Screw Orientation and Foam Density Interaction in Pullout of Anterior Lumbar Interbody Fusion Plates

Abstract

Introduction: Previous studies demonstrated increases in single screw pullout strength with increases in material density. Recent anterior cervical interbody fusion plate pullout studies utilizing a polyurethane foam block model have shown that alterations in screw insertion angle from straight-in are not associated with an increase in pullout strength. The purpose of this study is to characterize the pullout strength of an anterior lumber interbody fusion (ALIF) plate when installed at various screw angles in different simulated bone densities. Materials and methods:Ninety ALIF plate pullout tests were performed using three common screw insertion angles in polyurethane (PU) foam blocks of three densities: 0.08 g/cm 3 , 0.16 g/cm 3 and 0.24 g/cm 3 , simulating severely, mildly and nonosteoporotic cancellous bone, respectively. Plates were pulled out axially at 1 mm/min and pullout strength and stiffness compared.Results: Doubling foam density yielded 2.6-fold and 3.0-fold increases (p < 0.05) in mean pullout strength and stiffness, respectively. Tripling foam density yielded 4.5-fold and 5.3-fold increases (p < 0.05) in mean pullout strength and stiffness, respectively. Screw angle placement contributed relatively less to pullout strength and stiffness compared to PU foam density.Conclusion: In our model, ALIF plate pullout strength and stiffness appear to be more associated with increased foam block density than screw trajectory. Vertebral bone density should be a strong consideration in preoperative planning for ALIF with plating. Screw trajectory should be based on vascular anatomy and screw placement safety, rather than the classic lateral-to-medial trajectory.