Accuracy of cervical pedicle screw placement using the funnel technique

Study Design. This was a cadaver study assessing the accuracy of cervical pedicle screw placement. Objective. To evaluate the accuracy of the funnel technique of screw placement. Summary of Background Data. Although excellent results have been reported in clinical studies, with no major neurovascular injuries, several cadaveric studies have shown a high pedicle perforation rate during screw placement. Methods. Ten fresh frozen cervical spines (C2-C7) were used (120 pedicles, 20 pedicles per level). The average specimen age was 79.6 years (range 65-97); the average height was 159 cm (range 155-175). The male-to-female ratio was 3:7. Pedicle width and angulation were measured on preoperative axial computed tomography (1-mm slices). By use of four bony landmarks and the funnel technique, screws were placed under direct vision. Critical perforations (documented contact of a screw with, or an injury to, a spinal cord, nerve root, or vertebral artery) and noncritical perforations (a perforation with no critical contact) were recorded. Results. In seven pedicles (5.8%) the procedure was aborted because of a small or nonexistent pedicle medullary canal. Ninety-four pedicle screws (83.2%) were placed correctly, whereas 11 pedicles (9.7%) had noncritical perforations and 8 pedicles (7.1%) had critical perforations. The majority of the critical and noncritical perforations were at C3, C4, and C5. Conclusions. Axial computed tomography is necessary for the preoperative planning. Because of the small diameter and steep angulation of cervical pedicles, every spine surgeon who intends to use pedicle screws should first master the technique on cadavers.