Ossification patterns of the atlas vertebra.

Objective: The objective of this study was to characterize ossification patterns of the C1 (atlas) vertebra in children, to better differentiate normal variants from traumatic injury.

Materials And Methods: A retrospective review of all sinus and temporal bone CT examinations was performed for the period of 2002-2009. Patients 96 months old or younger for whom C1 level was at least partially imaged were included. Patients with a history of trauma or genetic disorder-associated spinal abnormalities were excluded.

Results: A total of 1270 CT examinations were reviewed. The anterior arch of C1 was completely imaged in 841 patients (66%), and the posterior arch was completely imaged in 378 patients (30%). Multiple anterior arch ossification centers were observed in 179 of 841 patients (21%), and posterior arch variants were present in nine of 378 patients (2%). At least partial ossification of the anterior arch was seen in 113 of 147 children (77%) younger than 25 months, whereas only 14 of the remaining 694 children (2%) older than 24 months failed to show any ossification. Incomplete ossification of the anterior arch was noted in 47 of 103 patients (46%) in the 85-96-month-old category. The posterior arches were at least partially ossified in all children. Incomplete fusion of the posterior synchondrosis was seen in 17 of 108 patients (16%) older than 60 months.

Conclusion: C1 ossification patterns and timing of synchondrosis fusion are variable. Knowledge of these patterns is important to better differentiate a normal variant from traumatic injury.