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

Retrospective cohort study.

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.

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.

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.

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).

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.