Computer-aided analysis for optimal screw insertion in lateral mass of C1: An anatomical study

Motion preserving techniques in C1 ring fractures are increasingly used especially in young patients. Therefore, lateral mass screws are inserted in the first vertebra and connected by a rod. The purpose of this study was to determine safe zones regarding the vertebral arteries and the medulla oblongata for optimal lateral mass screw positioning when fusing the C1-ring.

Images of the cervical spine of 50 patients (64-line CT scanner) were evaluated and virtual screws were positioned in both lateral masses of the first vertebra using 3D-reconstructions of CT scans. The length of the screws, the insertion angles in two planes, the distance to the vertebral artery, and the spinal canal was investigated. Descriptive statistics was used and gender-dependent differences were calculated using student t-test. A diameter of 4 mm was chosen for the screws.

The mean screw length was 30.0 ± 2.3 mm on the right and 30.1 ± 2.1 mm on the left side. The arithmetic mean for the transverse angle was 16.4 ± 5.6° on the right and 15.6 ± 6.3° on the left, the sagittal angle averaged 8.3 ± 3.8° on the right, and 11.0 ± 4.9° on the left side. The mean distance between screw and spinal canal has been determined on the right with 2.4 ± 0.7 mm and 2.2 ± 0.6 mm on the left side. The distance from the C1 lateral mass screw to the vertebral artery was on average 7.1 ± 1.5 mm on the right side (significant correlation with gender, p value: 0.03) and 7.4 ± 1.4 mm on the left side.

Screws should be positioned with a slightly converging angle of 16° and a slightly ascending angle of 10°. Due to the required high precision technique intraoperatively multiplanar 2 D or 3 D imaging is recommended to avoid harm to the vertebral artery or the spinal canal.