In Vitro Evaluation of Self-Centering Pedicle Screws Using Computer-Aided Surgery Techniques

Design of pedicle screw fixation systems has significantly improved. The SilhouetteTM System (Sulzer Spine–Tech, Minneapolis, MN) is claimed to self–center into the vertebral pedicle. The screw is designed with a flat non–cutting surface on the outer thread and thus less likely to breach the pedicle cortex. The purpose of this study was to test this claim.

Utilizing cadaveric specimens and a frameless stereotactic guided system (Brain Lab Vector Vision, Munich, Germany) pedicle screws from the SilhouetteTM system and the tiMexTM system (Depuy–Acromed, Raynham, MA) were placed in the pedicles from L–1 to L–5. An equal number of screws were placed under two different variables. The entry point was varied either laterally or medially and the offset distance recorded. The second variable (angle) was altered using the navigational system after an ideal entry point was determined. Both screws were placed using minimal force and without the use of a tap. The cadavers underwent pre and post insertional CT scans, and dissection of the soft tissues from the cadaver for final analysis. The angle and depth of the pedicle screws were recorded at regular intervals utilizing the navigational system. CT analysis and visual inspection of the cadaveric specimens determined the final screw position and cortical violations.

Both screws show a predilection to correct and remain within the pedicle, however the SilhouetteTM screws correct more and are less likely to breach the cortex of the pedicle. Data will be presented.