Abbreviation; OO (os odontoideum), AOA (atlanto-occipital assimilation), KFS (Klippel-Feil syndrome), Num (number), OH (odontoid hypoplasia), BI (basilar invagination), AC (Arnold Chiari), AOD (Atlanto-occipital dislocation), ADD (atlanto-dental dislocation), RAAD (rotatory atlantoaxial dislocation), MM (myelomalacia), Syr. (syringomyelia)