JBJS - 2026-07-21 - Journal Article
Impact of T4-L1-Hip Axis Normalization on Outcomes and Complications in Adult Spinal Deformity.
Mohanty S, Sardar ZM, Reyes JL, Hassan FM, Kelly MP, Coury JR, Geraghty E, Lee NJ, Scheer JK, Roth SG, Hung CW, Lehman RA, Lombardi JM, Lenke LG
Topics
Key Takeaway
Both under- and overcorrection of T4PA-L1PA mismatch independently increased mechanical complication risk in a nonlinear (U-shaped) fashion (OR 1.01 per unit increase in squared deviation, p<0.001), while normalization did not improve SRS-22r or ODI MCID attainment.
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Summary
This single-center retrospective study asked whether postoperative T4PA-L1PA mismatch predicts mechanical complications (implant-related reoperations, particularly PJF) and PRO improvement after >6-level posterior spinal fusion for ASD. In multivariable logistic regression (AUC=0.72), squared T4PA-L1PA deviation independently predicted mechanical complications (OR 1.01, p<0.001) alongside higher CCI (OR 1.21) and pelvic fixation (OR 2.78), demonstrating a U-shaped risk curve. T4PA-L1PA normalization was not associated with MCID attainment for SRS-22r or ODI.
Key Limitation
The 6-week postoperative timepoint for T4PA-L1PA measurement conflates intended surgical correction with early construct behavior, making it impossible to determine whether complications result from the planned alignment target or from postoperative alignment loss.
Original Abstract
BACKGROUND
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).
METHODS
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).
RESULTS
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.
CONCLUSIONS
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.
LEVEL OF EVIDENCE
Prognostic Level IV. See Instructions for Authors for a complete description of levels of evidence.