Thoracolumbar Biomechanical Analysis of Lenke Type 1 Adolescent Idiopathic Scoliosis Across Roussouly Classifications.

Background: Lenke type 1 is the most common adolescent idiopathic scoliosis (AIS), and its sagittal morphology critically influences progression and treatment. However, its biomechanical characteristics across Roussouly types remain unclear. Purpose: To quantify the biomechanical responses of Lenke type 1 AIS under pure bending moments across different Roussouly classifications. Methods: This study was based on a validated thoracolumbar finite element model. Using mesh morphing, spinal alignments and vertebral rotations were adjusted to construct finite element models of Lenke type 1 AIS with Roussouly types 1-4. Simulations were conducted under ±7.5 Nm pure bending moments for flexion-extension, lateral bending, and axial rotation. Spinal range of motion (ROM) and intervertebral disc loadings-including force, moment, and Von Mises stress-were quantified. Results: Compared to the normal model, the AIS model showed asymmetrical total ROM at the T7-T12 segment, whereas the T1-S1 segment remained relatively symmetrical. At the T9-T10 and T12-L1 discs, shear and compressive forces increased markedly, with peak values of 197 N and secondary moments reaching ~2.8 Nm. Stress in the T9-T10 disc exhibited a distinct concave-side concentration, with the maximum Von Mises stress reaching 7.7 MPa. The T1-S1 ROM during extension, right bending, and right rotation in Roussouly 1 and 2 was ~10% greater than in Roussouly 3 and 4, with markedly higher shear and compressive forces (up to 50-fold) at the T6-T7 and T9-T10 discs. Regarding stress distribution, Von Mises stress at the T6-T7 and T9-T10 discs was higher in Roussouly 3 and 4, whereas stress at the T12-L1 disc was more pronounced in Roussouly 1 and 2. Conclusion: The findings underscore the critical role of sagittal morphology in AIS biomechanics. Compared to Roussouly 1 and 2, Roussouly 3 and 4 exhibited reduced ROM, lower disc forces, and more favorable stress distributions, suggesting a biomechanically advantageous load-bearing pattern.