An open-mouth odontoid view in a victim of a diving accident shows bilateral outward displacement of the C1 lateral masses relative to C2, with total combined displacement of 8 mm. What is the most important implication of this measurement?

• A Isolated stable C1 ring fracture, treat with a soft collar
• B Rupture of the transverse atlantal ligament, indicating an unstable injury ✓
• C Type II odontoid fracture requiring halo immobilisation
• D Normal variant of C1-C2 articulation, no further imaging needed

Explanation

This is the rule of Spence applied to a Jefferson burst fracture of C1. When the sum of the lateral displacements of both C1 lateral masses over C2 exceeds 6.9 mm (approximately 7 mm) on the open-mouth view, the transverse atlantal ligament is presumed ruptured, rendering the injury unstable and mandating MRI and surgical consultation. Displacement below this threshold suggests an intact transverse ligament with a potentially stable fracture. An odontoid fracture would be seen at the dens itself, not as lateral mass offset.

Reference: Grainger and Allison's Diagnostic Radiology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.