Lumbar and Thoracolumbar Curves Are Associated with Coronal Lower Limb Malalignment in Adolescent Idiopathic Scoliosis

Background and Objectives: Adolescent idiopathic scoliosis (AIS) may influence pelvic orientation and lower-limb alignment; however, data on coronal lower-limb alignment after completion of spinal treatment remain limited. This study aimed to evaluate lower-limb radiographic alignment in AIS patients after spinal treatment and to determine whether these parameters differ according to main curve location. Materials and Methods: In this retrospective study, 70 AIS patients treated surgically (n = 52) or with brace therapy (n = 18) between 2010 and 2020 were analyzed. Patients were grouped according to main curve location as thoracic (n = 28), lumbar (n = 21), or thoracolumbar (n = 21). Pre-treatment standing full-spine radiographs were used to assess Cobb angle, coronal balance, and pelvic coronal obliquity angle (PCOA). After completion of spinal treatment, full-length weight-bearing lower-limb radiographs were evaluated for femoral and tibial lengths, mechanical axis deviation (MAD), femoral neck–shaft angle (NSA), anatomical lateral distal femoral angle (aLDFA), and mechanical lateral distal femoral angle (mLDFA). Additional treatment-stratified, treatment-adjusted, and threshold-based analyses were performed. Results: PCOA, coronal balance, bilateral MAD, right aLDFA, and right mLDFA differed significantly among the three curve-location groups. The lumbar group demonstrated more negative MAD values than the thoracic group, indicating a tendency toward valgus alignment (right MAD: −5.88 ± 8.8 mm vs. 3.65 ± 7.9 mm, p = 0.004; left MAD: −3.5 ± 7.5 mm vs. 3.75 ± 7.0 mm, p = 0.005). After adjustment for treatment modality, age, and main Cobb angle, curve location remained significantly associated with right MAD, left MAD, right aLDFA, and right mLDFA. However, the proportion of patients with clinically relevant malalignment, defined as MAD exceeding ±10 mm in at least one limb, did not differ significantly among the groups. Conclusions: AIS patients show subtle but measurable differences in coronal lower-limb alignment after completion of spinal treatment. Lumbar and thoracolumbar curves are associated with greater pelvic obliquity and a tendency toward more valgus mechanical-axis alignment, whereas limb lengths and NSA remain comparable among curve-location groups. These findings appear to represent mainly radiographic or biomechanical variations rather than overt clinically relevant deformity in most patients.