Morphometric trajectory analysis for the C2 crossing laminar screw technique

This paper reports a morphometric study of the C2 laminae to provide quantitative anatomical data for safe crossing laminar screw placement. A valid trajectory is essential for C2 crossing laminar screw placement. Although several clinical technique notes and modifications to define a safe screw trajectory have been introduced in the recent years, no morphometric analysis has been performed to confirm the accuracy of screw trajectory using this technique. In this study, morphometric analysis was performed on 100 Korean C2 three-dimensional reconstruction CT images. The reconstructive C2 vertebrae from the post-edge of the spinal canal to the spinal process were divided into several zones, 1 mm each. Each zone was chosen as the entry point to imitate a crossing laminar screw (3.5 mm diameter) placement. In each 1-mm zoned trajectory, the screw pass ratio (PR), safe screw angle range (SAR) and maximum screw length (MSL) were measured and compared with the data from the other zoned trajectories. The zone '5-6 mm posterior to the post-edge of the spinal canal' was found to be a more feasible and safer entry point for guiding a crossing laminar screw placement than the other zones because this zone could provide a trajectory with maximal PR (85%), SAR (9.57 ± 4.36°) and a larger MSL (21.74 ± 2.44 mm) than the other areas. The recommended safe screw angle in the axial plane is 49.68 ± 4.94 to 59.19 ± 4.70°. However, the screw angle can vary considerably according to the individual variance. A preoperative evaluation of the screw trajectory is essential for safe screw placement using this technique.