Vertebral Rotation Discrepancy for Distal Add-on when Selecting the Touched Vertebra as the Lowest Instrumented Vertebra in Thoracic Curves of Adolescent Idiopathic Scoliosis

Study Design Retrospective cohort study. Objectives To assess the influencing factors of the distal add-on when selecting the touched vertebra (TV) as the lowest instrumented vertebra (LIV) in thoracic idiopathic scoliosis (IS). Methods A total of 96 thoracic IS patients who underwent deformity correction with LIV = TV were retrospectively analyzed to evaluate the distal add-on. Patients were divided into the DA group and the non-DA group. Radiological variables included coronal (Cobb’s angle, coronal balance, sacral slanting angle, and L4 vertebral tilt), sagittal (sagittal vertical axis, thoracic kyphosis, and lumbar lordosis), and axial parameters (vertebral rotation angle). Results When selecting the TV as the LIV, the occurrence rate of distal add-on was 20.8% (20 of 96) at a mean follow-up of 3 years. The mean number of remnant motion segments was significantly higher in the DA group (4.7) compared to the non-DA group (3.8) ( P < 0.001). The difference in the vertebral rotation angle between the lumbar apex and the TV level was significantly higher in the DA group (6.6°) than in the non-DA group (1.3°) ( P < 0.001). Multivariate logistic regression analysis demonstrated that the distal add-on was significantly associated with remnant motion segments (OR = 3.69, 95% CI: 1.03–13.18) and the vertebral rotation discrepancy (OR = 4.09, 95% CI: 1.98–8.43). Conclusions Greater vertebral rotation discrepancy between the LIV and lumbar apex was consistently associated with distal add-on when the TV was selected as the LIV, suggesting that three-dimensional rotational mismatch may contribute to residual distal add-on risk.