Displaced fractures with widening greater than 6.9 mm (rule of Spence: overhang on open mouth odontoid view) suggests injury of the transverse atlantal ligament.
For the ATLAS fractures the Gehweiler classification is used together with the Dickman classification.
These injuries are associated with transverse atlantal ligament injury which can be either:
Pure ligament rupture (Dickman type 1)
Avulsion (Dickman type 2).
In the presence of ligament rupture, open reduction and C1-C2 fusion is required.
Avulsion fractures of the transverse ligament (the ligament pulls off a piece of bone from the lateral mass) will usually heal if the patient is immobilized with a halo vest. The diagnosis is made on an axial CT image.