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Rule of Spence


  • The rule of Spence is a radiographic method that attempts to determine the integrity of the transverse ligament when doing an open mouth (odontoid view) radiograph. 
  • If the combined measurement of the right and left lateral masses of C1 hang over the lateral masses of C2 by more than 6.9 mm then there should be concern for a possible transverse ligament injury and the patient should get an MRI or flexion or extension imaging.
  • While the data supporting its ability to correlate it can be one of several radiographic methods when assessing for cervical spine injuries. If there is any doubt or concern, an MRI is warranted.

Rule of Spence

    Radiographic Image: Rule of Spence

    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.

    How to Calculate
    • 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.
    Application of Rule
    • 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.
    Related Content

    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 & Reviewers

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