34. The effect of screw length on single-level anterior cervical fusion using finite element

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<h3>BACKGROUND CONTEXT</h3> Anterior cervical discectomy fusion and plating (ACDFP) is one of the most common neurosurgical procedures performed. Early in the use of plates to minimize postoperative orthosis duration and to increase healing rates, there was an attempt to use a bicortical screw purchase. Over time, we have typically converted to a lesser screw length to decrease any additional risks from the bicortical technique. Our study looks at the biomechanical effect of screw length at a single-level ACDFP and the effect on adjacent levels in flexion and extension using finite element modeling. <h3>PURPOSE</h3> To determine the effect of screw length on single-level anterior cervical discectomy, fusion and plating, and its clinical application. <h3>STUDY DESIGN/SETTING</h3> Finite element model. <h3>METHODS</h3> A previously validated three-dimensional osteoligamentous finite element model (FEM) of the cervical spine column (C2-T1) was used. The FEM simulated the vertebra of the body, posterior elements, intervertebral disc and ligaments. Each vertebral body consisted of a thin cortical shell, a softer cancellous bone, endplate, and a posterior bony structure. The cortical bone was modeled as a linear isotropic material of 0.5mm thick shell surrounding the cancellous bone and a 0.2mm thick endplate was placed on the superior and inferior surface of the intervertebral disc. Each single motion segment of the FEM consists of vertebral bodies, intervertebral disc and five major vertebral ligaments. The intervertebral disc was modeled and composed of the nucleus pulposus, annulus ground substance and annulus fibrosus. Material property parameters from literature were used as definitions in the model. The intact spine was modified at the C5-C6 functional spine unit level to simulate the biomechanics of ACDF with different screw lengths (12mm, 14mm, 16mm, 18mm). The standard surgical procedure was used to simulate ACDFP with different length screws at the C5-C6 level. The ALL was removed. The disc properties were altered at the surgical level to simulate the trabecular bone, and this represented complete fusion. The plate was placed at the C5-C6 level, and 4 titanium screws were simulated. All FEM were fixed at the first thoracic vertebra in all degrees of freedom, and load was applied at the C2 superior endplate vertebra. Bending moment (1.5 Nm) combined with a follower load (75N) magnitude under flexion and extension was applied to the intact model. The ROM was measured at each level and across the C3-C7 levels. The variable moment loading protocol (otherwise known as hybrid loading) was next applied. The bending moment was varied until the ACDF with different length screw models displayed the same total range of motion across the C3-C7 levels (although individual segments could have different range of motion). The ROM was obtained at the segmental levels under the same loading mode. <h3>RESULTS</h3> The C5-6 area of fusion demonstrated the least flexion (0.17°) and extension (0.36°) with the 18mm screw and the most flexion (0.75°) and extension (1.06°) with the 12 mm screws. The length of the screw effects the adjacent levels of the spine throughout with the largest increase in ROM in flexion (11.51° -11.98° with 12mm and 18mm screw length respectively) and extension (6.71°- 7.71° with 12mm and 18mm screw length respectively occurring at the cephalad C4-5 level. The caudal C6-7 adjacent level was more pronounced with an increase in flexion ROM with 12mm screws with 12.04° vs 11.88° with 18mm screws and increased but less pronounced ROM in extension with 6.34° with 18mm screws and 5.72° with 12mm screws. <h3>CONCLUSIONS</h3> Increased screw length results in higher flexion ROM at the C4-5 level. An inverse relationship is noted with 12mm screws at the C6-7 adjacent levels demonstrating more ROM than the 18mm screws in flexion. In extension both C4-5 and C6-7 demonstrate that increasing screw length results in increased ROM as compared to 12mm screws. This increased ROM would be expected to increase adjacent segment degeneration more prominently at the C4-5 level as compared to C6-7. <h3>FDA DEVICE/DRUG STATUS</h3> This abstract does not discuss or include any applicable devices or drugs.