Rule of Spence – Lateral Mass Dislocation (LMD)

Lateral Mass Displacement (Overhang)

Although it is not well-utilized in clinical settings, there are many medical references for the Sum of Lateral Mass Displacement (LMD).  This radiology measurement may also be referenced as Rule of Spence or Lateral Mass Overhang.

LDM is used in radiology for assessing the integrity of a key ligament in the area of the skull and upper cervical spine, known as the transverse ligament, but also referenced as Transverse Atlantal Ligament (TAL).  It refers to a radiographic method used to evaluate the alignment of the atlas (C1 vertebra) and axis (C2) un the upper cervical spine, just below the skull.

There is disagreement in the medical community as to the reliability of this measurement as an indication of TAL failure, and therefore, should be used as an adjunctive tool to CT and MRI scans, rather than an absolute rule.

If the combined LDM measurement of the right and lateral masses of C1 is hanging over the lateral masses of C2 by more than 6.9 mm, then it may indicate a transverse ligament injury.

Medical articles typically use Sum of the Rule of Spence, meaning the sum of both the left and right LDM measurements, although LDM can also be measured and reported separately. i.e. LDM Left, LDM Right.

It can be measured on XRay and MRI but it is best done on CT Scans or Cone Beam CT Scans (located in some upper cervical chiropractors offices) for greater visual quality and accuracy.  This is completed by the patient standing upright and flexing the head / ear to the shoulder, on the left side and then on the right side.

It has been determined that if the total LDM exceeds 6.9mm, there is a high likelihood of TAL injury / rupture.  If LDM is less than 5.7 mm, TAL injury is unlikely.