| 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. |