It is the lateral head tilt, not head rotation, causing an asymmetry of the odontoid-lateral mass interspace

Asymmetry in odontoid-lateral mass interspace in trauma patients is a common finding that regularly leads to additional diagnostic work-up, since its dignity is not entirely clear. There is little evidence in the literature that atlantoaxial asymmetry is associated with C1-C2 instability or (sub) luxation. Asymmetry in odontoid-lateral mass interspace seems to occur occasionally in healthy individuals and patients suffering a cervical spine injury. Congenital abnormalities in odontoid-lateral mass asymmetry may mimic an atlantoaxial asymmetry. The center of C1-C2 rotation is based in the peg of dens axis; therefore, a C1-C2 rotational influence seems unlikely. So far, no study examined the influence of C0-C1-C2 tilt to an asymmetry in odontoid-lateral mass interspace.

In order to determine if rotation or tilt influences the lateral atlantodental interval (LADI) and to estimate physiologic values, we examined 300 CT scans of the cervical spine.

The mean LADI was 3.57 mm and the mean odontoid-lateral mass asymmetry was 1.0 mm. Head position during CT examination was found to be rotated in 39 % of the cases in more than 5°. Subsequent mean C0/C2 rotation was 4.6°. There was no significant correlation between atlantoaxial asymmetry and head rotation (p = 0.437). The average tilt of C0-C1-C2 was found to be 2°. We found a significant correlation between tilt of C0-C1-C2 and asymmetry in odontoid-lateral mass interspace (p = 0.000).

We conclude that an atlantoaxial asymmetry revealed in CT scans of the cervical spine occurs occasionally. Since head tilt correlates with an atlantoaxial asymmetry, it is crucial to perform cervical spine CT scans in a precise straight head position.