Oral Presentation 51st International Society for the Study of the Lumbar Spine Annual Meeting 2025

Quantifying paraspinal muscle fibre orientation (a)symmetry in adolescent idiopathic scoliosis (115780)

Phoebe Duncombe 1 , Taylor Dick 1 , Bart Bolsterelee 2 , Kylie Tucker 1
  1. The University of Queensland, Brisbane, QUEENSLAND, Australia
  2. Neuroscience Research Australia, Sydney, New South Wales, Australia

Introduction: Adolescent idiopathic scoliosis (AIS) is characterized by an atypical 3D spinal curvature that develops and progresses between 10 and 18 years of age[1]. There is growing evidence of asymmetries in the paraspinal muscle size[2,3], quality[2] and activation[4] in adolescents with AIS, which are associated with AIS presence and curve progression. Given the potential asymmetries in paraspinal muscle force and the 3D spinal curvature, understanding the line of action of these muscular forces on vertebrae may provide insights into their contributions to curve progression[5]. This study aimed to quantify paraspinal muscle fibre architecture differences between adolescents with a primary right thoracic scoliotic curve and age-matched controls with symmetrical spines.

Methods: T1-weighted and diffusion tensor imaging (DTI) scans were conducted on 23 female adolescents with primary right-convex thoracic scoliosis [Cobb angle: 37±12°; age: 13.6±1.5 years] and 16 controls [age: 13.2±1.9 years]. Regions of interest for the multifidus and longissimus muscles were defined from vertebral levels T7 to L4. Muscle fibre architecture was reconstructed from DTI using anatomically constrained fibre tractography[6], and muscle fibre orientation was calculated as the relative angle (°) between the principal eigenvector of the diffusion tensor and the longitudinal axis of the muscle.

Results: AIS participants exhibited significant overall and regional asymmetries in paraspinal muscle fibre architecture compared to controls (p<0.05, Fig. 1).

Multifidus Muscle: AIS participants showed marked between-side asymmetry in the relative muscle fibre angle (Right-side: 37.8°±15.6°; Left-side: 45.2°±14.6°, p<0.05), with a predominance of greater fibre angles on the left-side, which was evident at most vertebral levels (T7-L1, L3-L4, p<0.05). Control participants displayed overall relative symmetry in muscle fibre angles (Right: 34.6°±19.2°; Left: 35.8°±15.3°, p>0.05), with minor vertebral level asymmetry (greater angles in the left muscle) observed only at T8.

Longissimus Muscle: While both groups exhibited symmetry in the overall left and right longissimus muscle relative fibre angles, AIS participants had overall greater muscle fibre angles than controls (AIS: Right-side: 28.5± 17.4°, Left-side: 27.9°±14.8°; Control: Right-side: 16.2°±7.8°, Left-side: 17.0°±9.7°, both p>0.05). However, significant vertebral level asymmetries in the AIS group were identified at vertebral levels T9, T10, T12, and L4, with predominantly greater angles on the left-side (p < 0.05). The control group showed minor vertebral level asymmetries at T8, L3, and L4.

Between-group analysis showed that AIS participants had significantly greater multifidus fibre angles than controls at vertebral levels T7, T9-T11, L1 and L3, primarily on the left-side (p < 0.05, Fig. 1A). Longissimus relative muscle fibre angles at vertebral levels T8-T9, T11-T12 and L4 were significantly greater in the AIS group than in the control group (p<0.05, Fig. 1B).

Discussion: Adolescents with AIS demonstrate significant asymmetries in multifidus and longissimus muscle fibre architecture, with greater relative fibre angles on the left-side of the spine than the right. These findings suggest an asymmetry in the paraspinal muscles' line of action and, therefore, an asymmetry in how these muscles may apply force to the growing vertebrae, potentially contributing to the asymmetrical loading and development of the spine.

 

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