Jefferson fractures: the role of magnification artifact in assessing transverse ligament integrity.

Thirty-five consecutive odontoid views were taken using a standardized technique. A 10-mm radiographic marker was positioned lateral to the tip of the C-1 transverse process. Data from three independent readers showed an 18% magnification error inherent to anteroposterior open-mouth odontoid radiographs. Integrity of the transverse ligament after Jefferson fractures has been inferred from the separation of the lateral masses measured on the odontoid view. Spence's anatomic study showed that lateral mass separation of > 6.9 mm implied rupture, but radiograph magnification was not taken into account. Transverse ligament rupture should not be inferred with lateral mass separation of < 8.1 mm.