Anatomic considerations for standard and modified techniques of cervical lateral mass screw placement

Background context: Posterior screw placement techniques have been previously described. Each technique differs with respect to starting point, lateral angulation and sagittal orientation. Purpose: To examine the potential for injury to critical anatomic structures, such as nerve roots and vertebral arteries, during posterior cervical screw placement and to determine safe screw placement. Study design/setting: An anatomic study was conducted to determine the optimal screw angulation for posterior cervical lateral mass screws. Specimen sample: Ten fresh-frozen human cadaveric cervical spine specimens were used, consisting of four females and six males, ranging in age from 32 to 68 years. Outcome measures: Angular measurements and distance from nerve root and vertebral arteries were measured with a single caliper and recorded. One millimeter of proximity to a vital structure was considered a violation of that structure. Methods: Ten fresh-frozen human cadaveric cervical spine specimens were instrumented from C2 to C7 by a single surgeon. Kirschner wires (2.0 mm) were used to reproduce the Roy-Camille, Anderson and Magerl screw trajectories. The wire was drilled through each lateral mass, simulating overdrill error. Each technique was instrumented according to the original description and with additional modifications. The modification consisted of varying the angle of screw placement in the axial plane of the original description from 0 to 30 degrees. Distances to the closest neurovascular structures were averaged for all assays. Results: The Magerl technique is safe at the standard position and modified positions of 20 degrees and 30 degrees from C3-C6. The Roy-Camille technique frequently violates neurovascular structures below C3, especially the nerve root with more lateral screw angulation. The standard technique is noted to have good bone purchase only at C2 and C3. The Anderson technique is safe at 20 degrees and 30 degrees modified positions from C3-C7. Posterior screw placement at the C7 vertebral level was safe only with a modified Anderson technique of 20 degrees and 30 degrees of lateral screw angulation. Conclusions: The present study indicates that there are significant differences of potential neurovascular injury, which is dependent on the technique used for screw entry, the level instrumented and the angle of screw trajectory in the parasagittal plane.