ANTERIOR VERTEBRAL BODY SCREW PULLOUT TESTING - A COMPARISON OF ZIELKE, KANEDA, UNIVERSAL SPINE SYSTEM, AND UNIVERSAL SPINE SYSTEM WITH PULLOUT-RESISTANT NUT

Study Design. A biomechanical study of pullout of anteriorly implanted screws in cadaveric vertebral bodies. Objectives. To investigate and compare the pullout strength of the Zielke, Kaneda, Universal Spine Sy stem (USS) pedicle screw, and USS pedicle screw with a new pullout-res istant nut. Summary of Background Data. A common problem with anterior purchase regardless of the implant system is screw pullout at the pro ximal and distal ends of multilevel constructs. There is limited infor mation on a solution to this problem. Methods. The L1 to L4 vertebral bodies from four cadavers had one each of Zielke and Kaneda pedicle sc rews (Acromed Corp, Cleveland, OH), USS pedicle screw (Synthes Spine, Paoli, PA), and USS pedicle screw with pullout-resistant nut implanted transversely across the center of the vertebral body with bicortical purchase in a similar fashion as would be used clinically. The screws were extracted using a servohydraulic material testing system. The max imum axial forces were recorded. Results. The Zielke and Kaneda screws had no significant difference in mean pullout strength (P = 0.542). T he USS screw alone was less strong (P = 0.009). The USS screw and pull out-resistant nut increased the pullout strength by twofold (P = 0.000 06). In the screw pullout tests, the mode of failure was at the screw thread's interface, The USS screw and pullout-resistant nut failed by imploding the body around the nut. With the USS screw and pullout-resi stant nut, the pullout strength was determined by the compressive stre ngth of the bone. Conclusions. The addition of a pullout-resistant nut to an anterior vertebral body screw improves the pullout strength by twofold and changes the mode of failure to rely ultimately on the inhe rent vertebral body strength rather than the screw's characteristics. The addition of a pullout-resistant nut may be applicable to multileve l implant constructs to prevent screw pullout at the top and bottom.