Imaging description

Pseudosubluxation refers to physiologic anterior spondylolisthesis of C2 on C3, caused by ligamentous laxity and a more horizontal position of the facet joints compared with adults. It is seen in children less than 16 years of age, with most patients less than eight years of age. Rarely, it may be seen in an adult patient [1].

Lateral radiographs will reveal anterior displacement of the C2 vertebral body relative to C3. Displacement is most conspicuous during flexion, and may resolve during extension. A posterior cervical line may be drawn between the anterior cortex of the C1 and C3 posterior arches. This line, referred to as Swischuk’s line, should pass within 2 mm of the anterior cortex of the C2 posterior arch (Figure 22.1) [2]. If it does not, injury should be suspected [3].

CT will reveal similar findings to those seen on radiography. However, one may more confidently exclude a fracture of the axis in the setting of malalignment. If there is concern for ligamentous injury, MRI should be obtained. The absence of ligamentous edema is reassuring, and further suggestive of the normal variant of C2–C3 pseudosubluxation (Figure 22.2).

An important discriminator is the age of the patient. Pseudosubluxation of C2–C3 is much more common in children less than eight years of age. As the age increases beyond eight, this variant becomes much less common. Therefore, if malalignment at C2–C3 is identified in an older child or adult, it should be viewed with a much higher suspicion of injury.