Optimal entry points and trajectories for cervical pedicle screw placement into subaxial cervical vertebrae

Abstract

The present study was performed to determine the optimal entry points and trajectories for cervical pedicle screw insertion into C3-7. The study involved 40 patients (M:F=20:20) with various cervical diseases. A surgical simulation program was used to construct three-dimensional spine models from cervical spine axial CT images. Axial, sagittal, and coronal plane data were simultaneously processed to determine the ideal pedicle trajectory (a line passing through the center of the pedicle on coronal, sagittal, and transverse CT images). The optimal entry points on the lateral masses were then identified. Horizontal offsets and vertical offsets of the optimal entry points were measured from three different anatomical landmarks: the lateral notch, the center of the superior edge and the center of lateral mass. The transverse angle and sagittal angles of the ideal pedicle trajectory were measured. Using those entry points and trajectory results, virtual screws were placed into the pedicles using the simulation program, and the outcomes were evaluated. We found that at C3-6, the optimal entry point was located 2.0-2.4mm medial and 0-0.8mm inferior to the lateral notch. Since the difference of 1mm is difficult to discern intra-operatively, for ease of remembrance, we recommend rounding off our findings to arrive at a starting point for the C3-6 pedicle screws to be 2mm directly medial to the lateral notch. At C7, by contrast, the optimal entry point was 1.6mm lateral and 2.5mm superior to the center of lateral mass. Again, for ease of remembrance, we recommend rounding off these numbers to use a starting point for the C7 pedicle screws to be 2mm lateral and 2mm superior to the center of lateral mass. The average transverse angles were 45° at C3-5, 38° at C6, and 28° at C7. The entry points for each vertebra should be adjusted according to the transverse angles of pedicles. The mean sagittal angles were 7° upward at C3, and parallel to the upper end plate at C4-7. The simulation study showed that the entry point and ideal pedicle trajectory led to screw placements that were safer than those used in other studies.