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STUDY DESIGN A retrospective analysis. OBJECTIVE To evaluate whether the rotation of the presumed lower instrumented vertebra (LIV) on baseline convex side-bending (SB) radiographs leads to distal adding-on in Lenke 5C adolescent idiopathic scoliosis (AIS) patients. SUMMARY OF BACKGROUND DATA In Lenke 1A patients, derotation of the presumed LIV on baseline bending films is important owing to the association between LIV rotation and postoperative distal adding-on. However, the relationship between distal adding-on and derotation of the presumed LIV in Lenke 5C patients remains unknown. METHODS 85 Lenke 5C patients with a minimum of 2-year follow-up for posterior fusion were enrolled. L3 was selected as the LIV in all patients. Patients were divided into the presumed LIV derotation (DR group) and non-derotation (NDR group) groups according to derotation of the presumed LIV on SB films. Radiographic parameters, including Cobb angle, coronal balance, lower disc angle of LIV, LIV tilt, and LIV translation, were measured pre- and post-operatively. RESULTS Distal adding-on occurred in 16 patients (18.8%) at the final follow-up: 2 patients (10%) in the DR group and 14 patients (21.5%) in the NDR group (p = 0.248). Distal adding-on incidence was not greatly reduced with derotation of the presumed LIV on SB films at baseline. In the NDR group, the immediately postoperative lower disc angle of the LIV, LIV tilt, and LIV translation were significantly smaller in patients without than in those with distal adding-on (all p < 0.05). Logistic regression analysis revealed that the immediately postoperative LIV tilt and LIV translation were significant predictors of distal adding-on. CONCLUSIONS Derotation of the presumed LIV on SB films may hint less risk of distal adding-on in Lenke 5C patients. Nevertheless, horizontalization of the LIV and minimizing LIV translation during correction could reduce the risk of distal adding-on despite the presence of LIV rotation at baseline. LEVEL OF EVIDENCE 4.