Early postoperative T1 slope is associated with 2-year cervical sagittal malalignment in Lenke 1–2 AIS with thoracic lordosis

Purpose To investigate postoperative cervicothoracic sagittal alignment changes in adolescent idiopathic scoliosis (AIS) patients with thoracic lordosis and to identify radiographic predictors of cervical sagittal malalignment (CSMA). Methods This retrospective cohort study included 72 patients with Lenke type 1 or 2 AIS with thoracic lordosis (T5–12 angle ≤ 0°) who underwent posterior spinal fusion. Radiographic parameters were evaluated preoperatively, at 1 month postoperatively (1 M), and at 2 years postoperatively (2Y). CSMA was defined as C2–7 lordosis ≤ − 10° or C2–7 sagittal vertical axis ≥ 40 mm at 2Y. Univariable and multivariable logistic regression analyses were performed to identify predictors of CSMA. Receiver operating characteristic (ROC) analysis was conducted to evaluate predictive performance and determine the optimal cutoff value of the T1 slope. Results CSMA was observed in 30 patients (41.7%) at 2Y. Multivariable logistic regression analysis demonstrated that a lower T1 slope at 1 M was independently associated with CSMA (OR 0.87, 95% CI 0.78–0.96, p = 0.01). ROC analysis showed that T1 slope predicted CSMA with an area under the curve (AUC) of 0.76. The optimal cutoff value was 7°, with a sensitivity of 73% and a specificity of 76%. Bootstrap resampling confirmed the stability of this threshold. Patients with a T1 slope ≤ 7° had a significantly increased risk of CSMA (adjusted OR 6.95, 95% CI 2.28–21.15, p < 0.01). Conclusions In AIS patients with thoracic lordosis, a low postoperative T1 slope was strongly associated with the development of CSMA. A T1 slope ≤ 7° at 1 month postoperatively predicted CSMA at 2 years with moderate accuracy. Achieving sufficient proximal thoracic kyphosis to maintain an adequate T1 slope may be important for preventing postoperative cervical sagittal malalignment.