What anteroposterior (AP) spinal canal diameter is considered abnormal for the lumbar, cervical, and thoracic spine?

Spinal Canal AP Diameter - Abnormal Thresholds

The cervical spinal canal is considered abnormal when the AP diameter is less than 13 mm, with absolute stenosis occurring below 12 mm; the lumbar canal is abnormal when the AP diameter is less than 10-11 mm on myelography or when the dural sac area is reduced below normal thresholds on CT/MRI. 1, 2

Cervical Spine

Critical Thresholds

• AP diameter <13 mm: Increased risk of developing myelopathy or spinal cord compression 1
• AP diameter <12 mm: Absolute stenosis - patients will develop myelopathy and spinal subarachnoid block without exception 1
• AP diameter <15 mm: Upper limit in patients with existing myelopathy or positive Queckenstedt's test, indicating significant risk 1

Ethnic Considerations

• Polynesian populations: Average cervical canal approximately 2.5 mm narrower than European populations 3
• Māori populations: Average cervical canal approximately 1.5 mm narrower than New Zealand Europeans 3
• These ethnic variations must be considered when diagnosing congenital stenosis to avoid misdiagnosis in normal individuals from certain ethnic groups 3

Myelographic Measurements

• Thecoperiosteal diameter (TPD) <11 mm: Upper limit in myelopathy 1
• TPD <10 mm: Upper limit in spinal subarachnoid block 1
• TPD <8 mm: Likely to develop myelopathy 1

Lumbar Spine

Measurement Standards

• AP diameter <10-11 mm: Indicates central spinal stenosis on myelography or CT 2
• Dural sac transverse area reduction: More reliable than AP diameter alone for assessing stenosis 2
• Measurements should be obtained at the level of the intervertebral disc space 2

Measurement Accuracy

• Ultrasound measurements differ by ±5 mm from myelography and ±25 mm² from CT for dural sac area 2
• The ellipse approximation method (using IPD and AP diameter) has 8.44-15.51% error across L1-L5 levels 4

Special Consideration - Isthmic Spondylolisthesis

• Sagittal canal ratio ≥1.25: Indicates abnormally increased AP diameter ("wide canal sign"), diagnostic of bilateral pars interarticularis defects 5
• This ratio is calculated as maximum AP diameter at the affected level divided by the diameter at L1 5
• Normal sagittal canal ratio is <1.25 at all lumbar levels 5
• This sign is 100% accurate in distinguishing isthmic from degenerative spondylolisthesis in patients over 40 years old 5

Thoracic Spine

No specific guideline-based thresholds were identified in the provided evidence for thoracic spine AP diameter abnormalities. The thoracic canal is typically the narrowest region of the spine, but standardized abnormal values are not well-established in the current literature.

Clinical Algorithm for Assessment

When to Suspect Abnormal Canal Diameter

• Cervical spine: Patients presenting with myelopathy, radiculomyelopathy, or positive Queckenstedt's test warrant measurement 1
• Lumbar spine: Patients with clinical signs of spinal stenosis (neurogenic claudication, bilateral leg symptoms) 2

Imaging Approach

• MRI without and with contrast: Most sensitive (96%) and specific (94%) for evaluating spinal canal compromise and cord compression 6
• Non-contrast MRI: Sufficient for detecting structural abnormalities, bone marrow edema, and gross canal narrowing 6
• CT with multiplanar reformations: Useful for precise bony measurements and surgical planning 4

Important Caveats

• AP diameter measurements are operator-dependent and require consistent technique 4
• Ethnic background significantly affects normal values and must be considered 3
• The presence of degenerative changes, osteophytes, or ligamentum flavum hypertrophy can reduce effective canal diameter beyond simple bony measurements 1
• Correlation with clinical symptoms is essential, as anatomic narrowing does not always produce symptoms 6