Effects of L4-L5 fusion on the adjacent levels in the lumbar spine

Spondylolisthesis and degenerative disc disease can, in some cases, be treated using lumbar fusion techniques; however, the effects of these procedures on the surgical site, adjacent vertebral bodies, and intervertebral discs are not fully understood. In this work, a three-dimensional finite element model of the L1-L5 lumbar spine was developed using Computed Tomography data from a healthy individual, to evaluate stress redistribution following L4–L5 fusion. Four models were evaluated: a healthy spine without instrumentation; a spine with pedicle screw fixation at L4-L5; a spine with pedicle screw fixation and interbody cages at L4-L5; and a spine with interbody fusion at L4-L5. The analysis considered three loading conditions: axial compression, flexion, and extension. Percentage changes in vertebrae cortical-bone and intervertebral lumbar disc stress were calculated relative to the healthy-spine value under the same loading condition. Average von Mises stresses in the L1-L3 cortical bone and the L1-L2 and L2-L3 intervertebral discs remained similar to the healthy spine. Fusion reduced stress in the L4 vertebra relative to the screw-only construct, whereas L5 cortical bone stress increased across all instrumented configurations. Screw only fixation produced the largest increases, while the fused screw and cage construct produced the smallest, demonstrating that construct design and fusion state influence load redistribution at the fused level. The superior adjacent L3–L4 disc experienced a consistent 34–42% increase in stress across all instrumented models, indicating elevated adjacent segment loading even when cage-based constructs reduced stresses at L5. Flexion generated the highest stresses, approximately 90% greater than extension and 35% greater than compression, highlighting the sensitivity of postoperative load sharing to loading direction. These findings show that L4–L5 fusion alters stress distribution within the fused segment and the superior adjacent level, providing comparative biomechanical insight into how construct design and loading conditions influence postoperative load transfer.