Roentgenological study of the sagittal diameter of the cervical spinal canal in normal adult Japanese.

The sagittal diameter of the cervical spinal canal on roentgenograms in normal adult Japanese aged 15 years or over, 505 males and 492 females, was investigated to define the normal distribution and lower limit. Lateral roentgenograms of cervical spinal canals were taken at a constant focus-film distance of 1.5 m. The mean +/- SD magnification coefficient was 1.17 +/- 0.02. The mean +/- SD sagittal diameters of the cervical spinal canals at each vertebral level were: C-1, 21.0 +/- 2.2 mm; C-2, 18.0 +/- 1.7 mm; C-3, 15.8 +/- 1.5 mm; C-4, 15.2 +/- 1.5 mm; C-5, 15.3 +/- 1.5 mm; C-6, 15.7 +/- 1.5 mm; and C-7, 15.9 +/- 1.4 mm. The lowest mean -2 SD values were: C-1, 16.6 mm; C-2, 14.6 mm; C-3: 12.8 mm; C-4, 12.2 mm; C-5, 12.3 mm; C-6, 12.7 mm; and C-7, 13.1 mm. The smallest diameter was at the C-4 level, but there was no significant difference between values at the C-4 and C-5 levels. Males had significantly larger diameters than females (mean difference 0.8 mm) (p < 0.01). Younger subjects had greater diameters than older subjects. The incidence of spondylotic changes was 40.1% in subjects aged 50-59 years, 57.7% in those aged 60-69 years, and 76.6% in those aged 70-79 years. Males had a higher incidence than females. Ossification of the posterior longitudinal ligament was observed in 2.1% of all subjects. This study suggests that patients with a sagittal diameter in the cervical spinal canal of less than 12 mm have a high risk of cervical myelopathy.