Normal Cervical Spinal Canal Diameter
The normal cervical spinal canal diameter ranges from approximately 15-20 mm in the sagittal (anteroposterior) plane, with values below 13 mm indicating congenital stenosis and values below 10 mm representing absolute stenosis. 1, 2
Standard Measurement Parameters
Sagittal Diameter Ranges
- Normal canal diameter: 15.33-20.46 mm across C1-C7 levels, with the narrowest point typically at C4 3
- Congenital stenosis threshold: <13 mm 1, 4, 3
- Absolute stenosis threshold: <10 mm 1
- Relative stenosis threshold: <13 mm but ≥10 mm 1
Transverse Diameter
- Normal transverse diameter: 24.45-27.00 mm across C1-C7 levels 3
Torg-Pavlov Ratio
- Normal ratio: Mean value of 0.96 3
- Stenosis indicator: Ratio ≤0.8 indicates presence of stenosis 5, 2
- This ratio is calculated as cervical canal diameter divided by cervical vertebral body width 1
Clinical Significance of Specific Measurements
Space Available for Spinal Cord (SAC)
The space available for the cord at injury levels correlates directly with neurological outcomes 2:
- Complete spinal cord injury: Mean SAC of 10.5 mm 2
- Incomplete spinal cord injury: Mean SAC of 13.1 mm 2
- Isolated nerve root injury: Mean SAC of 15.9 mm 2
- No neurological deficit: Mean SAC of 16.7 mm 2
Gender and Ethnic Variations
- Females sustain cervical canal narrowing more easily than males 3
- Ethnic variations exist in stenosis definitions: ≤14 mm in Whites, ≤12 mm in Japanese, ≤13.7 mm in Chinese 3
Risk Stratification Based on Canal Diameter
High-Risk Measurements
- Canal diameter <10 mm: Absolute stenosis requiring individualized assessment for return to collision sports 1
- Canal diameter <13 mm with T2 MRI changes: Contraindication for return to collision sports 1
- Minimum cervical diameter ≤8 mm: Present in 99% of patients with spinal cord injury without CT evidence of trauma 6
Moderate-Risk Measurements
- Canal diameter 10-13 mm: Relative stenosis requiring case-by-case evaluation 1
- Canal diameter <14 mm: Associated with 10.2% prevalence in asymptomatic subjects but increased risk of spinal cord compression 7
Low-Risk Measurements
- Canal diameter >13 mm without MRI changes: Generally safe for return to collision sports 1, 5
- Canal diameter 13-15 mm: Considered normal range 4
- Canal diameter >15 mm: Wide canal with lower risk of pathological changes 4
Important Clinical Caveats
Congenital stenosis (<13 mm) significantly increases risk of developing cervical spondylotic myelopathy and pathological disc changes, particularly at C3-4, C5-6, and C6-7 levels 4. The narrowest cervical canal level is consistently at C4 across all ethnicities 3.
Extension positioning results in statistically significant additional stenosis compared to flexed or neutral positions, which is critical when assessing athletes or trauma patients 3.
Cut-off values for spinal cord compression in asymptomatic subjects are 14.8 mm in males and 13.9 mm in females, meaning measurements below these thresholds significantly increase compression risk 7.