Computed tomographic morphometric analysis of lateral inclination C1 pedicle screw for atlantoaxial instability patients with a narrow C1 posterior arch.

Atlanto-axial instability (AAI) is a congenital or developmental condition that leads to instability/subluxation of the atlantoaxial joint. This study is performed to determine anatomic feasibility of trajectory L and help select an optimal screw trajectory in treating patients with AAI with a narrow C1 posterior arch. Sixty patients who underwent cervical three dimensional-computed tomography (3D-CT) were chosen from the hospital's picture archiving and communication system (PACS). Aquilion Multi 64 helical CT was used to perform CT examinations and measurements on patients. Based on CT images, the trajectory L and M for each C1 pedicle were established. Posterior arch height was measured on C1 coronal image. The medullary cavity width along trajectory M was narrower than trajectory L. The medullary cavity width that was less than 3.5 mm was found in 12.5% pedicles along trajectory M and 2.5% along trajectory L. The posterior arch height along trajectory L was significantly higher than that of trajectory M. Posterior arch height that was less than 3.5 mm was found in 74.2% pedicles along trajectory M and only 16.7% along trajectory L. The pedicle length along trajectory L was significantly longer than trajectory M. Trajectory angles for C1 pedicles had an average of 9.08° ± 4.66° along trajectory L and 14.53° ± 4.03° along trajectory M. The Optimal entry point of trajectory L was 5.4 mm medial to trajectory M. Based on our results, this study suggests that C1 pedicle screw trajectory with lateral inclination yielded a good outcome in the treatment of AAI.