Normal Pseudosubluxation Size on Cervical Radiography
Pseudosubluxation refers to the apparent anterior displacement of one cervical vertebral body on another, most commonly at C2/C3, resulting from normal ligamentous laxity rather than true instability. It is a well-recognized normal variant encountered predominantly in children under 8 years of age due to their inherently hypermobile cervical spine. Accurate recognition is clinically essential to avoid misdiagnosis of traumatic injury and unnecessary intervention.
Normal Reference Values
| Orientation | Location | Measurement |
|---|---|---|
| Lateral | C2/C3 | <3 mm |
Clinical Significance
Anterior displacement of C2 on C3 of less than 3 mm is considered physiologic pseudosubluxation and does not require further workup in the absence of neurological symptoms or other imaging red flags. Displacement at or exceeding 3 mm raises concern for true ligamentous injury or fracture-dislocation and warrants further evaluation.
A useful adjunct is Swischuk’s line (posterior cervical line), drawn from the anterior cortex of the posterior arch of C1 to C3. In pseudosubluxation, the posterior arch of C2 should fall within 1 mm of this line; deviation greater than 2 mm suggests true subluxation.
- Physiologic pseudosubluxation (normal variant, children)
- Ligamentous injury following cervical trauma
- Atlantoaxial instability (e.g., Down syndrome, rheumatoid arthritis)
- Fracture-dislocation at C2/C3
- Hangman’s fracture with anterior displacement
Reference: Weissleder R, M.D. JW, Chen JW. Primer of Diagnostic Imaging, Expert Consult- Online and Print. Mosby Incorporated. p. 625. (2011).
Imaging Notes
Pseudosubluxation is assessed on the lateral cervical radiograph, obtained with the patient in a neutral or slightly flexed position — the latter accentuating the appearance. Measurement is performed from the posterior margin of the C2 vertebral body to the posterior margin of the C3 vertebral body along the anterior spinal line. Adequate visualization of all seven cervical vertebrae is required; a swimmer’s view or CT should be obtained if the cervicothoracic junction is not well visualized.
Extension views typically reduce or eliminate pseudosubluxation, which can serve as a confirmatory maneuver in cooperative, neurologically intact patients. CT or MRI is indicated when clinical suspicion for true injury persists despite equivocal radiographic findings, or when the patient presents with neurological deficits.
