Rule of Spence – Lateral Mass Dislocation (LMD)

There are many reputable medical references across the internet for Sum of Lateral Mass Displacement (LMD) (also medically referred to as Lateral Mass Overhang).  This is also referenced as “Rule of Spence”.  

The Rule of Spence is used in radiology for assessing the integrity of the transverse ligament, also known as Transverse Atlantal Ligament (TAL). It refers to a radiographic method used to evaluate the alignment of the atlas (C1) and axis (C2) vertebrae in the cervical spine. If the combined measurement of the right and left lateral masses of C1 is hanging over the lateral masses of C2 by more than 6.9mm, then it may indicate a transverse ligament injury.

Medical articles typically use “Sum of Rule of Spence”, meaning the sum of both left and right measurements, although LMD can also be measured and reported separately such as lateral mass overhang on left, and lateral mass overhang on right.  It can be measured on XRay and MRI but is best done on CT Scans for greater visual quality and accuracy. 

It has been determined that if the total lateral mass displacement (LMD) exceeds 6.9 mm, there is high likelihood of Transverse Atlantal Ligament (TAL) rupture.  If Lateral Mass Displacement is less than 5.7 mm TAL injury is unlikely.  Several recent radiographic studies have questioned the accuracy and validity of the “rule of spence” and it lacks biomechanical support.
Study Results:   Eleven cadaveric specimens were tested (n = 11). The average Lateral Mass Displacement upon Transverse Atlantal Ligament failure was 3.2 mm (±1.2 mm). The average force required to cause failure of the TAL was 242 N (±82 N). From our data analysis, if Lateral Mass Displacement exceeds 3.8 mm, there is a high probability of Transverse Atlantal Ligament failure. Our findings suggest that although the rule of spence is a conceptually valid measure of Transverse Atlantal Ligament (TAL) integrity, TAL failure occurs at a significantly lower value than previously reported.  Based on our literature review and findings, Lateral Mass Displacement is not a reliable independent indicator for Transverse Atlantal Ligament failure and should be used as an adjuntive tool to magnetic imaging rather than an absolute rule.  
References
  1. The American Journal of Orthopaedics 2014 (pg. 722-726) https://pubmed.ncbi.nlm.nih.gov/25344597/
  2. Canadian Association of Radiologists 2018 reference Rule of Spence pg. 45 “− “Rule of Spence > 7mm unstable Transverse Ligament rupture” https://car-asm.ca/wp-content/uploads/2019/05/CAR-2018-Program.pdf 
  3. European Journal of Spine and Neurosurgery 2017 pg. 9  https://www.semanticscholar.org/paper/Cranio-Cervical-Trauma-Eidemiology%2C-Classification%2C-Saxena/6bb9cc4ae1ca12640e077910610d4ad57ca60c8c 
  4. Department of Neurosurgery and Spine India 2022 pg. 15, 17 & 22 https://journals.lww.com/isoj/fulltext/2022/05010/approach_to_upper_cervical_trauma.3.aspx 
Note: The above information has not been reviewed or approved by a physician. Use at your own discretion.
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