Biomechanical comparison of an interspinous fusion device and bilateral pedicle screw system as additional fixation for lateral lumbar interbody fusion

• Both posterior augmentations (interspinous fusion and pedicle screws) performed similarly in an in vitro model.
• Complete lateral discectomy reduced the overall rigidity.
• Standalone lateral interbody cage may need additional rigidity in Extension motions.

BackgroundThis investigation compares an interspinous fusion device with posterior pedicle screw system in a lateral lumbar interbody lumbar fusion.MethodsWe biomechanically tested six cadaveric lumbar segments (L1–L2) under an axial preload of 50 N and torque of 5 Nm in flexion–extension, lateral bending and axial rotation directions. We quantified range of motion, neutral zone/elastic zone stiffness in the following conditions: intact, lateral discectomy, lateral cage, cage with interspinous fusion, and cage with pedicle screws.FindingsA complete lateral discectomy and annulectomy increased motion in all directions compared to all other conditions. The lateral cage reduced motion in lateral bending and flexion/extension with respect to the intact and discectomy conditions, but had minimal effect on extension stiffness. Posterior instrumentation reduced motion, excluding interspinous augmentation in axial rotation with respect to the cage condition. Interspinous fusion significantly increased flexion and extension stiffness, while pedicle screws increased flexion/extension and lateral bending stiffness, with respect to the cage condition. Both posterior augmentations performed equivalently throughout the tests except in lateral bending stiffness where pedicle screws were stiffer in the neutral zone.InterpretationA lateral discectomy and annulectomy generates immediate instability. Stand-alone lateral cages restore a limited amount of immediate stability, but posterior supplemental fixation increases stability. Both augmentations are similar in a single level lateral fusion in-vitro model, but pedicle screws are more equipped for coronal stability. An interspinous fusion is a less invasive alternative than pedicle screws and is potentially a conservative option for various interbody cage scenarios.