Biomechanical Stability Afforded by Unilateral Versus Bilateral Pedicle Screw Fixation with and without Interbody Support Using Lateral Lumbar Interbody Fusion

Objective: To determine the stability of fusion constructs with unilateral pedicle screw (UPS) or bilateral pedicle screw (BPS) fixation with and without an interbody implant using the lateral lumbar interbody (LLIF) approach.

Methods: Standard nondestructive flexibility tests were performed on 13 cadaveric lumbar specimens to assess spinal stability of intact specimens and 5 configurations of posterior and interbody instrumentation. Spinal stability was determined as mean range of motion in flexion-extension, lateral bending, and axial rotation. Nonpaired comparisons were made for these 6 conditions: 1) intact; 2) unilateral interbody via the LLIF approach (LLIF construct); 3) unilateral interbody + unilateral pedicle screws (UPS) via the LLIF approach (LLIF + UPS); 4) unilateral interbody + bilateral pedicle screws (BPS) using the LLIF approach (LLIF+BPS); 5) UPS alone; and 6) BPS alone.

Results: UPS and BPS, with and without interbody support, significantly reduced range of motion during the majority of directions of loading. BPS alone provided greater stability than UPS alone and LLIF alone in all directions of motion except axial rotation. With interbody support, there was no significant difference in stability between BPS and UPS across all movement directions.

Conclusions: These biomechanical results suggest that fixation in the lumbar spine with an interbody support using an LLIF approach with UPS is a promising alternative to BPS. Although BPS provides greater immediate stability compared with UPS, in the presence of a lateral interbody implant, UPS and BPS provide equivalent stability. In addition, LLIF does not appear to contribute significantly to immediate stability when BPS is used.