Failure Mode of Pedicle Screw Fixation Depends Upon the Presence, Absence, and Position of Interbody Spacers. A Pilot Study

Transpedicular screw fixation permits segmental stabilization to supplement anterior, transforamenal, posterior lumbar interbody, or postero-lateral fusion techniques. This study was designed to evaluate the use of ASTM F1717-14 in determining the failure mode of pedicle screw constructs using these four different fusion constructs. Single level pedicle screw constructs were assembled and tested for axial stiffness and possible failure mode. The constructs included interbody spacers placed anteriorly, unilaterally, posteriorly, and with no spacer. The anterior specimens failed at supraphysiologic loads with rods bending in the sagittal plane. The unilateral specimens failed in physiologic range of loads with expulsion of the spacers and deformation of the constructs in the coronal plane. The posterior specimens failed at supraphysiologic loads and relatively large cycle numbers with screw fatigue fractures and subsequent set screw disengagement. The specimens with no spacers failed at physiologic loads with abutting of the anterior aspect of the test blocks. Failure of lumbar fusion constructs in this laboratory model occur in different modes depending on the fusion construct. In general, when the spacer is placed in line with the load axis, the pedicle instrumentation is "protected" from failure. When the spacer is off the load axis or no spacer is used, the pedicle screw system is subject to potential failure within physiologic loads. Considering the different modes of failure depending on placement of the interbody device, no single testing model is applicable to all clinically used constructs.