For a study, researchers sought to evaluate the relationship between radiographic measures, such as the lowest instrumented vertebral (LIV) tilt, and the magnitude and rate of change of residual lumbar curves (LCs) after posterior spinal fusion for adolescent idiopathic scoliosis. The postoperative magnitude and probable progression of unfused LCs were still unknown, even though numerous criteria have been established for thoracic curve fusion. It was also uncertain how the LIV tilt would influence the LCs that were left behind. After a minimum of 2 years, patients with Lenke type 1 to 4 curves who underwent a posterior spinal fusion with LIV at or above L1 were assessed. Multivariate linear regressions with specific radiography variables were used to create prediction models for residual LCs. The variables that best predicted the development of residual LCs were subjected to subgroup analysis, followed by sensitivity testing. There were a total of 130 participants. High correlation coefficients (R=0.93 and 0.77, respectively) were found. Results from sensitivity analyses indicated that an immediate postoperative LIV-tilt angle less than 10° and a correction rate of main thoracic curve Cobb angle of more than 53% were both predictors of progression of residual LCs; when combined as one criterion, they achieved moderate discrimination (odds ratio=16.3, 95% CI=5.3-50.1; sensitivity=89%, specificity=67%, positive predicted value=51%, negative predicted value=94%). LIV tilt was found to be a significant predictor of the development of residual LCs and a determinant in prediction models, demonstrating a strong association with the magnitude of LCs after surgery. As a combined criterion, “immediate postoperative LIV-tilt angle less than 10° and rectification rate of major thoracic curve Cobb angle more than 53%” may be used to foretell the development of residual LCs.