The Intersection Between Lateral Mass and Inferomedial Edge of the C1 Posterior Arch: A Reference Point for C1 Lateral Mass Screw Insertion

Objective: To determine the ideal Atlas (C1) lateral mass screw placement and trajectory using the intersection between the lateral mass and inferomedial edge of the posterior arch as an easily identifiable and reproducible medial reference point. Selection of an ideal entry point and trajectory of C1 lateral mass screw insertion can help to minimize neurovascular injuries. While various techniques for screw insertion have been proposed in the past, they all require extensive dissection of the C1 lateral mass, which can cause profuse bleeding. Methods: Ninety-three 3-dimensional computed tomography reconstructed images of C1 lateral masses in adult patients were utilized to simulate the placement of C1 lateral mass screws via 4 entry points and 2 trajectory angles referencing off of a medial reference point using Vero's VISI 17 software. The safety during screw insertion simulation, as well as the screw length, were evaluated. Results: We found that C1 lateral mass screws could be safely placed bilaterally at 3 mm lateral to the reference point in both 0 degrees and 15 degrees medial screw angulation without violation of the cortex. The 15 degrees medial angulation allowed for longer (18 mm) screws than the 0 degrees angulation. Conclusion: We recommend starting C1 lateral mass screws 3 mm lateral to the intersection between the lateral mass and inferomedial edge of the posterior arch at a 15 degrees medial angulation.