Dotted Lines:

Represent the line used when determining if there is any overhang of the C1 vertebral lateral masses in relation to the lateral masses of C2 vertebra.

• In general, the outer edges of the C1 lateral masses should not extend over (or "hang over") the outer edge of the C2 lateral masses. 
  
• If there is overhang, the degree of overhang should be measured on both sides and if the combined distance of overhang is > 6.9 mm, then the rule of Spence would suggest there is a concern for a transverse ligament injury that would stabilize the dens (odontoid process) within the spinal canal.

• It is important to recognize, that like many radiographic methods for interpretation of pathology, the rule of Spence may not always correlate with a transverse ligament injury and should be used with other methods of radiographic interpretation. This may be due to different types of C1 (Jefferson) fractures that can occur with some not always causing displacement to the degree to pick up on a standard radiograph, such as an odontoid view.  
• If there is any concern, an MRI should be done.  The preference to an MRI over a CT scan is due to the fact that a CT scan may not be able to show the position of maximal displacement of the fracture based on the slices obtained.

For additional related information, please choose an item from below:

                          
Lateral C-Spine       Digastric Lines             Bimastoid Lines       Dens (Odontoid) Fractures

1. Radcliff KE et al.  Does C1 fracture displacement correlate with transverse ligament integrity? Orthop Surg 2013;5(2):94-9.  PubMed

Editors:  Anthony J. Busti, MD, PharmD, FNLA, FAHA
Last Updated: June 2015