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

Author(s): Farid Amirouche, PhD, Ishani Patel, MPH, Roberto Leonardo, Diaz, PhD, Joe Mekhail, BS, Craig Forsthoefel, MD, James M, Mok, MD

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 threedimensional 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?