ANTERIOR VERTEBRAL BODY SCREW PULLOUT TESTING - A COMPARISON OF ZIELKE, KANEDA, UNIVERSAL SPINE SYSTEM, AND UNIVERSAL SPINE SYSTEM WITH PULLOUT-RESISTANT NUT
Ih. Lieberman et al., ANTERIOR VERTEBRAL BODY SCREW PULLOUT TESTING - A COMPARISON OF ZIELKE, KANEDA, UNIVERSAL SPINE SYSTEM, AND UNIVERSAL SPINE SYSTEM WITH PULLOUT-RESISTANT NUT, Spine (Philadelphia, Pa. 1976), 23(8), 1998, pp. 908-910
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.