Establishing Optimal L1 Pelvic Angle Targets to Minimize Both Proximal Junctional Kyphosis and Pelvic Nonresponse in Adult Spinal Deformity Surgery

Objective: To determine the optimal targets of the L1 pelvic angle (L1PA) that minimize the risk of both proximal junctional kyphosis (PJK) and pelvic nonresponse (PNR) following adult spinal deformity (ASD) surgery. Methods: A retrospective study was conducted on 323 patients who underwent fusion surgery from the low thoracic spine (T9–12) to the pelvis and were followed up for 2 years. Risk factors for PJK and PNR were evaluated separately using multivariate logistic regression analysis. Receiver operating characteristic (ROC) curve analyses were performed to identify L1PA cutoff values predictive of PJK and PNR across 3 pelvic incidence (PI) categories: < 45°, 45°–60°, and ≥ 60°. L1PA thresholds were defined to delineate “ideal” alignment. Results: Risk factor analyses revealed that low L1PA was an independent risk factor for PJK (odds ratio [OR], 0.927; p = 0.019), while high PI–LL (OR, 1.101; p < 0.001) and high L1PA (OR, 1.249; p < 0.001) were significant risk factors for PNR. On ROC curve analyses, optimal L1PA ranges were 2.5°–4.5° for PI < 45°, 8.7°–12.6° for PI 45°–60°, and 15.1°– 17.3° for PI ≥ 60°. Patients within these ideal L1PA ranges had significantly lower rates of both PJK and PNR compared to those exceeding ideal L1PA ranges. Conclusion: This study demonstrated that optimal correction based on these L1PA targets reduced the risk of both PJK and PNR. Therefore, these L1PA targets can serve as reliable alignment goals to optimize surgical outcomes in ASD surgery.