Mapping fusion zones to motion resistance: Finite element insights into return-to-activity after interbody fusion

Lumbar interbody fusion outcomes depend on the spatial distribution and maturity of the fusion mass relative to direction-specific loading. Using finite element analysis of a pedicle screw-rod instrumented L4-L5 construct with a central PEEK cage, we quantified how localized fusion in anterior (A), posterior (P), and left/right lateral (L/R) zones affect stiffness under flexion-extension, lateral bending, and torsion. 11 fusion configurations (one 4-zone; four 3-zone; and six 2-zone) were tested using 20 Latin Hypercube-sampled fusion moduli from a trabecular bone distribution. Stiffness ratios were benchmarked to full fusion using 90% equivalence bounds [0.90, 1.10], two one-sided tests (p < 0.05), and sub-equivalence probabilities. 3-zone patterns APL, APR, and ALR were equivalent in flexion-extension, while PLR was borderline. Lateral bending required bilateral lateral fusion (LR, ALR, PLR) to match full-fusion performance, whereas torsional equivalence demanded circumferential coverage with both lateral zones plus either A or P (ALR/PLR). Apart from lateral bending, 2-zone constructs were uniformly sub-equivalent, though higher-quality anterior-involved pairs approached sagittal equivalence. Overall, these findings support a functional stiffness-based definition of fusion that advocates deliberate circumferential grafting, especially robust bilateral lateral fusion, and inform graded load-specific rehabilitation that prioritizes anterior-posterior activities before side-bending and rotation.