Biomechanical Changes of Adjacent Segments Following Single- versus Two-level Oblique Lateral Interbody Fusion with Different Fixation Methods: A Finite Element study

Background This study investigated the biomechanical effects of single- and two-level oblique lateral interbody fusion (OLIF) with different internal fixation methods on adjacent segments in an osteoporotic lumbar spine model, focusing on range of motion (ROM), intervertebral disc stress, endplate stress, and facet joint contact forces. Methods A three-dimensional finite element (FE) model of the L1–S1 spine was developed from CT scans of a healthy 30-year-old male and modified to simulate osteoporosis. The model included vertebral bodies, posterior elements, endplates, discs, and major ligaments. Material properties were assigned based on established literature, with modifications to simulate osteoporosis. Nine surgical scenarios were simulated: standalone cages, bilateral pedicle screws (BPS), and cortical bone trajectory (CBT) screws for both single- and two-level OLIF. A 400 N follower load and 10 Nm pure moments in six directions were applied to L1 to replicate physiological loading. Results All fusion constructs increased ROM and mechanical stress at adjacent segments compared with the intact osteoporotic spine. Two-level fusion induced significantly greater biomechanical alterations than single-level fusion. In the L2–L3 segment, the L3–L5 + BPS model exhibited the highest flexion range of motion (ROM) of 6.70° and peak disc stress of 2.20 MPa, whereas the L3–L4 cage + BPS configuration demonstrated a ROM of 6.07° and peak disc stress of 2.02 MPa. Similarly, at the L5–S1 level, the L3–L5 + BPS construct produced the greatest flexion ROM (14.69°) and disc stress (3.67 MPa), compared with the L4–L5 + BPS construct, which yielded a ROM of 13.14° and disc stress of 3.34 MPa. CBT fixation consistently produced lower disc and endplate stresses compared with BPS fixation; however, it resulted in substantially higher facet joint stresses, particularly during axial rotation. In the L3–L5 + CBT construct, L5–S1 facet joint stress increased by 143% relative to the intact condition. Conclusion Two-level constructs impose greater cumulative loads than single-level ones. Compared with bilateral pedicle screws, cortical bone trajectory screws reduce anterior column loading but increase facet joint stress, especially during rotation.