Postoperative L1 tilt as a Predictor of Proximal Junctional Kyphosis Following Lower Thoracic Spine-To-Pelvis Fusion for Adult Spinal Deformity

Study Design. Retrospective study Objective. To investigate the association between postoperative orientation of the L1 vertebra and proximal junctional kyphosis (PJK) occurrence in adult spinal deformity (ASD) surgery Summary of Background Data. PJK remains a common complication, with various risk factors identified. However, the role of the orientation of L1 vertebra has not been well studied. Methods. A total of 312 patients who underwent fusion from the pelvis to the lower thoracic spine (T9-12) were analyzed. Patient, surgical, and radiographic variables were evaluated for risk factor analysis of PJK. L1 tilt and L1 slope at six weeks postoperatively were included to represent the L1 orientation. Multivariate logistic regression analysis was performed to identify risk factors for PJK. Receiver operating characteristics (ROC) curve analysis was used to calculate the cutoff value of predictors for PJK. Results. PJK developed in 109 patients (34.9%). Multivariate regression analysis identified postoperative L1 tilt as the only independent risk factor for PJK (odds ratio=1.173, P<0.001). The cutoff value of L1 tilt for predicting PJK was determined as 8.1° based on ROC curve analysis (area under the curve=0.736, P<0.001). The rates of PJK (50.7% vs. 22.1%, P<0.001) and revision surgery (17.1% vs. 5.2%, P<0.001) were significantly higher in the high L1 tilt group than in the low L1 tilt group. The high L1 tilt group also exhibited significantly greater pelvic tilt, thoracic kyphosis, and T1 pelvic angle, as well as worse clinical outcomes at two years compared to the low L1 tilt group. Conclusions. An L1 tilt greater than 8.1° was associated with a higher risk of PJK, suboptimal sagittal alignment, and worse clinical outcomes at 2 years. Therefore, optimizing L1 orientation may reduce PJK risk and improve long-term surgical outcomes.