Impact of T4-L1-Hip Axis Normalization on Outcomes and Complications in Adult Spinal Deformity

In asymptomatic adults, a harmonious T4-L1-hip axis requires a T4-pelvic angle (T4PA) within 4° of the L1-pelvic angle (L1PA). We evaluated whether T4PA-L1PA mismatch is associated with mechanical complications (MCs) and patient-reported outcomes (PROs) after long-segment surgery for adult spinal deformity (ASD).

This single-center retrospective study involved a cohort of patients with ASD undergoing >6-level posterior spinal fusion (PSF) for spinal deformity. T4PA - L1PA was assessed at 6 weeks and followed for ≥2 years. The primary outcome was MCs (implant-related reoperations, with a particular focus on proximal junctional failure [PJF]) events. Multivariable logistic regression included linear and quadratic terms for T4PA - L1PA, adjusted for the Charlson Comorbidity Index (CCI), preoperative alignment, upper instrumented vertebra (UIV), pelvic fixation, and correction magnitude. Predicted probability curves depicted MC risk across the T4PA - L1PA spectrum. Secondary outcomes were attainment of the minimal clinically important difference (MCID) for the Scoliosis Research Society (SRS)-22r and Oswestry Disability Index (ODI).

The study included 427 patients (mean age, 61.16 ± 14.82 years; 285 [66.7%] female; mean of 12.50 ± 4.16 instrumented levels), with 78.7% undergoing pelvic fixation; 66 (15.5%) underwent MC-related reoperations. In multivariable analysis (area under the curve [AUC] = 0.72, p < 0.001), higher CCI (OR = 1.21, p = 0.029), pelvic fixation (OR = 2.78, p = 0.022), and greater (T4PA - L1PA)2 (OR = 1.01, p < 0.001) independently predicted MCs; the modeled risk suggested that both under- and overcorrection increased the risk of MCs. (T4PA - L1PA)2 was not associated with MCID attainment for the PROs.

Construct- and patient-related factors-particularly pelvic fixation-were associated with MCs. T4PA-L1PA deviation conferred additional nonlinear (U-shaped) risk: normalization was associated with fewer MCs, undercorrection was associated with implant-related failure, and overcorrection was associated with proximal junctional complications. Normalization was not associated with PRO improvement or attainment of the MCIDs for the SRS-22r and ODI.

Prognostic Level IV. See Instructions for Authors for a complete description of levels of evidence.