Impact of L1PA and T4PA Errors on Pelvic Tilt Deviation in Spinal Deformity Surgery

Introduction: Spino-pelvic alignment plays a critical role in optimizing postoperative outcomes in spinal deformity surgery. The L1 Pelvic Angle (L1PA) and T4 Pelvic Angle (T4PA) are essential components of the T4-L1-Hip Axis, which guides surgical correction. While achieving target L1PA and T4PA values is known to optimize sagittal balance, the effect of errors in these angles on pelvic tilt (PT) deviation remains unclear. We hypothesize that greater errors in L1PA and T4PA-L1PA mismatch will result in larger deviations from ideal PT alignment.

Methods: We retrospectively reviewed 116 thoracolumbar fusion patients at an academic tertiary care center between 2015 and 2024 with at least 6 months of follow-up. L1PA error, T4PA-L1PA mismatch, and their absolute values were treated as continuous variables. The ideal PT for each patient was calculated based on their pelvic incidence (PI) using the formula provided by the Global Alignment & Proportion score: Ideal PT = PI - [(PI × 0.59) + 9]. The primary outcome was the deviation of observed PT from this ideal value. Linear regression analyses were performed to assess the relationship between L1PA and T4PA errors with PT deviation.

Results: Patients with larger T4PA-L1PA mismatch had significantly greater PT deviations (coefficient = 0.62, p < 0.001), while L1PA error was also associated with PT deviation, though less strongly (coefficient = 0.27, p = 0.004). The absolute values of both L1PA and T4PA errors were also significant predictors of PT deviation (L1PA error: p = 0.02, T4PA-L1PA Mismatch: p < 0.001).

Conclusion : Achieving target regional (L1PA) and global (T4PA-L1PA mismatch elimination) significantly contributes to eliminating sagittal imbalance compensatory mechanisms such as pelvic retroversion.