stenosis should be considered separately at the C1- and the subaxial level.
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