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fracture can sometimes be mistaken for the normal or physiologic subluxation that
exists in the C2–C3 or C3–C4 regions in approximately 25% of children younger
than 8 years and may also be seen up to 16 years of age. This radiographic
pseudosubluxation is caused by ligamentous laxity, relatively horizontal facet
joints, weak neck muscles, and cartilage artifact. Distinguishing between a subtle
hangman’s fracture and pseudosubluxation can be accomplished using Swischuk’s
“posterior cervical line,” as described in Figure 112.18 . A value of more than 1.5
to 2 mm suggests an occult hangman’s fracture as the source of the anterior
subluxation of C2 on C3. The increase in magnitude of the distance between the
cortex of the spinous process of C2 and the posterior cervical line in a hangman’s
fracture is the result of anterior displacement of the skull, C1, and the anterior
portion of C2 on the remainder of the lower cervical spine. Nontraumatic
subluxation has also been reported at the C5/C6 and C6/C7 spinal levels, although
subluxation at these lower levels should always be fully investigated for potential
ligamentous injury.