Purpose <p>Selective thoracic fusion (STF) in adolescent idiopathic scoliosis (AIS) corrects the primary thoracic curve while achieving spontaneous lumbar curve correction (SLCC) and preserving flexibility. This study introduces a novel preoperative radiographic parameter defined as the ratio of the tilt of the upper and lower end vertebrae of the lumbar curve to the corresponding lumbar Cobb angle as a predictor of SLCC.</p> Methods <p>A multicenter retrospective review identified Lenke 1–4 patients with lumbar modifiers B or C undergoing STF. Radiographic parameters through 2-year follow-up included lumbar Cobb angle, UEV/LEV tilt, lumbar flexibility, and apical translation. LEV:Cobb and UEV:Cobb ratios were calculated by dividing vertebral tilt by preoperative lumbar Cobb. Univariate and multivariate regression assessed predictors of SLCC. Subgroup analyses were performed by lumbar modifier and fusion level relative to the stable vertebra.</p> Results <p>193 patients met inclusions criteria (mean age at time of surgery 15.0 ± 2.0&#xa0;years, 86% female). Mean preoperative lumbar Cobb was 41.2° ± 7.2, with mean SLCC of 56.8% ± 17.6% at 2&#xa0;years. In multivariate analysis, lower LEV:Cobb ratio (<i>p</i> = 0.012) and greater lumbar flexibility (<i>p</i> = 0.001) independently predicted higher SLCC; pelvic tilt trended toward significance (<i>p</i> = 0.071). Subgroup analysis showed stronger associations between tilt ratios and SLCC in modifier B curves (UEV:Cobb <i>p</i> = 0.001; LEV:Cobb <i>p</i> = 0.003).</p> Conclusions <p>LEV:Cobb ratio and lumbar flexibility are independent, preoperative predictors of SLCC in AIS patients undergoing STF. Incorporating these simple metrics into preoperative planning may improve fusion level selection, reduce residual deformity, and enhance postoperative balance.</p> Level of evidence <p>Level IV.</p>