Persistent postoperative coronal imbalance (PCI) and distal adding-on (AO) remain clinically relevant complications in patients with Lenke 5C adolescent idiopathic scoliosis (AIS) with fusion to L3, despite adequate curve correction. Although factors such as lowest instrumented vertebra (LIV) level, LIV tilt angle, and lumbosacral fractional curve characteristics have been implicated in postoperative decompensation, their predictive significance remains incompletely defined. We hypothesized that lumbosacral fractional curve stiffness may be reflected by the LIV tilt angle measured on the preoperative supine right-side-bending (RSB) radiograph when adjusted for pelvic obliquity (PO). This study aimed to investigate the radiographic factors associated with persistent PCI and distal AO in Lenke 5C AIS patients when L3 was selected as the LIV. This retrospective study included 50 patients with Lenke 5C AIS who underwent posterior spinal fusion between 2016 and 2022. The LIV tilt angle measured on preoperative supine RSB radiograph (𝑥 angle) and adjusted for PO was defined as the α angle (α=𝑥+PO). Primary outcomes included coronal balance distance (CBD) and distal AO. Logistic regression analyses were performed to identify the predictive factors for persistent PCI and distal AO. Persistent PCI occurred in 12.0% of patients (n=6/50), while distal AO was observed in 10.0% (n=5/50). At the final follow-up, persistent PCI group had a greater magnitude of ⍺ angle (-8.0° [-10.8° – -5.0°]) compared with balanced patients (-0.5° [-3.0° – 0.0°]) (p<0.001), with a mean CBD of -28.8±7.3mm. Multivariate logistic regression identified ⍺ angle as the significant predictor of persistent PCI (OR=0.463; 95% CI:0.263 – 0.817; p=0.008), while preoperative thoracic kyphosis (T5-T12) was significantly associated with distal AO (OR=0.860; 95% CI:0.767 – 0.963; p=0.009). Each 1° decrease in ⍺ angle (more negative ⍺ angle) was associated with an approximately 2.2-fold increased risk of persistent PCI. In conclusion, a more negative ⍺angle, reflecting increased lumbosacral fractional curve stiffness, was associated with a higher risk of persistent PCI following fusion to L3 in Lenke 5C AIS patients. Meanwhile, lower preoperative thoracic kyphosis was associated with distal AO.