Research Article Volume 7 • Issue 2 50 Exploring Flexibility vs. Stability: A Biomechanical Study on Stand-Alone Cages vs. Unilateral and Bilateral Pedicle Screw Fixation in Multilevel Lateral Lumbar Interbody Fusion and the Impact on Slope Variations Farid Amirouche, PhD 1,2 *, Ishani Patel, MPH 1 , Roberto Leonardo, Diaz, PhD 1 , Joe Mekhail, BS 1 , Craig Forsthoefel, MD 1,2 , James M, Mok, MD 1,3 Abstract Background Context: Lateral lumbar interbody fusion (LLIF) is utilized to treat various lumbar spine conditions, including degenerative disc disease, spondylolisthesis, and spinal instability. Although pedicle screws and rods are commonly added for fusion stability, they pose risks such as adjacent facet joint issues and guidewire-related vascular problems. Previous research has identified a direct link between the level of instrumentation and a reduction in spinal flexibility, prompting a critical question: What's the ideal balance between spinal flexibility and stability for successful fusion, challenging the idea of completely restricting natural spinal motion? Methods: Eight human cadaveric L1-L5 specimens were utilized, affixed to a universal testing machine (MTS 30/G) and subjected to optical motion-tracking technology for three- dimensional range of motion assessment. The specimens underwent testing under four conditions: 1) intact, 2) 26 mm lateral interbody stand-alone cages (stand-alone LLIF), 3) 26 mm lateral interbody cages with unilateral rod fixation at L1-L5 (LLIF + unilateral rod), and 4) 26 mm lateral interbody cages with bilateral rods fixation at L1-L5 (LLIF + bilateral rods). Results: From the intact condition, stand-alone LLIF decreased the slope of flexion by 0.29, extension by 0.89, left lateral bending by 0.93, and right lateral bending by 0.18. Compared to the stand-alone cages, LLIF with unilateral rod and pedicle screw fixation further decreased the slope of flexion by 0.08-0.30. Conversely, the implementation of bilateral rods and pedicle screws decreased slope by an additional 0.24-0.42 compared to the stand-alone cages. Conclusions: Our study found that the differences in ROM between stand-alone LLIF and using additional instrumentation amount to changes in slope below 1. This raises the question: Is the incremental decrease in ROM, often expressed in fractions, genuinely pivotal in the larger context of patient outcomes and overall well-being? Affiliation: 1 Department of Orthopaedics, University of Illinois at Chicago, Chicago, IL, USA 2 Northshore University Health System, Skokie, IL, USA 3 Stanford University School of Medicine, Palo Alto, CA *Corresponding author: Farid Amirouche, PhD, Vice-Chairman Basic Science Research, Department of Orthopaedic Surgery, Northshore University HealthSystem 9669 Kenton Avenue Suite 305, Skokie. IL 60076. Citation: Farid Amirouche, PhD, Ishani Patel, MPH, Roberto Leonardo, Diaz, PhD, Joe Mekhail, BS, Craig Forsthoefel, MD, James M, Mok, MD. Exploring Flexibility vs. Stability: A Biomechanical Study on Stand-Alone Cages vs. Unilateral and Bilateral Pedicle Screw Fixation in Multilevel Lateral Lumbar Interbody Fusion and the Impact on Slope Variations. Journal of Spine Research and Surgery. 7 (2025): 50-56. Received: March 24, 2025 Accepted: April 03, 2025 Published: May 20, 2025 Keywords: bilateral pedicle screw fixation; unilateral pedicle screw fixation; stand-alone cage; biomechanical spinal stability; cadaveric spinal fixation; lateral lumbar interbody fusion Introduction Lateral lumbar interbody fusion (LLIF) is a surgical technique employed to achieve spinal fusion in patients affected by diverse lumbar spine conditions, such as degenerative disc disease, spondylolisthesis, and spinal instability [1,2]. The primary clinical goals of LLIF include achieving spinal fusion, restoring disc height and alignment, and alleviating pain by addressing instability or neural compression. These outcomes are facilitated through indirect decompression, segmental stabilization, and biomechanical load redistribution by fusing two or more vertebral levels [3-5]. During the LLIF procedure, the surgeon skillfully removes the damaged disc and introduces a biocompatible interbody cage into the disc space, serving as a spacer to restore disc height and offer support to adjacent vertebrae [1]. The cage's design, often constructed from materials like titanium or polyetheretherketone, may feature porous surfaces to encourage bone growth in and around the cage, promoting fusion [6,7].