Biomechanical Analysis of Spinal Range of Motion and Intervertebral Disc Loadings in Normal and Adolescent Idiopathic Scoliosis Models

Haikuan Wang ¹ Zhengwei MA ² Zhihua Wu ³ Yuanfang Lin ⁴ Jie Yu ⁴ Xin Qian ⁴ Sili Jian ⁴ Yue-li Sun ⁵ Wei Wei ^6* Xiang Yu ³ Ziyang Liang ^4,7

• ¹ Acupuncture, Moxibustion and Rehabilitation School of Clinical Medicine, Guangzhou University of Chinese Medicine, Guangzhou 510405, China
• ² College of Urban Transportation and Logistics, Shenzhen Technology University, Shenzhen, Guangdong 518118, China
• ³ The First Affiliated Hospital of Guangzhou University of Chinese Medicine / Guangdong Clinical Research Academy of Chinese Medicine, Guangzhou 510405, China
• ⁴ Department of Tuina and Spinal Orthopaedics in Chinese Medicine, Shenzhen Traditional Chinese Medicine Hospital, The Fourth Clinical Medical College of Guangzhou University of Chinese Medicine, Shenzhen, Guangdong 518033, China
• ⁵ Longhua Hospital, Shanghai University of Traditional Chinese Medicine / Spine Institute, Key Laboratory of the Ministry of Education of Chronic Musculoskeletal Disease, Shanghai University of Traditional Chinese Medicine, Shanghai 200032, China
• ⁶ Laboratoire de Biomécanique Appliquée (UMRT24), Aix-Marseille Université/Université Gustave Eiffel, Marseille 13284, France
• ⁷ Department of Orthopedics, The Second Xiangya Hospital of Central South University, Changsha, 410011, China

Objective: This study aims to quantitatively explore and compare biomechanical responses in normal thoracolumbar spines and those with various curvatures of Lenke types under pure bending conditions. Methods: The baseline thoracolumbar finite element (FE) model was derived from a comprehensive human body FE model, validated, and calibrated against spinal responses under dynamic compression and quasi-static bending conditions. Using mesh morphing, Adolescent idiopathic scoliosis (AIS) models of Lenke 1, Lenke 2, Lenke 3, and Lenke 5 were established to represent their respective spinal curvatures. Pure bending moments of ±7.5Nm in flexion-extension, lateral bending, and axial rotation were applied to normal and AIS models. Spinal range of motion (ROM) were measured from the spinal segments T1-T6, T7-T12, and L1-Sacrum under each loading condition. Intervertebral disc (IVD) mechanical loadings, including force, moment, and VonMises stress, were also evaluated and compared across all models. Results: AIS models showed higher principal ROM compared to the normal model, with Lenke 2 having the highest ROM from T1-Sacrum and Lenke 3 the highest ROM from T6-12. AIS models exhibited more asymmetry in segmental ROM, particularly in the lumbar spine during lateral bending and axial rotation. IVD mechanical loadings varied significantly between normal and AIS models, influenced by spinal curvature types. AIS models had higher secondary moments and shear forces, especially under flexion-extension. The highest stress was mostly observed in the frontal IVD regions under flexion which was greatly reduced under extension. Lateral bending caused the highest stress predominantly on the same side as the loading direction in the IVD regions. The IVDs of T6-T7 and T12-L1 showed even stress distribution under axial rotation, while the right IVD regions of L5-Sacrum sustained the highest stress under right axial rotation, and the left regions under left axial rotation. In Lenke 3 and Lenke 5 models, the right (concave) regions of the T12-L1 IVD consistently sustained higher stress levels, regardless of the loading conditions. Conclusions: This study underscores significant biomechanical differences between normal and AIS models, revealing intricate interactions within scoliotic spines and enhancing our understanding of AIS biomechanics. These insights can aid in better diagnosis, treatment planning, and prognosis.