P23. Inline axial forces associated with implantation of ALIF interbody devices with integrated supplemental fixation

Abstract

<h3>BACKGROUND CONTEXT</h3> Anterior lumbar interbody fusion (ALIF) allows for an optimal sized interboy device while providing correction for lumbar lordosis, restoration of disc height and indirect decompression. Spinal interbody devices with integrated fixation through blades, screws or other supplemental anchoring components mitigate the need for supplemental instrumentation, thus reducing OR time and preserving the stability of the functional spinal unit. Unfortunately, interbody devices have failed at the bone implant interface resulting in complications including implant migration and expulsion. Gaining a better understanding of the biomechanics associated with interbody placement may help to prevent these complications in the future. The aim of present study is to determine the magnitude and effect of axial forces during the deployment of an interbody device with supplemental integrated fixation. <h3>PURPOSE</h3> To understand the axial loading imparted to the functional spinal unit during lumbar interbody fusion surgery. <h3>STUDY DESIGN/SETTING</h3> Biomechanical axial load mesaurement in cadaveric functional spinal units during lumbar disc preparation and spinal interbody device implant procedures. <h3>METHODS</h3> Cadaveric functional spinal units, levels L2-L3, L3-L4, L4-L5, and L5-S1, were decorticated to the osteoligamentous structures. Posterior elements were removed to allow for MicroCT imaging of each specimen at a voxel size of 27 micron before and after instrumentation. A custom test fixture was used to apply 50N compressive force to stabilize the specimen and also record inline axial forces associated with the deployment of anchor plates during instrumentation procedures. A Chatillon DFS II force gauge was used to record axial forces during implantation of ENZA 1.0 and ENZA 2.0 interbody devices. Axial forces resulting from implant trialing, device insertion and anchor deployment were recorded. MicroCT scans were repeated to allow for quantification of damage to bony structures as a result of implantation procedures. <h3>RESULTS</h3> L2-L3 and L3-L4 force outputs are similar for both insertion, 218.82 N, 219.89 N, and deployment forces, 34.58 N, 44.88 N respectively. Insertion forces throughout all spinal levels were larger than both deployment and removal forces. Insertion forces were larger in the proximal levels, 218.82 N, 219.89 N, when compared to the distal levels, 178.96 N, 113.76 N. However, deployment force was larger in the distal levels, 99.03 N, 77.76 N, when compared to proximal 34.58 N, 44.88 N. <h3>CONCLUSIONS</h3> As evidence mounts for the cost-effectiveness of standalone cages with integrated supplemental fixation, it is imperative that design requirements of these devices continue to improve. Previous studies have shown that the vertebral endplate is susceptible to fracture under compressive loading, with the weakest area of the vertebrae being the center region. In 2001, Hasegawa et al showed a cylindrical-shaped implant caused endplate fracture between 510N and 1335N and that implant size is inversely related to the force required to cause fracture. Endplate fracture is difficult to detect with standard imaging modalities and leaves the patient vulnerable to implant subsidence and reduces segmental stability as a result of changes in load transfer. Furthermore, endplate failure may be a contributor to low back pain, further reducing the chance of a successful outcome. Our study showed interbody devices with integrated fixation plates could safely be inserted and deployed in the disc space without causing endplate fracture. The implant-endplate interface should be investigated in greater detail to allow for design of standalone interbody devices which do not cause unintended distraction or worse, damage to the vertebrae. <h3>FDA DEVICE/DRUG STATUS</h3> ENZA Lumbar Interbody Fusion Device (Approved for this indication).