Identifying Thresholds for Optimal Outcomes in Adult Spinal Deformity Patients with Isolated Pelvic Incidence Minus Lordosis (PI-LL) Mismatch

Introduction: The importance of spinopelvic harmony [pelvic incidence (PI) = lumbar lordosis (LL) ±10 degrees] is well established in the literature. However, patients presented with isolated increased PI-LL mismatch in the absence of global deformity, especially in regards to the often associated increase in sagittal vertical axis (SVA), is not well described in literature.

Methods: Surgical ASD patients (SVA≥5cm, PT≥25°, or TK ≥60°, >3 levels fused) ≥18 years old with available baseline (BL) radiographic data at baseline (BL) and 2-year (1Y) were isolated in the single-center database. Patients were isolated based upon meeting BL Lafage et al. SAAS TPA and PT metrics. Logistic predictive modeling and with achieving optimal outcomes, defined as ≥2 of the following: no PJK or PJF by 2Y, no major complication or reoperation by 2Y, achieving MCID or BCO in ODI. Conditional Inference Tree (CIT) analysis determined PI-LL thresholds for meeting optimal outcomes in patients considered matched in PT and TPA (pure PI-LL deformity).

Results: 355 included(61.38±13.66years, 77%female, 27.90±5.89kg/m2, CCI: 1.85±1.73). At baseline, 27.9% considered matched per SAAS. By SAAS components, 30.6% considered matched by PT, 30.1% considered matched in TPA, 17.5% considered matched in PI-LL. Of all included, 9.5% considered to have pure PI-LL mismatch. Logistic regression, accounting for BL age, gender, and levels fused, determined radiographic PI-LL and SAAS PI-LL modifiers to be significant predictors of achieving optimal outcomes. Linear regression identified a strong negative correlation between BL PI-LL mismatch and optimal outcomes score count (r2=.88, p<.001). CIT analysis revealed patients with a BL SAAS PI-LL point score of < 1, or mismatch of ≤ 15.44⁰ to be significantly more likely to achieve optimal outcomes by 2Y (p=.03, <.001, respectively).

Conclusion : Increased pelvic incidence minus lumbar lordosis (PI-LL) of 15.44⁰ or greater, or a baseline SAAS PI-LL modifier>1 is strongly associated with decreased likelihood of achieving optimal outcomes post-operatively, even in the absence of SAAS pelvic tilt or TPA mismatch. As such, operative treatment of ASD patients with pure PI-LL deformity may be clinically beneficial.