Increased C1 to C2 spinous process distance

(A) This figure demonstrates an example of an enlarged C1–2:C2–3 ratio (20:6) in a patient with tectorial membrane disruption as the bones would appear on lateral cervical spine radiograph or sagittal computed tomography cervical spine reconstruction.
(B) Disrupted. The enlarged C1–2:C2–3 ratio in tectorial membrane disruption may be created by the vectors illustrated on this sagittal figure. The craniocervical junction is flexed in an supine young child on a flat surface (long curved arrow). Without an intact tectorial membrane firmly strapping the flexed cranium to C-2, the occipital condyles push down on the anterior rim of cup-shaped C-1 articulation (red arrow) and rotate the posterior ring of C-1 superiorly (straight black arrow). This enlarges the C1–2 interspace relative to the C2–3 interspace to create the abnormal C1–2:C2–3 ratio.
(C) Normal. The entire Oc—C2 complex moves up as a unit with an intact tectorial membrane (solid black line). Movement of the occipital condyle is limited to rotation within the cup-shaped C-1 articulation (curved black arrow), and the joint relationship is maintained (straight black arrow).