Distance between the anterior surface of the dens and the posterior surface of the tubercle of C1 (predentate space, predental space, atlantodental distance):
Normal ≤ 3 mm in adults
Normal ≤ 5 mm in children
Atlantoaxial instability: increase in the predentate space of greater then 3 mm in adults and 5 mm in children
Anterior atlantoaxial dislocations may be associated with a fracture of the dens (~50% at autopsy. This injury is unstable. Associated fractures of the skull and/or facial bones are common.
Almost all atlantoaxial dislocations involve forward movement of C1 on C2; posterior dislocation is extremely rare.
Chronic atlantoaxial dislocations may be asymptomatic, and occure in different diseases (non traumatic): Down syndrome, Grisel syndrome, Rheumatoid arthritis, Osteogenesis imperfecta, Neurofibromatosis, Morquio syndrome, Psoriasis, Lupus.