Positional effects of transforaminal interbody spacer placement at the L5-S1 intervertebral disc space: a biomechanical study

BACKGROUND CONTEXT: Transforaminal lumbar interbody fusion (TLIF) is an increasingly used alternative fusion method over anterior and posterior lumbar interbody fusions. There are conflicting results on the optimal positioning of interbody devices. No study has addressed the lumbosacral segment, L5-S1, where the lordotic configuration presents unique challenges. PURPOSE: To determine if there are biomechanical and/or anatomical advantages related to the positioning of an interbody device at L5-S1, either anterior or posterior to the neutral axis. STUDY DESIGN: An in vitro biomechanical study using human cadaveric lumbar specimens. METHODS: Lumbar specimens were biomechanically tested using pure moments with and without compressive axial loading. Testing was performed in intact and after TLIF with the implant posterior (TLIF-post) and anterior (TLIF-ant) to neutral axis. Segmental range of motion (ROM) and stiffness were analyzed at the L5-S1 surgical level and the adjacent L4-L5 level. Neuroforaminal height measurements of L5-S1 were analyzed in neutral and end range positions. RESULTS: Compared with the intact condition, ROM decreased more than 75% at L5-S1 and stiffness increased up to 270% with TLIF. There was no significant difference between anterior or posterior placement for ROM and stiffness. There was a change in L5-S1 neuroforaminal height based on the placement, with posterior placement showing a significant increase compared with anterior placement. There were no relative changes in neuroforaminal height under loading after TLIF. Compressive load did not affect the magnitudes or resulting significance of outcome measures at L5-S1 after either TLIFs. CONCLUSIONS: An interbody spacer with the addition of posterior instrumentation significantly enhances the mechanical stability of L5-S1 regardless of interbody position. There were noticeable increases in terms of construct stability and stiffness after both TLIF-ant and TLIF-post in comparison with the intact condition. A posteriorly placed interbody implant did result in the distraction of the neuroforamin. Positioning an interbody implant at L5-S1 for TLIF with posterior instrumentation should be at the discretion of the surgeon without consequence to biomechanical stability. (C) 2014 Elsevier Inc. All rights reserved.