Is L5/S1 interbody fusion necessary with concurrent iliac fixation for long segment spinal fusion constructs?-a systematic review of biomechanical studies.

Background: Long segment fusion for adult spinal deformity (ASD) has a significant rate pseudoarthrosis, which may be in part due to the mechanical forces across the lumbosacral junction (LSJ). Placement of an interbody cage at the L5-S1 disc space and iliac fixation are two strategies to decrease strain at the L5-S1 level. This study assesses the current literature on biomechanics of the LSJ as it pertains to instrumentation in the context of long segment fusion for ASD. Methods: A systematic review of MEDLINE via the PubMed database and EMBASE was performed by two independent reviewers. Studies were included if they featured human cadaveric studies that had undergone multilevel spinal fusion involving the L5-S1 junction, and studies that measured biomechanical differences with and without iliac fixation and interbody fixation at L5-S1. Results: From a biomechanical standpoint, anterior lumbar interbody fusion (ALIF) and iliac fixation are similar in their reduction in range of motion (ROM) about the L5-S1 joint as well as S1 screw strain, compared to pedicle screws alone. Iliac fixation appears to reduce screw strain in more directions compared to ALIF. However, iliac fixation significantly increases posterior rod strain. No studies showed statistically significant biomechanical differences with concurrent iliac and interbody fixation, although 4 of 7 studies reported a trend towards decreased L5-S1 ROM. Conclusions: Both iliac fixation and ALIF cages decrease S1 screw strain and L5-S1 ROM in the setting of multilevel fusion constructs to the LSJ. There is no biomechanical evidence that concurrent iliac fixation and ALIF cages provide significant additional benefit. Larger biomechanical and clinical studies are warranted to better understand the relationship between the two strategies of reduction strain and successful arthrodesis across the LSJ.