Normal Pseudosubluxation Size on Cervical Spine Radiography
Pseudosubluxation refers to the physiologic anterior displacement of one cervical vertebral body relative to the one below, most commonly observed at C2/C3 and C3/C4 in children due to ligamentous laxity and immature facet joint angulation. Accurate recognition of this normal variant is clinically important to avoid misdiagnosis of traumatic subluxation or instability.
Normal Reference Values
| Orientation | Location | Measurement |
|---|---|---|
| Lateral | C3/C4 | <3 mm |
Clinical Significance
Pseudosubluxation is predominantly seen in children under 8 years of age and can mimic true traumatic subluxation on lateral cervical radiographs. The accepted upper limit of normal anterior displacement at C3/C4 is less than 3 mm. Displacement at or beyond this threshold warrants further evaluation, particularly in the setting of trauma.
A useful adjunct for differentiating pseudosubluxation from true subluxation is Swischuk's line (posterior cervical line), drawn along the anterior cortex of the posterior arch from C1 to C3. In pseudosubluxation, C2 typically falls within 1–2 mm of this line; a deviation of more than 2 mm raises concern for true instability or a hangman's fracture equivalent.
- Physiologic pseudosubluxation (normal variant in children)
- Traumatic subluxation or dislocation
- Ligamentous injury without bony fracture
- Flexion teardrop injury
- Os odontoideum with instability
Reference: Weissleder R, M.D. JW, Chen JW. Primer of Diagnostic Imaging, Expert Consult- Online and Print. Mosby Incorporated. p. 625. (2011).
Imaging Notes
Lateral cervical spine radiography is the primary modality for assessing pseudosubluxation. The measurement is taken on a true lateral projection in a neutral or slight flexion position, which is the natural resting posture in young children. Displacement is measured as the horizontal distance between the posterior cortex of the superior vertebral body and the posterior cortex of the inferior vertebral body. Ensure adequate positioning and that the image is obtained without forced extension, as extension reduces or eliminates physiologic displacement and may mask the finding.
When clinical suspicion for true injury remains high despite a measurement under 3 mm, CT or MRI can be employed to evaluate bony architecture and ligamentous integrity in greater detail. Pseudosubluxation typically reduces on extension views, which can help confirm its physiologic nature.