Cervical pedicle screws - Comparative accuracy of two insertion techniques

Study Design. Independently assessed radiographic and anatomic comparison o f device implantation methods. Objectives, To compare the relative accuracy of two techniques of inserting cervical pedicle screws. Summary of Background Data. In an attempt to define the anatomic risks of c ervical pedicle screw insertion, image-guided stereotactic technology was s hown to be superior to some other methods in vitro. Meanwhile,:in vivo expe rience with Abumi's technique of screw insertion has had few clinically rel evant instances of screw malposition. There has been no direct comparison b etween current image-guided technology and Abumi's fluoroscopically assiste d technique. Methods, The pedicles (C3-C7) of human cadaveric cervical spines were instr umented with 3.5-mm screws with either of two techniques. Cortical integrit y and potential neurovascular injury were independently assessed by compute d tomographic (CT) scans and anatomic dissection. A cortical breach was con sidered "critical" if the screw encroached on any vital structure. If any p art of the screw violated the cortex of the pedicle but no vital structure was at risk for injury, the breach was classified as "noncritical." Results. In Group I (StealthStation; Sofamor-Danek, Memphis, TN), 82% of sc rews were placed in the pedicle, and 18% had a critical breach. In Group II (Abumi technique), 88% of screws were placed in the pedicle, and 12% had a critical breach. No statistically significant differences were demonstrate d between each group (P=0.59). Regarding pedicle dimensions and safety of i nsertion, a critical pedicle diameter of 4.5 mm was determined to be the si ze below which a critical breach was likely, but above which there was a si gnificantly greater likelihood for safe screw placement, The most common st ructure injured in each group was the vertebral artery. Conclusions. The use of a computer-assisted image guidance system did not e nhance safety or accuracy in placing pedicle screws compared with Abumi's t echnique. Both techniques have a noteworthy risk of injuring a critical str ucture if inserted into the pedicles with a diameter of less than 4.5 mm. U nder laboratory conditions, pedicles with a diameter of more than 4.5 mm ha ve a significantly greater likelihood of being safely instrumented by eithe r technique. These data indicate that cervical pedicle screw placement is f easible, but it should be reserved for selected circumstances with clear in dications and in the presence of suitable pedicle morphology.