Explaining Mechanical Failures in Adult Spinal Deformity Patients with Pelvic Incidence-Lumbar Lordosis Within 10°: Is the Roussouly Classification the Answer?

OBJECTIVES: Pelvic incidence (PI)-lumbar lordosis (LL) is a non-specific alignment goal that doesn't account for lordosis distribution, whereas the Roussouly classification describes distinct sagittal spinal shapes. Among adult spinal deformity (ASD) patients instrumented to pelvis, with PI-LL≤10°, we evaluated: 1)mechanical complication rates, 2)postoperative Roussouly type, and 3)whether Roussouly classification explains their complications. METHODS: A retrospective cohort-study included ASD surgery patients (2009-21) with:≥5-level-fusion, sagittal/coronal deformity, fusion-to-sacrum/pelvis, and 2-year follow-up. Independent variables were postoperative PI-LL≤10°, sacral slope-based current Roussouly type, PI-based theoretical Roussouly type, lumbar apex, inflection point, and pelvic-retroversion. Primary outcome was mechanical complications. RESULTS: Of 53 patients with postoperative PI-LL≤10°, 32 (60%) experienced mechanical complications, with 15 (28%) requiring reoperation. Among patients with/without mechanical complications, most common current Roussouly was type-2 (59% vs. 52%), and theoretical Roussouly was type-4 (47%) vs. 3 (43%), respectively. Roussouly mismatch was non-significantly higher in patients with mechanical complications (78% vs. 57%,p=0.104), and significantly higher in patients with PJK/F (86% vs. 57%,p=0.033). Patients with mechanical complications were less likely to have postoperative lordosis apex at L4/5 (6% vs. 33%,p=0.010); lordosis apex moved distally in 16%, rostrally in 59%, and stayed same in 25%. False Type-2 was non-significantly higher in patients with mechanical complications (38% vs 19%,p=0.152). CONCLUSION: Among ASD patients with PI-LL≤10°, mechanical complications occurred in 60%, and 28% required reoperation. Roussouly mismatch was more common in patients with PJK/F, and False Type-2 in patients with mechanical complications. PI-LL≤10° does not fully explain ASD alignment goals, and Roussouly classification provides insight into avoiding these complications.