Global Alignment Masks Residual Segmental Malalignment in Short-Segment Lumbar Fusions.

STUDY DESIGN: Retrospective cohort study. OBJECTIVE: To assess iatrogenic sagittal malalignment after short-segment lumbar fusions at L4-5, L5-S1, and L4-S1 using segmental alignment criteria and identify preoperative predictors. SUMMARY OF BACKGROUND DATA: PI-LL mismatch is commonly used to assess sagittal correction after lumbar fusion, but may mask segmental under-correction due to compensatory mechanisms, risking iatrogenic flatback deformity. METHODS: A multicenter database of patients undergoing fusion at L4-5, L5-S1, or L4-S1 was retrospectively analyzed. Global alignment was defined as PI-LL <10°, and segmental alignment as L4-5≥15°, L5-S1≥20°, or L4-S1≥35°. Alignment changes were categorized as preserved, restored, not corrected, or worsened. McNemar's test compared global versus segmental alignment proportions, and logistic regression identified predictors of postoperative segmental alignment. RESULTS: Of 516 patients analyzed (273 L4-5, 122 L5-S1, 121 L4-S1), segmental criteria classified far fewer patients as aligned than PI-LL at L5-S1 (42.3% vs. 82.7%) and L4-S1 (33.6% vs. 72.0%; both P<.001). Segmental assessment also revealed markedly higher worsening rates at L5-S1 (44.2% vs. 12.5%) and L4-S1 (55.1% vs. 22.4%; both P<.001), while rates were identical at L4-5 (20.2% vs. 20.2%, P=1.0). Anterior interbody approaches and greater preoperative segmental lordosis predicted restoration; baseline lumbar lordosis was the only independent predictor of preserved alignment at L5-S1 (OR 1.25, 95% CI 1.05-1.49, P=.01). CONCLUSION: Segmental assessment revealed substantially higher rates of malalignment and postoperative worsening than PI-LL alone, particularly in fusions including L5-S1. Anterior interbody techniques and greater preoperative lordosis were associated with improved correction. Surgeons should incorporate segmental alignment targets when planning distal lumbar fusions to avoid inadvertent flatback deformity.