33. Comparison of the biomechanics of lumbar spine instrumented with standalone interbody fixation constructs vs interbody with supplemental fixation: a finite element investigation

Abstract

<h3>BACKGROUND CONTEXT</h3> Low fusion rates and cage subsidence have been reported as the main drawbacks of lumbar fixation with static interbody cages. Although several clinical and biomechanical studies have evaluated the efficacy of 360 interbody fixation constructs (anterior cage plus posterior fixation), no study has reported the biomechanical comparison between such constructs and more novel techniques which use standalone fixation implants. A cadaver validated computational model of lumbar spine was used to compare the biomechanics of spine instrumented with 360 fixations vs standalone cage and screw and cage with lateral plate systems. <h3>PURPOSE</h3> To compare the mechanical stability of different interbody fixation techniques in lumbar spinal segments with standalone interbody vs static cage with posterior fixation or lateral plate system. <h3>STUDY DESIGN/SETTING</h3> Finite element modeling. <h3>OUTCOME MEASURES</h3> Range of motion across instrument segments and load sharing on vertebral endplate. <h3>METHODS</h3> An experimentally validated finite element (FE) model of L1-pelvic segment was used to simulate ALIF and LIF lumbar fixation techniques including: ALIF cage at L5-S1 plus posterior screw-rod fixation (360 construct) vs ALIF standalone (screw through the cage). LIF cage at L4-L5 vs LIF cage with integrated two-hole lateral plate system. 4WEB Medical's Truss ALIF (27mm x 25mm), Lateral Truss (22mm X 5mm) cages and 2-hole integrated plate systems were used for simulation of the surgical procedure. For 360 constructs, a generic posterior rod and screw system was used. All models were subjected to a 400N compressive pre-load followed by an 8Nm moment to simulation flexion-extension, left and right bending and axial rotation motions. The segmental kinematics and the load sharing at the inferior endplate were compared among cases. <h3>RESULTS</h3> The segmental motion in standalone ALIF construct was 1.1°(Flex-Ext), 0.9° (LB) and 0.4° vs 0.8°, 0.8° and 0.6° in 360 ALIF in the same planes of motion. When comparing lateral cases, the motions were 1.2° (Flex-Ext), 1.4° (LB) and 0.5° (AR) in lateral cage with plate vs 0.9°, 1.0° and 0.8° in the 360 lateral construct for the same loads. The peak stresses in extension for the LIF stand-alone cage were somewhat higher than the posterior instrumented cases, but not significant. When comparing the mechanical stress on the inferior endplate of the index segment, the standalone ALIF had almost 20% higher peak stress compared to the 360 ALIF. In the lateral construct, the cage-plate segment experienced 15% lower stresses on the endplate compared to the 360 lateral construct. <h3>CONCLUSIONS</h3> Our data suggest that standalone ALIF cage with screw through the cage is able to provide kinematic stability in the fixation construct comparable to the cage plus posterior fixation, although the 360 construct is able to provide slightly more stability in the sagittal plane. Also, the standalone cage results in higher stresses applied to the endplate compared the cage in 360 constructs. The lateral cage with integrated plate had superior stability in axial rotation compared to the 360 lateral construct. The loads on the endplate were also slightly lower compared to the 360-fixation case. Standalone ALIF and LIF with lateral plate are biomechanically effecient alternatives to 360-fixation constructs at least under the controlled conditions analyzed in the present study. Clinical data are required to support the findings and define the further role and application of standalone cages. <h3>FDA DEVICE/DRUG STATUS</h3> 4WEB ALIF Truss System (Approved for this indication), 4WEB Lateral Plate System (Approved for this indication), 4WEB Lateral Truss System (Approved for this indication)