Type I

No subluxation. Fixed rotation of atlas and axis. Anterior displacement < 3 mm.

Type II

Unilateral, atlantoaxial subluxation of one atlantoaxial joint. The contralateral joint is pivotal.

Type III

Ventral subluxation of C1 in both joints. Anterior displacement > 5 mm.

Type IV

Dorsal subluxation of C1. Combination with fractured dens axis or congenital dens aplasia is possible.