Central and juxta-endplate vertebral body screw placement - A biomechanical analysis in a human cadaveric model

Study Design. In vitro biomechanical testing of transvertebral body screws in different positions in both axial pull-out and toggle. Objectives, To determine the relative strength of unicortical versus bicortical screw fixation within the vertebral body and to determine comparative strength of juxta-endplate and central screw positions with and without staples in both axial pull-out and toggle modes. Summary of Background Data. Loss of fixation is common in centrally placed screws at the rostral end of a construct. To preserve segmental vessels, juxta-endplate screw positions are often used. The biomechanical strength of such screw placement methods has not been measured. Methods. Eighty-three human cadaveric vertebral dies were tested for axial pull-out and toggle with and without staples. Screw positions included central, juxta and inferior endplate. Juxta-endplate screws were toggled in both the rostral and caudal directions perpendicular to the screw axes. Results. Unicortical fixation resulted in a 93% decrease In axial pull-out strength compared with bicortical fixation. Centrally placed screws and juxta-endplate screws were equivalent in axial pull-out if no staples were used. The juxta-endplate screw with a staple that was toggled away from the endplate had the highest yield strength, followed by the central screw with a staple, and then the juxta-end plate screw without a staple toggled away from the endplate. Conclusions. Bicortical fixation is much stronger than unicortical fixation. Centrally placed screws are significantly, stronger when used with a staple. When preservation of segmental vessels is desirable, juxta-endplate screws should be placed in such a manner that compressive forces are directed away from the endplate.