1) Description of Measurement

Translation on sagittal reconstruction quantifies the anteroposterior displacement of one cervical vertebral body relative to an adjacent vertebra on sagittal CT images. It is a direct indicator of spinal instability and reflects disruption of the anterior and/or posterior ligamentous complexes, often in the setting of trauma.

This measurement is particularly useful for identifying subluxation, fracture–dislocation, and unstable injury patterns, and it complements facet-based assessments (facet overlap, facet step-off) and global alignment lines.

2) Instructions to Measure

• Obtain thin-slice cervical CT with high-quality sagittal reconstructions centered on the midline and parasagittal planes.

• Identify the vertebral level of interest, particularly where malalignment is suspected.

• On the sagittal image:

• Identify the posterior cortical margin of the inferior vertebral body (reference vertebra).

• Identify the posterior cortical margin of the superior vertebral body (translated vertebra).

• Draw two vertical reference lines parallel to the sagittal axis:

• One along the posterior margin of the inferior vertebral body.

• One along the posterior margin of the superior vertebral body.
  
  

•  Measure the horizontal distance (mm) between these two lines.

• This distance represents anteroposterior translation at that level.

  Perform measurements at suspicious adjacent levels and record the greatest translation observed.

3) Normal vs. Pathologic Ranges

• Normal or physiologic alignment: ≤ 2 mm

• Borderline translation: 2 - 3.5 mm

• Pathologic translation: > 3.5 mm; unstable injury

Key points:

• > 3.5 mm of translation is a classic threshold for cervical instability

• Larger degrees of translation correlate with disco-ligamentous disruption and increased neurologic risk

4) Important References

White AA, Panjabi MM. Clinical Biomechanics of the Spine. 2nd ed. Lippincott; 1990.

Harris JH Jr, Mirvis SE. The Radiology of Acute Cervical Spine Trauma. 3rd ed. Williams & Wilkins; 1996.

Dvorak MF, Fisher CG, Fehlings MG, Rampersaud YR, Oner FC, Aarabi B, Vaccaro AR. The surgical approach to subaxial cervical spine injuries: an evidence-based algorithm based on the SLIC classification system. Spine (Phila Pa 1976). 2007 Nov 1;32(23):2620-9. doi: 10.1097/BRS.0b013e318158ce16.

Daffner RH, Sciulli RL, Rodriguez A, Protetch J. Imaging for evaluation of suspected cervical spine trauma: a 2-year analysis. Injury. 2006 Jul;37(7):652-8. doi: 10.1016/j.injury.2006.01.018.

5) Other info....

Translation on sagittal CT is a direct, reproducible measure of instability

Particularly useful for detecting:

• Fracture–dislocations

• Facet dislocations

• Ligamentous injuries not immediately obvious on axial images

CT is optimal for bony alignment; MRI is recommended when translation is present to assess:

• Disc injury

• Posterior ligamentous complex disruption

• Spinal cord compression

Even translation just above the threshold may be clinically significant when associated with neurologic symptoms or additional instability markers