Evaluating differences in the upper instrumented vertebra (UIV) tilt angle between adolescent idiopathic scoliosis patients with Lenke 1-ve (flexible) and Lenke 1 +ve (stiff) curves.

PURPOSE: To analyse the mean optimal upper instrumented vertebra (UIV) tilt angle in Lenke 1 subgroups with UIV at T2, T3, and T4, as derived from preoperative supervised cervical supine side-bending radiographs.

METHODS: This retrospective study included 288 Lenke 1 adolescent idiopathic scoliosis (AIS) patients who underwent posterior spinal fusion between 2020 and 2023, with UIV selected at T2, T3, or T4 levels. All patients were categorised into two subgroups: Lenke 1-ve (flexible) (proximal thoracic side bending (PTSB) Cobb angle: <15°) and Lenke 1 + ve (stiff) (PTSB Cobb angle: 15°-24.9°). Primary radiological parameters analysed included optimal UIV tilt angle (derived from preoperative supervised cervical supine side-bending radiographs), intraoperative UIV tilt angle, proximal thoracic (PT), main thoracic (MT), and thoracolumbar/lumbar (TL/L) Cobb angles, T1 tilt, clavicle angle (Cla-A), cervical axis (CA), and coronal balance (CB).

RESULTS: The optimal UIV tilt angle demonstrated excellent inter-rater and intra-rater reliability (ICC:0.890; 0.863). Lenke 1-ve and 1 + ve groups demonstrated different UIV level distributions (p < 0.001). In Lenke 1-ve group, the UIV levels were primarily at T3 (74.4%), followed by T4 (16.5%), and T2 (9.0%). Meanwhile, Lenke 1 + ve group had UIV at T3 (54.8%), followed by T2 (41.9%), and T4 (3.2%). The mean optimal UIV tilt differed significantly at T2, T3, and T4 for Lenke 1-ve group (-2.9°±3.9°, -5.2°±2.8°, -5.4°±1.5°) and Lenke 1 + ve group (-3.3°±2.7°; -5.4°±2.9°, -7.4°±5.2°) (p < 0.05). UIV level, apical vertebra level, and preoperative MT Cobb angle were significant predictive factors for optimal UIV tilt angle (p < 0.05). Patients with a negative preoperative T1 tilt had a more distal UIV level selected.

CONCLUSION: Lenke 1 + ve (stiff) group had a more proximal UIV level selected than Lenke 1-ve (flexible) group. Notably, UIV levels at T2, T3, and T4 demonstrated significantly different mean optimal UIV tilt angles, with more negative tilt magnitudes as the selection progressed caudally.