The annulus vacuum sign.

An annulus vacuum sign at the anterosuperior or anteroinferior margin of a cervical interspace in extension may be due to an acute hyperextension injury with tearing of Sharpey fibers allowing the annulus fibrosus to separate from the adjacent bone. The sign is not pathognomonic of this injury as has been implied in the literature. It can be due to an old hyperextension injury and is seen in degenerative disease of the annulus without any acute trauma. In addition, a pseudovacuum sign may be very difficult to distinguish from a true vacuum sign.