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June 18, 2026Open Access

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

IYIyan YounusNeurological Surgery HJHarsh JainNeurological Surgery RARanbir AhluwaliaNeurological Surgery

Key Points

This study aims to evaluate mechanical complication rates in adult spinal deformity patients with pelvic incidence-lumbar lordosis ≤10° and assess the relevance of the Roussouly classification.
Retrospective cohort study of adult spinal deformity surgery patients (2009-2021) with ≥5-level fusion and 2-year follow-up.
Analyzed postoperative pelvic incidence-lumbar lordosis ≤10°, Roussouly type, and mechanical complications.
Patient data included sacral slope, lumbar apex, and classification based on Roussouly types.
60% of patients (32/53) experienced mechanical complications; 28% (15/53) required reoperation.
Roussouly mismatch was observed more frequently in patients with PJK/F (86% vs. 57%, p=0.033).
Patients with complications were less likely to have lordosis apex at L4/5, showing a significant difference (6% vs. 33%, p=0.010).

Abstract

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.

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