Oblique, Unilateral, or Bilateral Rods Configurations for Single-Level Interbody Fusion and Posterior Spinal Fixation: A Finite Element Study

AIM: To compare three different posterior mono-segmental instrumented models with a Lateral Lumbar Interbody Fusion (LLIF) cage in L4-L5 based on finite element (FE) analysis. MATERIAL and METHODS: Three different configurations of posterior instrumentation were created: 1. Bilateral posterior screws with 2 rods: Bilateral (B); 2. Left posterior rod and left pedicle screws in L4-L5: Unilateral (U); 3. Oblique posterior rod, left pedicle screw in L4, and right pedicle screw in L5: Oblique (O). The models were compared regarding the range of motion (ROM), stresses in the L4 and L5 pedicle screws, and posterior rods.RESULTS: The Oblique and Unilateral models showed a lower decrease in ROM than the Bilateral model (O vs U vs B; 92% vs 95% vs 96%). In the L4 screw, a higher stress level was identified in the O than in the B model. Still, lower if compared to U. In the L5 screw, the highest stress values were observed with the O model in extension and flexion and the U model in lateral bending and axial rotation. The highest stress values for the rods were observed for the O model in extension, flexion, and axial rotation and the U model in lateral bending.CONCLUSION: The FE analysis showed that the three configurations significantly reduced the ROM. The stress analysis identified a substantially higher value for the rod and pedicle screws in oblique or unilateral configuration systems compared to the standard bilateral one. In particular, the oblique configuration has stress properties similar to the unilateral in lateral bending and axial rotation but is significantly higher in flexion-extension.