Fielding-Hawkins classification

Atlanto-axial instability

Type I

Anterior displacement of C1 on lateral X-ray <3 mm

Type II

Anterior displacement of C1 on lateral X-ray 3-5 mm

Type III

Anterior displacement of C1 on lateral X-ray >5 mm

Type IV

Posterior displacement of C1 on C2

Reference

The Neurosurgeon's Handbook. Ed. G Samandouras. Oxford University Press, 2010.